alias operators: '<<' same as '<+', '>>' same as '+>'. Insert and append operation optimized with linked lists

kern/fraction-type.go: changed some syntax
restructured some test files
2026-05-16 17:18:23 +02:00 · 2026-05-13 08:01:18 +02:00 · 2026-05-11 14:01:22 +02:00 · 2026-05-09 17:42:04 +02:00 · 2026-05-09 12:15:59 +02:00 · 2026-05-09 10:34:46 +02:00
143 changed files with 2510 additions and 1036 deletions
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // function.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-base.go
@@ -255,24 +255,31 @@ func setFunc(ctx kern.ExprContext, name string, args map[string]any) (result any
 	return
 }

-// func unsetFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-// 	var varName string
-// 	var ok bool
+func charFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+	var ord int

-// 	if varName, ok = args[kern.ParamName].(string); !ok {
-// 		return nil, kern.ErrWrongParamType(name, kern.ParamName, kern.TypeString, args[kern.ParamName])
-// 	} else {
-// 		ctx.GetParent().DeleteVar(varName)
-// 		result = nil
-// 	}
-// 	return
-// }
+	if n, ok := args[kern.ParamValue].(byte); ok {
+		ord = int(n)
+	} else if n, ok := args[kern.ParamValue].(int64); ok {
+		ord = int(n)
+	} else if n, ok := args[kern.ParamValue].(int); ok {
+		ord = n
+	} else {
+		return nil, kern.ErrWrongParamType(name, kern.ParamName, kern.TypeString, args[kern.ParamName])
+	}
+	if ord < 0 || ord > 255 {
+		err = kern.ErrFuncInvalidArg(name, fmt.Sprintf("character code must be in range 0-255, got %d", ord))
+	} else {
+		result = string(rune(ord))
+	}
+	return
+}

 //// import

 func ImportBuiltinsFuncs(ctx kern.ExprContext) {
 	anyParams := []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamValue),
+		kern.NewFuncParam(kern.ParamValue),
 	}

 	ctx.RegisterFunc("isNil", kern.NewGolangFunctor(isNilFunc), kern.TypeBoolean, anyParams)
@@ -290,28 +297,27 @@ func ImportBuiltinsFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("dec", kern.NewGolangFunctor(decFunc), kern.TypeFloat, anyParams)
 	ctx.RegisterFunc("string", kern.NewGolangFunctor(stringFunc), kern.TypeString, anyParams)
 	ctx.RegisterFunc("fract", kern.NewGolangFunctor(fractFunc), kern.TypeFraction, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamValue),
-		NewFuncParamFlagDef(ParamDenominator, PfDefault, int64(1)),
+		kern.NewFuncParam(kern.ParamValue),
+		kern.NewFuncParamFlagDef(ParamDenominator, kern.PfDefault, int64(1)),
 	})

 	ctx.RegisterFunc("eval", kern.NewGolangFunctor(evalFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamSource),
 	})

 	ctx.RegisterFunc("var", kern.NewGolangFunctor(varFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamName),
-		NewFuncParamFlagDef(kern.ParamValue, PfDefault, nil),
+		kern.NewFuncParam(kern.ParamName),
+		kern.NewFuncParamFlagDef(kern.ParamValue, kern.PfDefault, nil),
 	})

 	ctx.RegisterFunc("set", kern.NewGolangFunctor(setFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamName),
-		NewFuncParam(kern.ParamValue),
+		kern.NewFuncParam(kern.ParamName),
+		kern.NewFuncParam(kern.ParamValue),
 	})

-	// ctx.RegisterFunc("unset", kern.NewGolangFunctor(unsetFunc), kern.TypeAny, []kern.ExprFuncParam{
-	// 	NewFuncParam(kern.ParamName),
-	// 	NewFuncParam(kern.ParamValue),
-	// })
+	ctx.RegisterFunc("char", kern.NewGolangFunctor(charFunc), kern.TypeString, []kern.ExprFuncParam{
+		kern.NewFuncParam(kern.ParamValue),
+	})
 }

 func init() {
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-fmt.go
@@ -47,10 +47,10 @@ func printLnFunc(ctx kern.ExprContext, name string, args map[string]any) (result

 func ImportFmtFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("print", kern.NewGolangFunctor(printFunc), kern.TypeInt, []kern.ExprFuncParam{
-		NewFuncParamFlag(kern.ParamItem, PfRepeat),
+		kern.NewFuncParamFlag(kern.ParamItem, kern.PfRepeat),
 	})
 	ctx.RegisterFunc("println", kern.NewGolangFunctor(printLnFunc), kern.TypeInt, []kern.ExprFuncParam{
-		NewFuncParamFlag(kern.ParamItem, PfRepeat),
+		kern.NewFuncParamFlag(kern.ParamItem, kern.PfRepeat),
 	})
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-import.go
@@ -69,10 +69,10 @@ func doImport(ctx kern.ExprContext, name string, dirList []string, it kern.Itera

 func ImportImportFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("import", kern.NewGolangFunctor(importFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParamFlag(kern.ParamFilepath, PfRepeat),
+		kern.NewFuncParamFlag(kern.ParamFilepath, kern.PfRepeat),
 	})
 	ctx.RegisterFunc("importAll", kern.NewGolangFunctor(importAllFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParamFlag(kern.ParamFilepath, PfRepeat),
+		kern.NewFuncParamFlag(kern.ParamFilepath, kern.PfRepeat),
 	})
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-iterator.go
@@ -94,9 +94,9 @@ func runFunc(ctx kern.ExprContext, name string, args map[string]any) (result any

 func ImportIterFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("run", kern.NewGolangFunctor(runFunc), kern.TypeAny, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamIterator),
-		NewFuncParamFlag(iterParamOperator, PfOptional),
-		NewFuncParamFlag(iterParamVars, PfOptional),
+		kern.NewFuncParam(kern.ParamIterator),
+		kern.NewFuncParamFlag(iterParamOperator, kern.PfOptional),
+		kern.NewFuncParamFlag(iterParamVars, kern.PfOptional),
 	})
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-math-arith.go
@@ -172,11 +172,11 @@ func mulFunc(ctx kern.ExprContext, name string, args map[string]any) (result any

 func ImportMathFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("add", kern.NewGolangFunctor(addFunc), kern.TypeNumber, []kern.ExprFuncParam{
-		NewFuncParamFlagDef(kern.ParamValue, PfDefault|PfRepeat, int64(0)),
+		kern.NewFuncParamFlagDef(kern.ParamValue, kern.PfDefault|kern.PfRepeat, int64(0)),
 	})

 	ctx.RegisterFunc("mul", kern.NewGolangFunctor(mulFunc), kern.TypeNumber, []kern.ExprFuncParam{
-		NewFuncParamFlagDef(kern.ParamValue, PfDefault|PfRepeat, int64(1)),
+		kern.NewFuncParamFlagDef(kern.ParamValue, kern.PfDefault|kern.PfRepeat, int64(1)),
 	})
 }

@@ -0,0 +1,102 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// builtin-os-file.go
+package expr
+
+import (
+	"git.portale-stac.it/go-pkg/expr/file"
+	"git.portale-stac.it/go-pkg/expr/kern"
+)
+
+const fileByteIteratorType = "fileByteIterator"
+
+type fileFileByteIterator struct {
+	fileIterBase
+	b byte
+}
+
+func newFileByteIterator(r *file.Reader, autoClose bool) *fileFileByteIterator {
+	return &fileFileByteIterator{
+		fileIterBase: fileIterBase{reader: r, index: -1, count: 0, autoClose: autoClose},
+		b:            0}
+}
+
+func (it *fileFileByteIterator) TypeName() string {
+	return fileByteIteratorType
+}
+
+func (it *fileFileByteIterator) String() string {
+	return it.repr(fileByteIteratorType)
+}
+
+func (it *fileFileByteIterator) Next() (item any, err error) { // must return io.EOF after the last item
+	if it.b, err = it.reader.ReadByte(); err == nil {
+		it.increment()
+		item = it.b
+	} else if it.autoClose {
+		it.Clean()
+	}
+	return
+}
+
+func (it *fileFileByteIterator) Current() (item any, err error) {
+	item = it.b
+	return
+}
+
+func (it *fileFileByteIterator) Reset() (err error) {
+	if err = it.reader.Reset(); err == nil {
+		it.reset()
+		it.b = 0
+	}
+	return
+}
+
+func (it *fileFileByteIterator) Clean() (err error) {
+	if it.reader.Valid() {
+		if err = it.reader.GetFile().Close(); err == nil {
+			it.reader = nil
+		}
+	}
+	it.reset()
+	it.b = 0
+	return
+}
+
+func (it *fileFileByteIterator) CallOperation(name string, args map[string]any) (v any, err error) {
+	switch name {
+	case kern.NextName:
+		v, err = it.Next()
+	case kern.ResetName:
+		err = it.Reset()
+	case kern.CleanName:
+		err = it.Clean()
+	case kern.IndexName:
+		v = int64(it.Index())
+	case kern.CurrentName:
+		v, err = it.Current()
+	case kern.CountName:
+		v = it.count
+	default:
+		err = kern.ErrNoOperation(name)
+	}
+	return
+}
+
+func fileByteIteratorFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+	var handle *file.Reader
+	var invalidFileHandle any
+	var autoClose bool
+
+	if handle, invalidFileHandle, autoClose, err = initFileHandle(ctx, name, args); err == nil {
+		if handle != nil {
+			result = newFileByteIterator(handle, autoClose)
+		}
+	}
+
+	if err == nil && (handle == nil || invalidFileHandle != nil) {
+		err = errInvalidFileHandle(name, invalidFileHandle)
+	}
+	return
+}
@@ -0,0 +1,105 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// builtin-os-file.go
+package expr
+
+import (
+	"git.portale-stac.it/go-pkg/expr/file"
+	"git.portale-stac.it/go-pkg/expr/kern"
+)
+
+const fileLineIteratorType = "fileLineIterator"
+
+type fileFileLineIterator struct {
+	fileIterBase
+	line string
+}
+
+func newFileLineIterator(r *file.Reader, autoClose bool) *fileFileLineIterator {
+	return &fileFileLineIterator{
+		fileIterBase: fileIterBase{reader: r, index: -1, count: 0, autoClose: autoClose},
+		line:         "",
+	}
+}
+
+func (it *fileFileLineIterator) TypeName() string {
+	return fileLineIteratorType
+}
+
+func (it *fileFileLineIterator) String() string {
+	return it.repr(fileLineIteratorType)
+}
+
+func (it *fileFileLineIterator) Next() (item any, err error) { // must return io.EOF after the last item
+	if it.line, err = it.reader.ReadString('\n'); err == nil {
+		it.increment()
+		item = it.line[0 : len(it.line)-1]
+	} else if it.autoClose {
+		it.Clean()
+	}
+	return
+}
+
+func (it *fileFileLineIterator) Current() (item any, err error) {
+	if len(it.line) > 0 {
+		item = it.line[0 : len(it.line)-1]
+	}
+	return
+}
+
+func (it *fileFileLineIterator) Reset() (err error) {
+	if err = it.reader.Reset(); err == nil {
+		it.reset()
+		it.line = ""
+	}
+	return
+}
+
+func (it *fileFileLineIterator) Clean() (err error) {
+	if it.reader != nil {
+		if err = it.reader.Close(); err == nil {
+			it.reader = nil
+		}
+	}
+	it.reset()
+	it.line = ""
+	return
+}
+
+func (it *fileFileLineIterator) CallOperation(name string, args map[string]any) (v any, err error) {
+	switch name {
+	case kern.NextName:
+		v, err = it.Next()
+	case kern.ResetName:
+		err = it.Reset()
+	case kern.CleanName:
+		err = it.Clean()
+	case kern.IndexName:
+		v = int64(it.Index())
+	case kern.CurrentName:
+		v, err = it.Current()
+	case kern.CountName:
+		v = it.count
+	default:
+		err = kern.ErrNoOperation(name)
+	}
+	return
+}
+
+func fileLineIteratorFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+	var handle *file.Reader
+	var invalidFileHandle any
+	var autoClose bool
+
+	if handle, invalidFileHandle, autoClose, err = initFileHandle(ctx, name, args); err == nil {
+		if handle != nil {
+			result = newFileLineIterator(handle, autoClose)
+		}
+	}
+
+	if err == nil && (handle == nil || invalidFileHandle != nil) {
+		err = errInvalidFileHandle(name, invalidFileHandle)
+	}
+	return
+}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-os-file.go
@@ -6,146 +6,66 @@ package expr

 import (
 	"fmt"
-	"io"
 	"slices"

+	"git.portale-stac.it/go-pkg/expr/file"
 	"git.portale-stac.it/go-pkg/expr/kern"
 )

 const paramHandleOrPath = "handle-or-path"
-const fileReadTextIteratorType = "fileReadTextIterator"

-type fileReadTextIterator struct {
-	osReader  *osReader
+type fileIterBase struct {
+	reader    *file.Reader
 	index     int64
 	count     int64
-	line      string
 	autoClose bool
 }

-func newReadTextIterator(r *osReader, autoClose bool) *fileReadTextIterator {
-	return &fileReadTextIterator{osReader: r, index: -1, autoClose: autoClose}
-}
-
-func (it *fileReadTextIterator) TypeName() string {
-	return fileReadTextIteratorType
-}
-
-func (it *fileReadTextIterator) String() string {
-	if it.osReader != nil && it.osReader.fh != nil {
-		return fmt.Sprintf("$(%s@%q)", fileReadTextIteratorType, it.osReader.fh.Name())
-	}
-	return fmt.Sprintf("$(%s@<nil>)", fileReadTextIteratorType)
-}
-
-func (it *fileReadTextIterator) Count() int64 {
+func (it *fileIterBase) Count() int64 {
 	return it.count
 }

-func (it *fileReadTextIterator) Next() (item any, err error) { // must return io.EOF after the last item
-	if it.osReader.fh != nil {
-		if it.line, err = it.osReader.reader.ReadString('\n'); err == nil {
-			it.index++
-			it.count++
-			item = it.line[0 : len(it.line)-1]
-		} else if it.autoClose {
-			it.Clean()
-		}
-	}
-	return
-}
-
-func (it *fileReadTextIterator) Current() (item any, err error) {
-	if len(it.line) > 0 {
-		item = it.line[0 : len(it.line)-1]
-	}
-	return
-}
-
-func (it *fileReadTextIterator) Index() int64 {
+func (it *fileIterBase) Index() int64 {
 	return it.index
 }

-func (it *fileReadTextIterator) Reset() (err error) {
-	if _, err = it.osReader.fh.Seek(0, io.SeekStart); err == nil {
-		it.index = -1
-		it.count = 0
-		it.line = ""
-	}
-	return
-}
-
-func (it *fileReadTextIterator) HasOperation(name string) bool {
+func (it *fileIterBase) HasOperation(name string) bool {
 	return slices.Contains([]string{kern.NextName, kern.ResetName, kern.IndexName, kern.CountName, kern.CurrentName, kern.CleanName}, name)
 }

-func (it *fileReadTextIterator) Clean() (err error) {
-	if it.osReader.fh != nil {
-		if err = it.osReader.fh.Close(); err == nil {
-			it.osReader = nil
-		}
-	}
-	return nil
+func (it *fileIterBase) reset() {
+	it.index = -1
+	it.count = 0
 }

-func (it *fileReadTextIterator) CallOperation(name string, args map[string]any) (v any, err error) {
-	switch name {
-	case kern.NextName:
-		v, err = it.Next()
-	case kern.ResetName:
-		err = it.Reset()
-	case kern.CleanName:
-		err = it.Clean()
-	case kern.IndexName:
-		v = int64(it.Index())
-	case kern.CurrentName:
-		v, err = it.Current()
-	case kern.CountName:
-		v = it.count
-	default:
-		err = kern.ErrNoOperation(name)
-	}
-	return
+func (it *fileIterBase) increment() {
+	it.index++
+	it.count++
 }

-func fileReadIteratorFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-	var handle *osReader
-	var invalidFileHandle any
-	var ok, autoClose bool
+func (it *fileIterBase) repr(typeName string) string {
+	if it.reader.Valid() {
+		return fmt.Sprintf("$(%s@%q)", typeName, it.reader.GetName())
+	}
+	return fmt.Sprintf("$(%s@<nil>)", typeName)
+}

-	result = nil
-	if handle, ok = args[paramHandleOrPath].(*osReader); !ok {
+func initFileHandle(ctx kern.ExprContext, name string, args map[string]any) (handle *file.Reader, invalidFileHandle any, autoClose bool, err error) {
+	var ok bool
+
+	if handle, ok = args[paramHandleOrPath].(*file.Reader); !ok {
 		if fileName, ok := args[paramHandleOrPath].(string); ok && len(fileName) > 0 {
 			var handleAny any
 			if handleAny, err = openFileFunc(ctx, name, map[string]any{kern.ParamFilepath: fileName}); err != nil {
 				return
 			}
 			if handleAny != nil {
-				handle = handleAny.(*osReader)
+				handle = handleAny.(*file.Reader)
 				autoClose = true
 			}
 		} else {
 			invalidFileHandle = args[paramHandleOrPath]
 		}
 	}
-
-	if handle != nil {
-		result = newReadTextIterator(handle, autoClose)
-
-	}
-
-	if err == nil && (handle == nil || invalidFileHandle != nil) {
-		err = errInvalidFileHandle(name, invalidFileHandle)
-	}
 	return
 }
-
-// func ImportOsIterFuncs(ctx ExprContext) {
-// 	ctx.RegisterFunc("fileReadIterator", NewGolangFunctor(fileReadIteratorFunc), TypeIterator, []ExprFuncParam{
-// 		NewFuncParam(paramHandleOrPath),
-// 	})
-// }
-
-// func init() {
-// 	RegisterBuiltinModule("os.file", ImportOsIterFuncs, "Operating system file iterator functions")
-// }
@@ -1,15 +1,14 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-os-file.go
 package expr

 import (
-	"bufio"
 	"fmt"
 	"io"
-	"os"

+	"git.portale-stac.it/go-pkg/expr/file"
 	"git.portale-stac.it/go-pkg/expr/kern"
 )

@@ -17,44 +16,6 @@ const (
 	osLimitCh = "limitCh"
 )

-type osHandle interface {
-	getFile() *os.File
-}
-
-type osWriter struct {
-	fh     *os.File
-	writer *bufio.Writer
-}
-
-func (h *osWriter) TypeName() string {
-	return "osWriter"
-}
-
-func (h *osWriter) String() string {
-	return "writer"
-}
-
-func (h *osWriter) getFile() *os.File {
-	return h.fh
-}
-
-type osReader struct {
-	fh     *os.File
-	reader *bufio.Reader
-}
-
-func (h *osReader) TypeName() string {
-	return "osReader"
-}
-
-func (h *osReader) String() string {
-	return "reader"
-}
-
-func (h *osReader) getFile() *os.File {
-	return h.fh
-}
-
 func errMissingFilePath(funcName string) error {
 	return fmt.Errorf("%s(): missing or invalid file path", funcName)
 }
@@ -67,24 +28,26 @@ func errInvalidFileHandle(funcName string, v any) error {
 	}
 }

-func createFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+func openFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
 	if filePath, ok := args[kern.ParamFilepath].(string); ok && len(filePath) > 0 {
-		var fh *os.File
-		if fh, err = os.Create(filePath); err == nil {
-			result = &osWriter{fh: fh, writer: bufio.NewWriter(fh)}
-		}
+		// var fh *os.File
+		// if fh, err = os.Open(filePath); err == nil {
+		// 	result = file.NewReader(fh)
+		// }
+		result, err = file.OpenReader(filePath)
 	} else {
 		err = errMissingFilePath(name)
 	}
 	return
 }

-func openFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+func createFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
 	if filePath, ok := args[kern.ParamFilepath].(string); ok && len(filePath) > 0 {
-		var fh *os.File
-		if fh, err = os.Open(filePath); err == nil {
-			result = &osReader{fh: fh, reader: bufio.NewReader(fh)}
-		}
+		// var fh *os.File
+		// if fh, err = os.Create(filePath); err == nil {
+		// 	result = file.NewWriter(fh)
+		// }
+		result, err = file.CreateWriter(filePath)
 	} else {
 		err = errMissingFilePath(name)
 	}
@@ -93,10 +56,11 @@ func openFileFunc(ctx kern.ExprContext, name string, args map[string]any) (resul

 func appendFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
 	if filePath, ok := args[kern.ParamFilepath].(string); ok && len(filePath) > 0 {
-		var fh *os.File
-		if fh, err = os.OpenFile(filePath, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0660); err == nil {
-			result = &osWriter{fh: fh, writer: bufio.NewWriter(fh)}
-		}
+		// var fh *os.File
+		// if fh, err = os.OpenFile(filePath, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0660); err == nil {
+		// 	result = file.NewWriter(fh)
+		// }
+		result, err = file.AppendWriter(filePath)
 	} else {
 		err = errMissingFilePath(name)
 	}
@@ -104,18 +68,18 @@ func appendFileFunc(ctx kern.ExprContext, name string, args map[string]any) (res
 }

 func closeFileFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-	var handle osHandle
+	var handle file.Handle
 	var invalidFileHandle any
 	var ok bool

-	if handle, ok = args[kern.ParamHandle].(osHandle); !ok {
+	if handle, ok = args[kern.ParamHandle].(file.Handle); !ok {
 		invalidFileHandle = args[kern.ParamHandle]
 	}

 	if handle != nil {
-		if fh := handle.getFile(); fh != nil {
-			if w, ok := handle.(*osWriter); ok {
-				err = w.writer.Flush()
+		if fh := handle.GetFile(); fh != nil {
+			if w, ok := handle.(*file.Writer); ok {
+				err = w.Flush()
 			}

 			if err == nil {
@@ -131,19 +95,20 @@ func closeFileFunc(ctx kern.ExprContext, name string, args map[string]any) (resu
 }

 func fileWriteTextFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-	var handle osHandle
+	var handle file.Handle
 	var invalidFileHandle any
 	var ok bool

-	if handle, ok = args[kern.ParamHandle].(osHandle); !ok {
+	if handle, ok = args[kern.ParamHandle].(file.Handle); !ok {
 		invalidFileHandle = args[kern.ParamHandle]
 	}

 	if handle != nil {
-		if w, ok := handle.(*osWriter); ok {
+		if w, ok := handle.(*file.Writer); ok {
 			if v, exists := args[kern.ParamItem]; exists {
 				argv := v.([]any)
-				result, err = fmt.Fprint(w.writer, argv...)
+				// result, err = fmt.Fprint(w.writer, argv...)
+				result, err = w.Write(argv...)
 			}
 		} else {
 			invalidFileHandle = handle
@@ -157,24 +122,24 @@ func fileWriteTextFunc(ctx kern.ExprContext, name string, args map[string]any) (
 }

 func fileReadTextFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-	var handle osHandle
+	var handle file.Handle
 	var invalidFileHandle any
 	var ok bool

 	result = nil
-	if handle, ok = args[kern.ParamHandle].(osHandle); !ok || args[kern.ParamHandle] == nil {
+	if handle, ok = args[kern.ParamHandle].(file.Handle); !ok || args[kern.ParamHandle] == nil {
 		invalidFileHandle = args[kern.ParamHandle]
 	}

 	if handle != nil {
-		if r, ok := handle.(*osReader); ok {
+		if r, ok := handle.(*file.Reader); ok {
 			var limit byte = '\n'
 			var v string
 			if s, ok := args[osLimitCh].(string); ok && len(s) > 0 {
 				limit = s[0]
 			}

-			v, err = r.reader.ReadString(limit)
+			v, err = r.ReadString(limit)
 			if err == io.EOF {
 				err = nil
 			}
@@ -197,19 +162,19 @@ func fileReadTextFunc(ctx kern.ExprContext, name string, args map[string]any) (r
 }

 func fileReadTextAllFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
-	var handle osHandle
+	var handle file.Handle
 	var invalidFileHandle any
 	var ok bool

 	result = nil
-	if handle, ok = args[kern.ParamHandle].(osHandle); !ok || args[kern.ParamHandle] == nil {
+	if handle, ok = args[kern.ParamHandle].(file.Handle); !ok || args[kern.ParamHandle] == nil {
 		invalidFileHandle = args[kern.ParamHandle]
 	}

 	if handle != nil {
-		if r, ok := handle.(*osReader); ok {
+		if r, ok := handle.(*file.Reader); ok {
 			var b []byte
-			b, err = io.ReadAll(r.reader)
+			b, err = r.ReadAll()
 			result = string(b)
 		} else {
 			invalidFileHandle = handle
@@ -224,38 +189,43 @@ func fileReadTextAllFunc(ctx kern.ExprContext, name string, args map[string]any)

 func ImportOsFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("fileOpen", kern.NewGolangFunctor(openFileFunc), kern.TypeFileHandle, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamFilepath),
+		kern.NewFuncParam(kern.ParamFilepath),
 	})

 	ctx.RegisterFunc("fileAppend", kern.NewGolangFunctor(appendFileFunc), kern.TypeFileHandle, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamFilepath),
+		kern.NewFuncParam(kern.ParamFilepath),
 	})

 	ctx.RegisterFunc("fileCreate", kern.NewGolangFunctor(createFileFunc), kern.TypeFileHandle, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamFilepath),
+		kern.NewFuncParam(kern.ParamFilepath),
 	})

 	ctx.RegisterFunc("fileClose", kern.NewGolangFunctor(closeFileFunc), kern.TypeBoolean, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamHandle),
+		kern.NewFuncParam(kern.ParamHandle),
 	})

 	ctx.RegisterFunc("fileWriteText", kern.NewGolangFunctor(fileWriteTextFunc), kern.TypeInt, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamHandle),
-		NewFuncParamFlagDef(kern.ParamItem, PfDefault|PfRepeat, ""),
+		kern.NewFuncParam(kern.ParamHandle),
+		kern.NewFuncParamFlagDef(kern.ParamItem, kern.PfDefault|kern.PfRepeat, ""),
 	})

 	ctx.RegisterFunc("fileReadText", kern.NewGolangFunctor(fileReadTextFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamHandle),
-		NewFuncParamFlagDef(osLimitCh, PfDefault, "\n"),
+		kern.NewFuncParam(kern.ParamHandle),
+		kern.NewFuncParamFlagDef(osLimitCh, kern.PfDefault, "\n"),
 	})

 	ctx.RegisterFunc("fileReadTextAll", kern.NewGolangFunctor(fileReadTextAllFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamHandle),
+		kern.NewFuncParam(kern.ParamHandle),
 	})

-	ctx.RegisterFunc("fileReadIterator", kern.NewGolangFunctor(fileReadIteratorFunc), kern.TypeIterator, []kern.ExprFuncParam{
-		NewFuncParam(paramHandleOrPath),
+	ctx.RegisterFunc("fileLineIterator", kern.NewGolangFunctor(fileLineIteratorFunc), kern.TypeIterator, []kern.ExprFuncParam{
+		kern.NewFuncParam(paramHandleOrPath),
 	})
+
+	ctx.RegisterFunc("fileByteIterator", kern.NewGolangFunctor(fileByteIteratorFunc), kern.TypeIterator, []kern.ExprFuncParam{
+		kern.NewFuncParam(paramHandleOrPath),
+	})
+
 }

 func init() {
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtin-string.go
@@ -225,44 +225,44 @@ func lowerStrFunc(ctx kern.ExprContext, name string, args map[string]any) (resul
 // Import above functions in the context
 func ImportStringFuncs(ctx kern.ExprContext) {
 	ctx.RegisterFunc("strJoin", kern.NewGolangFunctor(joinStrFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSeparator),
-		NewFuncParamFlag(kern.ParamItem, PfRepeat),
+		kern.NewFuncParam(kern.ParamSeparator),
+		kern.NewFuncParamFlag(kern.ParamItem, kern.PfRepeat),
 	})

 	ctx.RegisterFunc("strSub", kern.NewGolangFunctor(subStrFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
-		NewFuncParamFlagDef(kern.ParamStart, PfDefault, int64(0)),
-		NewFuncParamFlagDef(kern.ParamCount, PfDefault, int64(-1)),
+		kern.NewFuncParam(kern.ParamSource),
+		kern.NewFuncParamFlagDef(kern.ParamStart, kern.PfDefault, int64(0)),
+		kern.NewFuncParamFlagDef(kern.ParamCount, kern.PfDefault, int64(-1)),
 	})

 	ctx.RegisterFunc("strSplit", kern.NewGolangFunctor(splitStrFunc), "list of "+kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
-		NewFuncParamFlagDef(kern.ParamSeparator, PfDefault, ""),
-		NewFuncParamFlagDef(kern.ParamCount, PfDefault, int64(-1)),
+		kern.NewFuncParam(kern.ParamSource),
+		kern.NewFuncParamFlagDef(kern.ParamSeparator, kern.PfDefault, ""),
+		kern.NewFuncParamFlagDef(kern.ParamCount, kern.PfDefault, int64(-1)),
 	})

 	ctx.RegisterFunc("strTrim", kern.NewGolangFunctor(trimStrFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamSource),
 	})

 	ctx.RegisterFunc("strStartsWith", kern.NewGolangFunctor(startsWithStrFunc), kern.TypeBoolean, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
-		NewFuncParam(kern.ParamPrefix),
-		NewFuncParamFlag(strParamOther, PfRepeat),
+		kern.NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamPrefix),
+		kern.NewFuncParamFlag(strParamOther, kern.PfRepeat),
 	})

 	ctx.RegisterFunc("strEndsWith", kern.NewGolangFunctor(endsWithStrFunc), kern.TypeBoolean, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
-		NewFuncParam(kern.ParamSuffix),
-		NewFuncParamFlag(strParamOther, PfRepeat),
+		kern.NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamSuffix),
+		kern.NewFuncParamFlag(strParamOther, kern.PfRepeat),
 	})

 	ctx.RegisterFunc("strUpper", kern.NewGolangFunctor(upperStrFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamSource),
 	})

 	ctx.RegisterFunc("strLower", kern.NewGolangFunctor(lowerStrFunc), kern.TypeString, []kern.ExprFuncParam{
-		NewFuncParam(kern.ParamSource),
+		kern.NewFuncParam(kern.ParamSource),
 	})
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // builtins-register.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // data-cursors.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // dict-iterator.go
@@ -116,8 +116,11 @@ func NewDictIterator(dict *kern.DictType, args []any) (it *DictIterator, err err
 		}
 	}

-	dictIt.makeKeys(*dict, sortType)
-	return dictIt, err
+	if err == nil {
+		dictIt.makeKeys(*dict, sortType)
+		it = dictIt
+	}
+	return
 }

 func NewMapIterator(m map[any]any) (it *DictIterator) {
@@ -139,7 +142,6 @@ func (it *DictIterator) TypeName() string {
 }

 func (it *DictIterator) HasOperation(name string) bool {
-	// yes := name == NextName || name == ResetName || name == IndexName || name == CountName || name == CurrentName
 	yes := slices.Contains([]string{kern.NextName, kern.ResetName, kern.IndexName, kern.CountName, kern.CurrentName, kern.CleanName, kern.KeyName, kern.ValueName}, name)
 	return yes
 }
@@ -34,7 +34,7 @@ Expressions calculator

 toc::[]

-#TODO: Work in progress (last update on 2026/04/21, 6:49 p.m.)#
+#TODO: Work in progress (last update on 2026/05/08)#

 == Expr
 _Expr_ is a GO package that can analyze, interpret and calculate expressions.
@@ -698,9 +698,9 @@ The value on the left side of [blue]`=` must be a variable identifier or an expr
 === Selector operator [blue]`? : ::`
 The _selector operator_ is very similar to the _switch/case/default_ statement available in many programming languages.

-.Selector literal Syntax
+.Selector literal syntax
 ====
-_selector-operator_ = _select-expression_ "*?*" _selector-case_ { "*:*" _selector-case_ } ["*::*" _default-multi-expression_] +
+*_selector-operator_* = _select-expression_ "*?*" _selector-case_ { "*:*" _selector-case_ } ["*::*" _default-multi-expression_] +
 _selector-case_ = [_match-list_] _case-value_ +
 _match-list_ = "*[*" _item_ {"*,*" _items_} "*]*" +
 _item_ = _expression_ +
@@ -737,19 +737,38 @@ The [blue]`:` symbol (colon) is the separator of the selector-cases. Note that i
 `>>>` [blue]`10 ? {"a"} : {"b"}` +
 [red]`Eval Error: [1:3] no case catches the value (10) of the selection expression`

+==== Triple special case of the selector operator
+If the _select-expression_ is a boolean expression, the selector operator can be used as a sort of _if-then-else_ statement. In this case, the first case is evaluated if the _select-expression_ is true, and the second case is evaluated if the _select-expression_ is false. In this special case, the _match-list_ of both cases must be empty.
+
+.Example
+`>>>` [blue]`(true) ? {"T"}: {"F"}` +
+[green]`T` +
+`>>>` [blue]`(2 > 1) ? {"a"} : {"b"}` +
+[green]`a` +
+`>>>` [blue]`(2 < 1) ? {"a"} : {"b"}` +
+[green]`b`
+
+[WARNING]
+====
+The triple special case of the selector operator is very useful, but it only works with boolean expressions.
+
+.Example of confusion
+`>>>` [blue]`int(true) ? {"T"}: {"F"}` +
+[green]`F`
+====

 === Variable default value [blue]`??`, [blue]`?=`, and [blue]`?!`
-The left operand of the first two operators, [blue]`??` and [blue]`?=`, must be a variable. The right operator can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.
+The left operand of the first two operators, [blue]`??` and [blue]`?=`, must be a variable. The right operatand can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.

 IMPORTANT: If the left variable is defined, the right expression is not evaluated at all.

 The [blue]`??` operator do not change the status of the left variable.

-The [blue]`?=` assigns the calculated value of the right expression to the left variable.
+The [blue]`?=` assigns the calculated value of the right expression to the variable on the left side.

 The third one, [blue]`?!`, is the alternate operator. If the variable on the left size is not defined, it returns [blue]_nil_. Otherwise it returns the result of the expressione on the right side.

-IMPORTANT: If the left variable is NOT defined, the right expression is not evaluated at all.
+IMPORTANT: If the variable [blue]`?!` is NOT defined, the expression is not evaluated at all.

 .Examples
 `>>>` [blue]`var ?? (1+2)` +
@@ -836,8 +855,14 @@ The table below shows all supported operators by decreasing priorities.
 .2+|*INSERT*| [blue]`+>` | _Infix_ | _Prepend_ | _any_ `+>` _list_ -> _list_
 | [blue]`<+` | _Infix_ | _Append_ | _list_ `<+` _any_ -> _list_
 .2+|*ASSIGN*| [blue]`=` | _Infix_ | _Assignment_ | _identifier_ `=` _any_ -> _any_
-4+| _See also the table of special allocation operators below_
+4+| _See also the table of special assignment operators below_
 .1+|*BUT*| [blue]`but` | _Infix_ | _But_ | _any_ `but` _any_ -> _any_
+.6+|*ITER-OP*| [blue]`digest` | _Infix_ | _Item-digesting_ | _iterable_ `digest` _expr_ -> _any_
+| [blue]`filter` | _Infix_ | _Item-filtering_ | _iterable_ `filter` _expr_ -> _list_
+| [blue]`groupby` | _Infix_ | _Dict-grouping_ | _iterable_ `groupby` _key-expr_ -> _dict_
+| [blue]`join` | _Infix_ | _Item-joining_ | _iterable_ `join` _iterable_ -> _list_
+| [blue]`map` | _Infix_ | _Item-mapping_ | _iterable_ `map` _-expr_ -> _list_
+4+| _See iterators section for examples_
 .1+|*RANGE*| [blue]`:` | _Infix_ | _Index-range_ | _integer_ `:` _integer_ -> _integer-pair_
 |===

@@ -921,12 +946,12 @@ _param-name_ = _identifier_
 [green]`fib(n):any{}`

 `>>>` [gray]_// Required and optional parameters_ +
-`>>>` [blue]`measure = func(value, unit="meter"){ value + " " + unit + (value > 1) ? [true] {"s"} :: {""}}` +
+`>>>` [blue]`measure = func(value, unit="meter"){ value + " " + unit + (value > 1) ? {"s"} :: {""}}` +
 [green]`measure(value, unit="meter"):any{}`


 === _Golang_ function definition
-Description of how to define Golang functions and how to bind them to _Expr_ are topics covered in another document that I'll write, one day, maybe.
+Description of how to define Golang functions and how to bind them to _Expr_ are topics covered in another documents that I'll write, one day, maybe.

 === Function calls
 To call a function, either Expr or Golang type, it is necessary to specify its name and, at least, its required parameters.
@@ -1010,7 +1035,7 @@ Clone variables are normal local variables. The only diffence will appear when t
 .Example
 `>>>` [blue]`f = func() { @x = 3; x = 5 }` [gray]_// f() declares two *different* local variables: ``@x`` and ``x``_ +
 [green]`f():any{}` +
-`>>>` [blue]`f()` [gray]_// The multi-expression (two) in f() is calculated and the last result is returned_ +
+`>>>` [blue]`f()` [gray]_// The multi-expression (two expressions) in f() is calculated and the last result is returned_ +
 [green]`5` +
 `>>>` [blue]`x` [gray]_// The `x` variable was not defined in the main context before the f() invocation. It appears in the main context by cloning the `@x` variable, local to f() after its termnation._ +
 [green]`3`
@@ -1048,7 +1073,7 @@ Builtins activation is done by using the [blue]`BUILTIN` operator. All modules e

 .Builtin activation syntax
 ====
-*_builtin-activation_* = [blue]`BUILTIN` (_builtin-name_ | _list-of-builtin-names_) +
+*_builtin-activation_* = [blue]`BUILTIN` (_builtin-name_ | _list-of-builtin-names_ | **"*"**) +
 _builtin-name_ = _string_ +
 _list-of-builtin-names_ = **[** _string_ { "**,**" _string_ } **]**
 ====
@@ -1086,9 +1111,9 @@ The "base" builtin module provides functions for type checking and type conversi
 * <<_fract,fract()>>

 .Other functions
+* <<_char,char()>>
 * <<_eval,eval()>>
 * <<_set,set()>>
-* <<_unset,unset()>>
 * <<_var,var()>>


@@ -1100,7 +1125,9 @@ Returns _true_ if the value type of _<expr>_ is boolean, false otherwise.
 `>>>` [blue]`isBool(true)` +
 [green]`true` +
 `>>>` [blue]`isBool(3==2)` +
-[green]`true`
+[green]`true` +
+`>>>` [blue]`isBool(3 + 2)` +
+[green]`false`

 ===== isDict()
 Syntax: `isDict(<expr>) -> bool` +
@@ -1178,7 +1205,7 @@ Returns _true_ if the value type of _<expr>_ is fraction or int, false otherwise

 ===== isString()
 Syntax: `isString(<expr>) -> bool` +
-Returns a boolean value , false otherwise.
+Returns _true_ if the value type of _<expr>_ is string, false otherwise.

 .Examples
 `>>>` [blue]`isString("ciao")` +
@@ -1190,7 +1217,7 @@ Returns a boolean value , false otherwise.

 ===== bool()
 Syntax: `bool(<expr>) -> bool` +
-Returns a _boolean_ value consisent to the value of the expression.
+Returns a _boolean_ value consisent with the value of the expression.

 .Examples
 `>>>` [blue]`bool(1)` +
@@ -1295,6 +1322,16 @@ Returns a _fraction_ value consistent with the value of the expression.
 `>>>` [blue]`fract(true)` +
 [green]`1:1`

+===== char()
+Syntax: `char(<intexpr>) -> string` +
+Returns the character whose ASCII (soon Unicode too) code point is specified by the integer expression.
+
+.Examples
+`>>>` [blue]`char(65)` +
+[green]`"A"` +
+`>>>` [blue]`char(97)` +
+[green]`"a"`
+
 ===== eval()
 Syntax: `eval(<string-expr>) -> any` +
 Computes and returns the value of the [.underline]#string# expression.
@@ -1308,12 +1345,12 @@ Syntax: +
 `{4sp}var(<string-expr>, <expr>) -> any` +
 `{4sp}var(<string-expr>) -> any`

-This function allows you to define variables whose names must include special characters. The first form of the function allows you to define a variable with a name specified by the first parameter and assign it the value of the second parameter. The second form only returns the value of the variable with the specified name.
+This function allows you to define variables whose names can include special characters. The first form of the function allows you to define a variable with a name specified by the first parameter and assign it the value of the second parameter. The second form only returns the value of the variable with the specified name.

 .Examples
 `>>>` [blue]`var("$x", 3+9)` +
 [green]`12` +
-`>>>` [blue]`var("$x")` +
+`>>>` [blue]`var("$"+"x")` +
 [green]`12` +
 `>>>` [blue]`var("gain%", var("$x"))` +
 [green]`12` +
@@ -1334,26 +1371,17 @@ It is equivalent to the first form of the var() function, but it is more explici
 `>>>` [blue]`var("$x")` +
 [green]`100` +

-===== unset()
-Syntax: +
- `{4sp}unset(<string-expr>) -> any`
-
-This function allows you to unset a variable whose name can include special characters. The parameter is the name of the variable to unset.
-
-.Examples
-`>>>` [blue]`unset("$x")` +
-[green]`nil` +
-`>>>` [blue]`var("$x")` +
-[red]`Eval Error: var(): unknown variable "$x"`

 ==== Module "fmt"
+#to-do#

 ===== print()

 ===== println()

 ==== Module "import"
-Module actiovation: +
+Module activation: +
+`{4sp}BUILTIN "import"`

 ===== _import()_
 Syntax: +
@@ -1364,6 +1392,8 @@ Loads the multi-expression contained in the specified source and returns its val
 ===== _importAll()_

 ==== Module "iterator"
+Module activation: +
+`{4sp}BUILTIN "iterator"`

 ===== run()
 Syntax: +
@@ -1372,6 +1402,9 @@ Syntax: +
 Iterates over the specified iterator and applies the specified operator to the current value of the iterator.

 ==== Module "math.arith"
+Module activation: +
+`{4sp}BUILTIN "math.arith"`
+
 Currently, the "math.arith" module provides two functions, add() and mul(), that perform addition and multiplication of an arbitrary number of parameters. More functions will be added in the future.

 * <<_add,add()>>
@@ -1421,13 +1454,12 @@ Same as <<_add,add()>> but returns the product of the values of the parameters.
 [green]`24`

 ==== Module "os.file"
+Module activation: +
+`{4sp}BUILTIN "os.file"`
+
 The "os.file" module provides functions for working with files.

-Activation: +
-`{4sp}builtin "os.file"`
-
-Currently available functions:
-
+.File related functions
 * <<_fileOpen,fileOpen()>>
 * <<_fileAppend,fileAppend()>>
 * <<_fileCreate,fileCreate()>>
@@ -1436,6 +1468,10 @@ Currently available functions:
 * <<_fileReadText,fileReadText()>>
 * <<_fileReadTextAll,fileReadTextAll()>>

+.Iterator functions for files
+* <<_fileByteIterator,fileByteIterator()>>
+* <<_fileLineIterator,fileLineIterator()>>
+
 More functions will be added in the future.

 ---
@@ -1447,6 +1483,10 @@ Syntax: +
 Returns a file handle for the specified file path. The file is opened in read-write mode. If the file does not exist, it is created.

 ===== fileAppend()
+Syntax: +
+`{4sp}fileAppend(<file-path>) -> any`
+
+Like <<_fileCreate,fileCreate()>> but write operations happen at the end of the file.

 ===== fileCreate()
 Syntax: +
@@ -1455,21 +1495,30 @@ Syntax: +
 Creates or truncates the named _<file-path>_. If the file already exists, it is truncated. If the file does not exist, it is created with mode 0o666 (before umask). The associated file descriptor has mode [O_RDWR]. The directory containing the file must already exist.

 ===== fileClose()
+#to-do#

 ===== fileWriteText()
+#to-do#

 ===== fileReadText()
+#to-do#

 ===== fileReadTextAll()
+#to-do#
+
+===== fileByteIterator()
+#to-do#
+
+===== fileLineIterator()
+#to-do#


 ==== Module "string"
+Module activation: +
+`{4sp}BUILTIN "string"`
+
 This module provides functions for working with strings.

-Activation: +
-`{4sp}builtin "string"`
-
-
 Currently available functions:

 * <<_strJoin,strJoin()>>
@@ -0,0 +1,322 @@
+
+<!DOCTYPE html>
+<html>
+	<head>
+		<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+		<title>file: Go Coverage Report</title>
+		<style>
+			body {
+				background: black;
+				color: rgb(80, 80, 80);
+			}
+			body, pre, #legend span {
+				font-family: Menlo, monospace;
+				font-weight: bold;
+			}
+			#topbar {
+				background: black;
+				position: fixed;
+				top: 0; left: 0; right: 0;
+				height: 42px;
+				border-bottom: 1px solid rgb(80, 80, 80);
+			}
+			#content {
+				margin-top: 50px;
+			}
+			#nav, #legend {
+				float: left;
+				margin-left: 10px;
+			}
+			#legend {
+				margin-top: 12px;
+			}
+			#nav {
+				margin-top: 10px;
+			}
+			#legend span {
+				margin: 0 5px;
+			}
+			.cov0 { color: rgb(192, 0, 0) }
+.cov1 { color: rgb(128, 128, 128) }
+.cov2 { color: rgb(116, 140, 131) }
+.cov3 { color: rgb(104, 152, 134) }
+.cov4 { color: rgb(92, 164, 137) }
+.cov5 { color: rgb(80, 176, 140) }
+.cov6 { color: rgb(68, 188, 143) }
+.cov7 { color: rgb(56, 200, 146) }
+.cov8 { color: rgb(44, 212, 149) }
+.cov9 { color: rgb(32, 224, 152) }
+.cov10 { color: rgb(20, 236, 155) }
+
+		</style>
+	</head>
+	<body>
+		<div id="topbar">
+			<div id="nav">
+				<select id="files">
+				
+				<option value="file0">git.portale-stac.it/go-pkg/expr/file/file.go (88.9%)</option>
+				
+				<option value="file1">git.portale-stac.it/go-pkg/expr/file/reader.go (77.8%)</option>
+				
+				<option value="file2">git.portale-stac.it/go-pkg/expr/file/writer.go (100.0%)</option>
+				
+				</select>
+			</div>
+			<div id="legend">
+				<span>not tracked</span>
+			
+				<span class="cov0">not covered</span>
+				<span class="cov8">covered</span>
+			
+			</div>
+		</div>
+		<div id="content">
+		
+		<pre class="file" id="file0" style="display: none">// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// file.go
+package file
+
+import (
+        "os"
+
+        "git.portale-stac.it/go-pkg/expr/kern"
+)
+
+type Handle interface {
+        kern.Typer
+        GetFile() *os.File
+        GetName() string
+        Valid() bool
+        Close() error
+}
+
+type handleBase struct {
+        fh *os.File
+}
+
+func (h *handleBase) GetFile() *os.File <span class="cov0" title="0">{
+        return h.fh
+}</span>
+
+func (h *handleBase) GetName() (name string) <span class="cov8" title="1">{
+        if h.fh != nil </span><span class="cov8" title="1">{
+                name = h.fh.Name()
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (h *handleBase) Valid() bool <span class="cov8" title="1">{
+        return h.fh != nil
+}</span>
+
+func (h *handleBase) Close() (err error) <span class="cov8" title="1">{
+        if h.fh != nil </span><span class="cov8" title="1">{
+                err = h.fh.Close()
+                h.fh = nil
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+</pre>
+		
+		<pre class="file" id="file1" style="display: none">// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// reader.go
+package file
+
+import (
+        "bufio"
+        "io"
+        "os"
+)
+
+type Reader struct {
+        // fh     *os.File
+        handleBase
+        reader *bufio.Reader
+}
+
+func NewReader(fh *os.File) *Reader <span class="cov8" title="1">{
+        return &amp;Reader{handleBase: handleBase{fh: fh}, reader: bufio.NewReader(fh)}
+}</span>
+
+func OpenReader(filePath string) (r *Reader, err error) <span class="cov8" title="1">{
+        var fh *os.File
+        if fh, err = os.Open(filePath); err == nil </span><span class="cov8" title="1">{
+                r = NewReader(fh)
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (h *Reader) TypeName() string <span class="cov8" title="1">{
+        return "fileReader"
+}</span>
+
+func (h *Reader) String() string <span class="cov8" title="1">{
+        return "reader"
+}</span>
+
+func (h *Reader) Valid() bool <span class="cov8" title="1">{
+        return h.handleBase.Valid() &amp;&amp; h.reader != nil
+}</span>
+
+func (w *Reader) Close() (err error) <span class="cov8" title="1">{
+        w.reader = nil
+        err = w.handleBase.Close()
+        return
+}</span>
+
+func (h *Reader) ReadByte() (b byte, err error) <span class="cov8" title="1">{
+        if h.reader != nil </span><span class="cov8" title="1">{
+                b, err = h.reader.ReadByte()
+        }</span> else<span class="cov0" title="0"> {
+                err = io.ErrClosedPipe
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (h *Reader) ReadAll() (p []byte, err error) <span class="cov8" title="1">{
+        if h.reader != nil </span><span class="cov8" title="1">{
+                p, err = io.ReadAll(h.reader)
+        }</span> else<span class="cov0" title="0"> {
+                err = io.ErrClosedPipe
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (h *Reader) ReadString(delim byte) (s string, err error) <span class="cov0" title="0">{
+        if h.reader != nil </span><span class="cov0" title="0">{
+                s, err = h.reader.ReadString(delim)
+        }</span> else<span class="cov0" title="0"> {
+                err = io.ErrClosedPipe
+        }</span>
+        <span class="cov0" title="0">return</span>
+}
+
+func (h *Reader) Reset() (err error) <span class="cov8" title="1">{
+        if h.fh != nil </span><span class="cov8" title="1">{
+                if _, err = h.fh.Seek(0, 0); err == nil </span><span class="cov8" title="1">{
+                        h.reader.Reset(h.fh)
+                }</span>
+        }
+        <span class="cov8" title="1">return</span>
+}
+</pre>
+		
+		<pre class="file" id="file2" style="display: none">// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// writer.go
+package file
+
+import (
+        "bufio"
+        "fmt"
+        "os"
+)
+
+type Writer struct {
+        // fh     *os.File
+        handleBase
+        writer *bufio.Writer
+}
+
+func NewWriter(fh *os.File) *Writer <span class="cov8" title="1">{
+        return &amp;Writer{handleBase: handleBase{fh: fh}, writer: bufio.NewWriter(fh)}
+}</span>
+
+func CreateWriter(filePath string) (w *Writer, err error) <span class="cov8" title="1">{
+        var fh *os.File
+        if fh, err = os.Create(filePath); err == nil </span><span class="cov8" title="1">{
+                w = NewWriter(fh)
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func AppendWriter(filePath string) (w *Writer, err error) <span class="cov8" title="1">{
+        var fh *os.File
+        if fh, err = os.OpenFile(filePath, os.O_APPEND|os.O_WRONLY, 0644); err == nil </span><span class="cov8" title="1">{
+                w = NewWriter(fh)
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (w *Writer) TypeName() string <span class="cov8" title="1">{
+        return "fileWriter"
+}</span>
+
+func (w *Writer) String() string <span class="cov8" title="1">{
+        return "writer"
+}</span>
+
+func (w *Writer) Valid() bool <span class="cov8" title="1">{
+        return w.handleBase.Valid() &amp;&amp; w.writer != nil
+}</span>
+
+func (w *Writer) Close() (err error) <span class="cov8" title="1">{
+        var err1 error
+        if w.writer != nil </span><span class="cov8" title="1">{
+                err1 = w.Flush()
+                w.writer = nil
+        }</span>
+        <span class="cov8" title="1">if err = w.handleBase.Close(); err == nil </span><span class="cov8" title="1">{
+                err = err1
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (w *Writer) Flush() (err error) <span class="cov8" title="1">{
+        if w.writer != nil </span><span class="cov8" title="1">{
+                err = w.writer.Flush()
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (w *Writer) Write(args ...any) (n int, err error) <span class="cov8" title="1">{
+        if w.writer != nil </span><span class="cov8" title="1">{
+                n, err = fmt.Fprint(w.writer, args...)
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+
+func (w *Writer) Writef(format string, args ...any) (n int, err error) <span class="cov8" title="1">{
+        if w.writer != nil </span><span class="cov8" title="1">{
+                n, err = fmt.Fprintf(w.writer, format, args...)
+        }</span>
+        <span class="cov8" title="1">return</span>
+}
+</pre>
+		
+		</div>
+	</body>
+	<script>
+	(function() {
+		var files = document.getElementById('files');
+		var visible;
+		files.addEventListener('change', onChange, false);
+		function select(part) {
+			if (visible)
+				visible.style.display = 'none';
+			visible = document.getElementById(part);
+			if (!visible)
+				return;
+			files.value = part;
+			visible.style.display = 'block';
+			location.hash = part;
+		}
+		function onChange() {
+			select(files.value);
+			window.scrollTo(0, 0);
+		}
+		if (location.hash != "") {
+			select(location.hash.substr(1));
+		}
+		if (!visible) {
+			select("file0");
+		}
+	})();
+	</script>
+</html>
@@ -0,0 +1,46 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// file.go
+package file
+
+import (
+	"os"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
+)
+
+type Handle interface {
+	kern.Typer
+	GetFile() *os.File
+	GetName() string
+	Valid() bool
+	Close() error
+}
+
+type handleBase struct {
+	fh *os.File
+}
+
+func (h *handleBase) GetFile() *os.File {
+	return h.fh
+}
+
+func (h *handleBase) GetName() (name string) {
+	if h.fh != nil {
+		name = h.fh.Name()
+	}
+	return
+}
+
+func (h *handleBase) Valid() bool {
+	return h.fh != nil
+}
+
+func (h *handleBase) Close() (err error) {
+	if h.fh != nil {
+		err = h.fh.Close()
+		h.fh = nil
+	}
+	return
+}
@@ -0,0 +1,83 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// reader.go
+package file
+
+import (
+	"bufio"
+	"io"
+	"os"
+)
+
+type Reader struct {
+	// fh     *os.File
+	handleBase
+	reader *bufio.Reader
+}
+
+func NewReader(fh *os.File) *Reader {
+	return &Reader{handleBase: handleBase{fh: fh}, reader: bufio.NewReader(fh)}
+}
+
+func OpenReader(filePath string) (r *Reader, err error) {
+	var fh *os.File
+	if fh, err = os.Open(filePath); err == nil {
+		r = NewReader(fh)
+	}
+	return
+}
+
+func (h *Reader) TypeName() string {
+	return "fileReader"
+}
+
+func (h *Reader) String() string {
+	return "reader"
+}
+
+func (h *Reader) Valid() bool {
+	return h.handleBase.Valid() && h.reader != nil
+}
+
+func (w *Reader) Close() (err error) {
+	w.reader = nil
+	err = w.handleBase.Close()
+	return
+}
+
+func (h *Reader) ReadByte() (b byte, err error) {
+	if h.reader != nil {
+		b, err = h.reader.ReadByte()
+	} else {
+		err = io.ErrClosedPipe
+	}
+	return
+}
+
+func (h *Reader) ReadAll() (p []byte, err error) {
+	if h.reader != nil {
+		p, err = io.ReadAll(h.reader)
+	} else {
+		err = io.ErrClosedPipe
+	}
+	return
+}
+
+func (h *Reader) ReadString(delim byte) (s string, err error) {
+	if h.reader != nil {
+		s, err = h.reader.ReadString(delim)
+	} else {
+		err = io.ErrClosedPipe
+	}
+	return
+}
+
+func (h *Reader) Reset() (err error) {
+	if h.fh != nil {
+		if _, err = h.fh.Seek(0, 0); err == nil {
+			h.reader.Reset(h.fh)
+		}
+	}
+	return
+}
@@ -0,0 +1,62 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// reader.go
+package file
+
+import "testing"
+
+func TestOpenReader(t *testing.T) {
+	r, err := OpenReader("t_reader_test.go")
+	if err != nil {
+		t.Fatalf("OpenReader failed: %v", err)
+	}
+	defer r.Close()
+
+	if !r.Valid() {
+		t.Fatal("Reader should be valid after opening")
+	}
+
+	if r.TypeName() != "fileReader" {
+		t.Fatalf("Expected TypeName 'fileReader', got '%s'", r.TypeName())
+	}
+
+	if r.String() != "reader" {
+		t.Fatalf("Expected String 'reader', got '%s'", r.String())
+	}
+
+	// GetName may return either "t_reader_test.go" or "./t_reader_test.go" depending on the environment
+	name := r.GetName()
+	if (name != "t_reader_test.go") && (name != "./t_reader_test.go") {
+		t.Fatalf("Expected GetName 't_reader_test.go' or './t_reader_test.go', got '%s'", name)
+	}
+
+	// Test reading a byte
+	b, err := r.ReadByte()
+	if err != nil {
+		t.Fatalf("ReadByte failed: %v", err)
+	}
+	if b == 0 {
+		t.Fatal("ReadByte should not return zero byte")
+	}
+
+	err = r.Reset()
+	if err != nil {
+		t.Fatalf("Reset failed: %v", err)
+	}
+
+	if s, err := r.ReadString('\n'); err != nil {
+		t.Fatalf("ReadString failed: %v", err)
+	} else {
+		t.Logf("ReadString: %s", s)
+	}
+
+	// Test reading all content
+	content, err := r.ReadAll()
+	if err != nil {
+		t.Fatalf("ReadAll failed: %v", err)
+	}
+	if len(content) == 0 {
+		t.Fatal("ReadAll should return non-empty content")
+	}
+}
@@ -0,0 +1,69 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// writer.go
+package file
+
+import "testing"
+
+func TestCreateWriter(t *testing.T) {
+	w, err := CreateWriter("/tmp/test_writer.txt")
+	if err != nil {
+		t.Fatalf("CreateWriter failed: %v", err)
+	}
+	defer w.Close()
+
+	if !w.Valid() {
+		t.Fatal("Writer should be valid after creation")
+	}
+
+	if w.TypeName() != "fileWriter" {
+		t.Fatalf("Expected TypeName 'fileWriter', got '%s'", w.TypeName())
+	}
+
+	if w.String() != "writer" {
+		t.Fatalf("Expected String 'writer', got '%s'", w.String())
+	}
+
+	name := w.GetName()
+	if name != "/tmp/test_writer.txt" {
+		t.Fatalf("Expected GetName '/tmp/test_writer.txt', got '%s'", name)
+	}
+
+	if n, err := w.Write("Hello, World!\n"); err != nil {
+		t.Fatalf("Write failed: %v", err)
+	} else if n != len("Hello, World!\n") {
+		t.Fatalf("Expected to write %d bytes, wrote %d", len("Hello, World!\n"), n)
+	}
+
+	if n, err := w.Writef("This is a %s.\n", "test"); err != nil {
+		t.Fatalf("Writef failed: %v", err)
+	} else if n != len("This is a test.\n") {
+		t.Fatalf("Expected to write %d bytes, wrote %d", len("This is a test.\n"), n)
+	}
+}
+
+func TestAppendWriter(t *testing.T) {
+	w, err := AppendWriter("/tmp/test_writer.txt")
+	if err != nil {
+		t.Fatalf("AppendWriter failed: %v", err)
+	}
+	defer w.Close()
+
+	if !w.Valid() {
+		t.Fatal("Writer should be valid after opening for append")
+	}
+
+	if w.TypeName() != "fileWriter" {
+		t.Fatalf("Expected TypeName 'fileWriter', got '%s'", w.TypeName())
+	}
+
+	if w.String() != "writer" {
+		t.Fatalf("Expected String 'writer', got '%s'", w.String())
+	}
+
+	name := w.GetName()
+	if name != "/tmp/test_writer.txt" {
+		t.Fatalf("Expected GetName '/tmp/test_writer.txt', got '%s'", name)
+	}
+}
@@ -0,0 +1,82 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// writer.go
+package file
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+type Writer struct {
+	// fh     *os.File
+	handleBase
+	writer *bufio.Writer
+}
+
+func NewWriter(fh *os.File) *Writer {
+	return &Writer{handleBase: handleBase{fh: fh}, writer: bufio.NewWriter(fh)}
+}
+
+func CreateWriter(filePath string) (w *Writer, err error) {
+	var fh *os.File
+	if fh, err = os.Create(filePath); err == nil {
+		w = NewWriter(fh)
+	}
+	return
+}
+
+func AppendWriter(filePath string) (w *Writer, err error) {
+	var fh *os.File
+	if fh, err = os.OpenFile(filePath, os.O_APPEND|os.O_WRONLY, 0644); err == nil {
+		w = NewWriter(fh)
+	}
+	return
+}
+
+func (w *Writer) TypeName() string {
+	return "fileWriter"
+}
+
+func (w *Writer) String() string {
+	return "writer"
+}
+
+func (w *Writer) Valid() bool {
+	return w.handleBase.Valid() && w.writer != nil
+}
+
+func (w *Writer) Close() (err error) {
+	var err1 error
+	if w.writer != nil {
+		err1 = w.Flush()
+		w.writer = nil
+	}
+	if err = w.handleBase.Close(); err == nil {
+		err = err1
+	}
+	return
+}
+
+func (w *Writer) Flush() (err error) {
+	if w.writer != nil {
+		err = w.writer.Flush()
+	}
+	return
+}
+
+func (w *Writer) Write(args ...any) (n int, err error) {
+	if w.writer != nil {
+		n, err = fmt.Fprint(w.writer, args...)
+	}
+	return
+}
+
+func (w *Writer) Writef(format string, args ...any) (n int, err error) {
+	if w.writer != nil {
+		n, err = fmt.Fprintf(w.writer, format, args...)
+	}
+	return
+}
@@ -1,388 +0,0 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
-// All rights reserved.
-
-// function.go
-package expr
-
-import (
-	"fmt"
-	"strings"
-
-	"git.portale-stac.it/go-pkg/expr/kern"
-	"git.portale-stac.it/go-pkg/expr/scan"
-)
-
-// ---- Function template
-// type FuncTemplate func(ctx expr.ExprContext, name string, args map[string]any) (result any, err error)
-
-// ---- Common functor definition
-type BaseFunctor struct {
-	info kern.ExprFunc
-}
-
-func (functor *BaseFunctor) ToString(opt kern.FmtOpt) (s string) {
-	if functor.info != nil {
-		s = functor.info.ToString(opt)
-	} else {
-		s = "func(){}"
-	}
-	return s
-}
-
-func (functor *BaseFunctor) GetParams() (params []kern.ExprFuncParam) {
-	if functor.info != nil {
-		return functor.info.Params()
-	} else {
-		return []kern.ExprFuncParam{}
-	}
-}
-
-func (functor *BaseFunctor) SetFunc(info kern.ExprFunc) {
-	functor.info = info
-}
-
-func (functor *BaseFunctor) GetFunc() kern.ExprFunc {
-	return functor.info
-}
-
-func (functor *BaseFunctor) GetDefinitionContext() kern.ExprContext {
-	return nil
-}
-
-// ---- Function Parameters
-type paramFlags uint16
-
-const (
-	PfDefault paramFlags = 1 << iota
-	PfOptional
-	PfRepeat
-)
-
-type funcParamInfo struct {
-	name         string
-	flags        paramFlags
-	defaultValue any
-}
-
-func NewFuncParam(name string) kern.ExprFuncParam {
-	return &funcParamInfo{name: name}
-}
-
-func NewFuncParamFlag(name string, flags paramFlags) kern.ExprFuncParam {
-	return &funcParamInfo{name: name, flags: flags}
-}
-
-func NewFuncParamFlagDef(name string, flags paramFlags, defValue any) *funcParamInfo {
-	return &funcParamInfo{name: name, flags: flags, defaultValue: defValue}
-}
-
-func (param *funcParamInfo) Name() string {
-	return param.name
-}
-
-func (param *funcParamInfo) Type() string {
-	return kern.TypeAny
-}
-
-func (param *funcParamInfo) IsDefault() bool {
-	return (param.flags & PfDefault) != 0
-}
-
-func (param *funcParamInfo) IsOptional() bool {
-	return (param.flags & PfOptional) != 0
-}
-
-func (param *funcParamInfo) IsRepeat() bool {
-	return (param.flags & PfRepeat) != 0
-}
-
-func (param *funcParamInfo) DefaultValue() any {
-	return param.defaultValue
-}
-
-// --- Functions
-
-// funcInfo implements expr.ExprFunc
-type funcInfo struct {
-	name         string
-	minArgs      int
-	maxArgs      int
-	functor      kern.Functor
-	formalParams []kern.ExprFuncParam
-	returnType   string
-}
-
-func newFuncInfo(name string, functor kern.Functor, returnType string, params []kern.ExprFuncParam) (info *funcInfo, err error) {
-	var minArgs = 0
-	var maxArgs = 0
-	for _, p := range params {
-		if maxArgs == -1 {
-			return nil, fmt.Errorf("no more params can be specified after the ellipsis symbol: %q", p.Name())
-		}
-		if p.IsDefault() || p.IsOptional() {
-			maxArgs++
-		} else if maxArgs == minArgs {
-			minArgs++
-			maxArgs++
-		} else {
-			return nil, fmt.Errorf("can't specify non-optional param after optional ones: %q", p.Name())
-		}
-		if p.IsRepeat() {
-			minArgs--
-			maxArgs = -1
-		}
-	}
-
-	info = &funcInfo{
-		name: name, minArgs: minArgs, maxArgs: maxArgs, functor: functor, returnType: returnType, formalParams: params,
-	}
-	functor.SetFunc(info)
-	return info, nil
-}
-
-func (info *funcInfo) Params() []kern.ExprFuncParam {
-	return info.formalParams
-}
-
-func (info *funcInfo) ReturnType() string {
-	return info.returnType
-}
-
-func (info *funcInfo) ToString(opt kern.FmtOpt) string {
-	var sb strings.Builder
-	if len(info.Name()) == 0 {
-		sb.WriteString("func")
-	} else {
-		sb.WriteString(info.Name())
-	}
-	sb.WriteByte('(')
-	if info.formalParams != nil {
-		for i, p := range info.formalParams {
-			if i > 0 {
-				sb.WriteString(", ")
-			}
-			sb.WriteString(p.Name())
-
-			if p.IsDefault() {
-				sb.WriteByte('=')
-				if s, ok := p.DefaultValue().(string); ok {
-					sb.WriteByte('"')
-					sb.WriteString(s)
-					sb.WriteByte('"')
-				} else {
-					sb.WriteString(fmt.Sprintf("%v", p.DefaultValue()))
-				}
-			}
-		}
-	}
-	if info.maxArgs < 0 {
-		sb.WriteString(" ...")
-	}
-	sb.WriteString("):")
-	if len(info.returnType) > 0 {
-		sb.WriteString(info.returnType)
-	} else {
-		sb.WriteString(kern.TypeAny)
-	}
-	sb.WriteString("{}")
-	return sb.String()
-}
-
-func (info *funcInfo) Name() string {
-	return info.name
-}
-
-func (info *funcInfo) MinArgs() int {
-	return info.minArgs
-}
-
-func (info *funcInfo) MaxArgs() int {
-	return info.maxArgs
-}
-
-func (info *funcInfo) Functor() kern.Functor {
-	return info.functor
-}
-
-func (info *funcInfo) AllocContext(parentCtx kern.ExprContext) (ctx kern.ExprContext) {
-	if defCtx := info.functor.GetDefinitionContext(); defCtx != nil {
-		ctx = defCtx.Clone()
-		ctx.SetParent(defCtx)
-	} else {
-		ctx = parentCtx.Clone()
-		ctx.SetParent(parentCtx)
-	}
-	return
-}
-
-func (info *funcInfo) ParamSpec(paramName string) kern.ExprFuncParam {
-	for _, spec := range info.formalParams {
-		if spec.Name() == paramName {
-			return spec
-		}
-	}
-	return nil
-}
-
-func initActualParams(ctx kern.ExprContext, info kern.ExprFunc, callTerm *scan.Term) (actualParams map[string]any, err error) {
-	var varArgs []any
-	var varName string
-
-	namedParamsStarted := false
-
-	formalParams := info.Params()
-	actualParams = make(map[string]any, len(formalParams))
-	if callTerm == nil {
-		return
-	}
-
-	for i, tree := range callTerm.Children {
-		var paramValue any
-		paramCtx := ctx.Clone()
-		if paramValue, err = tree.Compute(paramCtx); err != nil {
-			break
-		}
-		if paramName, namedParam := kern.GetAssignVarName(tree); namedParam {
-			if info.ParamSpec(paramName) == nil {
-				err = fmt.Errorf("%s(): unknown param %q", info.Name(), paramName)
-				break
-			}
-			actualParams[paramName] = paramValue
-			namedParamsStarted = true
-		} else if !namedParamsStarted {
-			if varArgs != nil {
-				varArgs = append(varArgs, paramValue)
-			} else if i < len(formalParams) {
-				spec := formalParams[i]
-				if spec.IsRepeat() {
-					varArgs = make([]any, 0, len(callTerm.Children)-i)
-					varArgs = append(varArgs, paramValue)
-					varName = spec.Name()
-				} else {
-					actualParams[spec.Name()] = paramValue
-				}
-			} else {
-				err = kern.ErrTooManyParams(info.Name(), len(formalParams), len(callTerm.Children))
-				break
-			}
-		} else {
-			err = fmt.Errorf("%s(): positional param nr %d not allowed after named params", info.Name(), i+1)
-			break
-		}
-	}
-	if err == nil {
-		if varArgs != nil {
-			actualParams[varName] = varArgs
-		}
-	}
-	return
-}
-
-func (info *funcInfo) PrepareCall(name string, actualParams map[string]any) (err error) {
-	passedCount := len(actualParams)
-	if info.MinArgs() > passedCount {
-		err = kern.ErrTooFewParams(name, info.MinArgs(), info.MaxArgs(), passedCount)
-		return
-	}
-
-	if passedCount < len(info.formalParams) {
-		for _, p := range info.formalParams {
-			if _, exists := actualParams[p.Name()]; !exists {
-				if !p.IsDefault() {
-					break
-				}
-				if p.IsRepeat() {
-					varArgs := make([]any, 1)
-					varArgs[0] = p.DefaultValue()
-					actualParams[p.Name()] = varArgs
-				} else {
-					actualParams[p.Name()] = p.DefaultValue()
-				}
-			}
-		}
-	}
-
-	if info.MaxArgs() >= 0 && info.MaxArgs() < len(actualParams) {
-		err = kern.ErrTooManyParams(name, info.MaxArgs(), len(actualParams))
-	}
-	return
-}
-
-// ----- Call a function ---
-
-// func getAssignVarName(t *term) (name string, ok bool) {
-// 	if ok = t.symbol() == SymEqual; ok {
-// 		name = t.children[0].source()
-// 	}
-// 	return
-// }
-
-// func CallFunctionByTerm(parentCtx expr.ExprContext, name string, callTerm *term) (result any, err error) {
-// 	var actualParams map[string]any
-// 	if info, exists := GetFuncInfo(parentCtx, name); exists {
-// 		if actualParams, err = initActualParams(parentCtx, info, callTerm); err == nil {
-// 			ctx := info.AllocContext(parentCtx)
-// 			if err = info.PrepareCall(name, actualParams); err == nil {
-// 				functor := info.Functor()
-// 				result, err = functor.InvokeNamed(ctx, name, actualParams)
-// 				exportObjectsToParent(ctx)
-// 			}
-// 		}
-// 	} else {
-// 		err = fmt.Errorf("unknown function %s()", name)
-// 	}
-// 	return
-// }
-
-// func CallFunctionByArgs(parentCtx expr.ExprContext, name string, args []any) (result any, err error) {
-// 	var actualParams map[string]any
-// 	if info, exists := GetFuncInfo(parentCtx, name); exists {
-// 		functor := info.Functor()
-// 		actualParams = bindActualParams(functor, args)
-// 		ctx := info.AllocContext(parentCtx)
-// 		if err = info.PrepareCall(name, actualParams); err == nil {
-// 			result, err = functor.InvokeNamed(ctx, name, actualParams)
-// 			exportObjectsToParent(ctx)
-// 		}
-// 	} else {
-// 		err = fmt.Errorf("unknown function %s()", name)
-// 	}
-// 	return
-// }
-
-// func CallFunctionByParams(parentCtx expr.ExprContext, name string, actualParams map[string]any) (result any, err error) {
-// 	//var actualParams map[string]any
-// 	if info, exists := GetFuncInfo(parentCtx, name); exists {
-// 		functor := info.Functor()
-// 		ctx := info.AllocContext(parentCtx)
-// 		if err = info.PrepareCall(name, actualParams); err == nil {
-// 			result, err = functor.InvokeNamed(ctx, name, actualParams)
-// 			exportObjectsToParent(ctx)
-// 		}
-// 	} else {
-// 		err = fmt.Errorf("unknown function %s()", name)
-// 	}
-// 	return
-// }
-
-// func GetParam(args map[string]any, paramName string, paramNum int) (value any, exists bool) {
-// 	if value, exists = args[paramName]; !exists {
-// 		if paramNum > 0 && paramNum <= len(args) {
-// 			value, exists = args["arg"+strconv.Itoa(paramNum)]
-// 		}
-// 	}
-// 	return
-// }
-
-// func bindActualParams(functor Functor, args []any) (actualParams map[string]any) {
-// 	formalParams := functor.GetParams()
-// 	actualParams = make(map[string]any, len(args))
-// 	for i, arg := range args {
-// 		if i < len(formalParams) {
-// 			actualParams[formalParams[i].Name()] = arg
-// 		} else {
-// 			actualParams["arg"+strconv.Itoa(i+1)] = arg
-// 		}
-// 	}
-// 	return
-// }
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // global-context.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 //go:build graph
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // helpers.go
@@ -45,7 +45,7 @@ func EvalStringV(source string, args []Arg) (result any, err error) {
 				functor := kern.NewGolangFunctor(f)
 				// ctx.RegisterFunc(arg.Name, functor, 0, -1)
 				ctx.RegisterFunc(arg.Name, functor, kern.TypeAny, []kern.ExprFuncParam{
-					NewFuncParamFlagDef(kern.ParamValue, PfDefault|PfRepeat, 0),
+					kern.NewFuncParamFlagDef(kern.ParamValue, kern.PfDefault|kern.PfRepeat, 0),
 				})
 			} else {
 				err = fmt.Errorf("invalid function specification: %q", arg.Name)
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // import-utils.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // int-iterator.go
@@ -1,14 +1,17 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // iter-factory.go
 package expr

 import (
+	"slices"
+
 	"git.portale-stac.it/go-pkg/expr/kern"
+	"git.portale-stac.it/go-pkg/expr/scan"
 )

-func NewIterator(value any) (it kern.Iterator, err error) {
+func NewIterator(ctx kern.ExprContext, value any, ops []*scan.Term) (it kern.Iterator, err error) {
 	if value == nil {
 		return NewArrayIterator([]any{}), nil
 	}
@@ -21,9 +24,21 @@ func NewIterator(value any) (it kern.Iterator, err error) {
 	case []any:
 		it = NewArrayIterator(v)
 	case kern.Iterator:
-		it = v
+		// var exprs []*scan.Term
+		it, err = NewIterIter(v, ctx, ops)
 	default:
 		it = NewArrayIterator([]any{value})
 	}
 	return
 }
+
+func HasIterStandardOperations(name string) bool {
+	return slices.Contains([]string{kern.NextName, kern.ResetName, kern.IndexName, kern.CountName, kern.CurrentName, kern.CleanName}, name)
+}
+
+func HasIterOperations(name string, ops ...string) bool {
+	return slices.Contains([]string{
+		kern.NextName, kern.ResetName, kern.IndexName, kern.CountName, kern.CurrentName, kern.CleanName,
+	}, name) ||
+		slices.Contains(ops, name)
+}
@@ -0,0 +1,135 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// iter-iter.go
+package expr
+
+import (
+	"fmt"
+	"io"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
+	"git.portale-stac.it/go-pkg/expr/scan"
+)
+
+const iterIterType = "IterIter"
+
+type IterIter struct {
+	it       kern.Iterator
+	count    int64
+	index    int64
+	ctx      kern.ExprContext
+	exprList []*scan.Term
+	current  any
+}
+
+func NewIterIter(it kern.Iterator, ctx kern.ExprContext, exprs []*scan.Term) (iter kern.Iterator, err error) {
+	if ctx == nil {
+		err = fmt.Errorf("context is required for %s", iterIterType)
+	} else if it == nil {
+		err = fmt.Errorf("source iterator is required for %s", iterIterType)
+	} else {
+		iter = &IterIter{it: it, count: 0, index: -1, ctx: ctx, exprList: exprs, current: nil}
+	}
+	return
+}
+
+func (it *IterIter) String() string {
+	return fmt.Sprintf("$(%s)", it.it)
+}
+
+func (it *IterIter) TypeName() string {
+	return iterIterType
+}
+
+func (it *IterIter) HasOperation(name string) bool {
+	return HasIterStandardOperations(name)
+}
+
+func (it *IterIter) CallOperation(name string, args map[string]any) (v any, err error) {
+	switch name {
+	case kern.NextName:
+		v, err = it.Next()
+	case kern.ResetName:
+		err = it.Reset()
+	case kern.CleanName:
+		err = it.Clean()
+	case kern.IndexName:
+		v = int64(it.Index())
+	case kern.CurrentName:
+		v, err = it.Current()
+	case kern.CountName:
+		v = it.count
+	default:
+		err = kern.ErrNoOperation(name)
+	}
+	return
+}
+
+func (it *IterIter) Current() (item any, err error) {
+	if it.current != nil {
+		item = it.current
+	} else if len(it.exprList) > 0 {
+		// Evaluate the expression list and use the result as the current item
+		var exprValue any
+		for _, expr := range it.exprList {
+			if exprValue, err = expr.Compute(it.ctx); err != nil {
+				break
+			}
+			it.ctx.UnsafeSetVar(kern.ControlLastResult, exprValue)
+		}
+		if err == nil {
+			item = exprValue
+		}
+	} else {
+		var exists bool
+		if it.current, exists = it.ctx.GetVar("_"); !exists {
+			err = fmt.Errorf("current item not available")
+		} else {
+			item = it.current
+		}
+	}
+	return
+}
+
+func (it *IterIter) Next() (item any, err error) {
+	var src any
+
+	it.current = nil
+	ctx := it.ctx
+	for src, err = it.it.Next(); src == nil && err == nil; src, err = it.it.Next() {
+	}
+
+	if err == nil {
+		if src == nil {
+			err = io.EOF
+		} else {
+			ctx.UnsafeSetVar("_", src)
+			ctx.UnsafeSetVar("__", it.it.Index())
+			ctx.UnsafeSetVar("_#", it.it.Count())
+			item, err = it.Current()
+			ctx.DeleteVar("_#")
+			ctx.DeleteVar("__")
+			ctx.DeleteVar("_")
+		}
+	}
+	return
+}
+
+func (it *IterIter) Index() int64 {
+	return it.index
+}
+
+func (it *IterIter) Count() int64 {
+	return it.count
+}
+
+func (it *IterIter) Reset() error {
+	it.index = -1
+	it.count = 0
+	return nil
+}
+
+func (it *IterIter) Clean() error {
+	return nil
+}
@@ -1,23 +1,23 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // bind-go-function.go
 package kern

 // ---- Linking with Go functions
-type golangFunctor struct {
+type GolangFunctor struct {
 	BaseFunctor
 	f FuncTemplate
 }

-func NewGolangFunctor(f FuncTemplate) *golangFunctor {
-	return &golangFunctor{f: f}
+func NewGolangFunctor(f FuncTemplate) *GolangFunctor {
+	return &GolangFunctor{f: f}
 }

-func (functor *golangFunctor) TypeName() string {
+func (functor *GolangFunctor) TypeName() string {
 	return "GoFunctor"
 }

-func (functor *golangFunctor) InvokeNamed(ctx ExprContext, name string, args map[string]any) (result any, err error) {
+func (functor *GolangFunctor) InvokeNamed(ctx ExprContext, name string, args map[string]any) (result any, err error) {
 	return functor.f(ctx, name, args)
 }
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // common-errors.go
@@ -84,6 +84,10 @@ func ErrUnknownVar(funcName, varName string) error {
 	return fmt.Errorf("%s(): unknown variable %q", funcName, varName)
 }

+func ErrFuncInvalidArg(funcName, details string) error {
+	return fmt.Errorf("%s(): invalid argument -- %s", funcName, details)
+}
+
 // --- Operator errors

 func ErrLeftOperandMustBeVariable(leftTerm, opTerm Term) error {
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // common-params.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // common-type-names.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // context-helpers.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // control.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // dict-type.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // expr-context.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // expr-function.go
@@ -37,3 +37,8 @@ type ExprFunc interface {
 	PrepareCall(name string, actualParams map[string]any) (err error)
 	AllocContext(parentCtx ExprContext) (ctx ExprContext)
 }
+
+func IsFunctor(v any) (ok bool) {
+	_, ok = v.(Functor)
+	return
+}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // expr.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // formatter.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // fraction-type.go
@@ -44,10 +44,11 @@ func MakeGeneratingFraction(s string) (f *FractionType, err error) {
 	if len(s) == 0 {
 		goto exit
 	}
-	if s[0] == '-' {
+	switch s[0] {
+	case '-':
 		sign = int64(-1)
 		s = s[1:]
-	} else if s[0] == '+' {
+	case '+':
 		s = s[1:]
 	}
 	//	if strings.HasSuffix(s, "()") {
@@ -0,0 +1,164 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// func-info.go
+package kern
+
+import (
+	"fmt"
+	"strings"
+)
+
+// --- Functions
+
+// FuncInfo implements expr.ExprFunc
+type FuncInfo struct {
+	name         string
+	minArgs      int
+	maxArgs      int
+	functor      Functor
+	formalParams []ExprFuncParam
+	returnType   string
+}
+
+func NewFuncInfo(name string, functor Functor, returnType string, params []ExprFuncParam) (info *FuncInfo, err error) {
+	var minArgs = 0
+	var maxArgs = 0
+	for _, p := range params {
+		if maxArgs == -1 {
+			return nil, fmt.Errorf("no more params can be specified after the ellipsis symbol: %q", p.Name())
+		}
+		if p.IsDefault() || p.IsOptional() {
+			maxArgs++
+		} else if maxArgs == minArgs {
+			minArgs++
+			maxArgs++
+		} else {
+			return nil, fmt.Errorf("can't specify non-optional param after optional ones: %q", p.Name())
+		}
+		if p.IsRepeat() {
+			minArgs--
+			maxArgs = -1
+		}
+	}
+
+	info = &FuncInfo{
+		name: name, minArgs: minArgs, maxArgs: maxArgs, functor: functor, returnType: returnType, formalParams: params,
+	}
+	functor.SetFunc(info)
+	return info, nil
+}
+
+func (info *FuncInfo) Params() []ExprFuncParam {
+	return info.formalParams
+}
+
+func (info *FuncInfo) ReturnType() string {
+	return info.returnType
+}
+
+func (info *FuncInfo) ToString(opt FmtOpt) string {
+	var sb strings.Builder
+	if len(info.Name()) == 0 {
+		sb.WriteString("func")
+	} else {
+		sb.WriteString(info.Name())
+	}
+	sb.WriteByte('(')
+	if info.formalParams != nil {
+		for i, p := range info.formalParams {
+			if i > 0 {
+				sb.WriteString(", ")
+			}
+			sb.WriteString(p.Name())
+
+			if p.IsDefault() {
+				sb.WriteByte('=')
+				if s, ok := p.DefaultValue().(string); ok {
+					sb.WriteByte('"')
+					sb.WriteString(s)
+					sb.WriteByte('"')
+				} else {
+					sb.WriteString(fmt.Sprintf("%v", p.DefaultValue()))
+				}
+			}
+		}
+	}
+	if info.maxArgs < 0 {
+		sb.WriteString(" ...")
+	}
+	sb.WriteString("):")
+	if len(info.returnType) > 0 {
+		sb.WriteString(info.returnType)
+	} else {
+		sb.WriteString(TypeAny)
+	}
+	sb.WriteString("{}")
+	return sb.String()
+}
+
+func (info *FuncInfo) Name() string {
+	return info.name
+}
+
+func (info *FuncInfo) MinArgs() int {
+	return info.minArgs
+}
+
+func (info *FuncInfo) MaxArgs() int {
+	return info.maxArgs
+}
+
+func (info *FuncInfo) Functor() Functor {
+	return info.functor
+}
+
+func (info *FuncInfo) AllocContext(parentCtx ExprContext) (ctx ExprContext) {
+	if defCtx := info.functor.GetDefinitionContext(); defCtx != nil {
+		ctx = defCtx.Clone()
+		ctx.SetParent(defCtx)
+	} else {
+		ctx = parentCtx.Clone()
+		ctx.SetParent(parentCtx)
+	}
+	return
+}
+
+func (info *FuncInfo) ParamSpec(paramName string) ExprFuncParam {
+	for _, spec := range info.formalParams {
+		if spec.Name() == paramName {
+			return spec
+		}
+	}
+	return nil
+}
+
+func (info *FuncInfo) PrepareCall(name string, actualParams map[string]any) (err error) {
+	passedCount := len(actualParams)
+	if info.MinArgs() > passedCount {
+		err = ErrTooFewParams(name, info.MinArgs(), info.MaxArgs(), passedCount)
+		return
+	}
+
+	if passedCount < len(info.formalParams) {
+		for _, p := range info.formalParams {
+			if _, exists := actualParams[p.Name()]; !exists {
+				if !p.IsDefault() {
+					break
+				}
+				if p.IsRepeat() {
+					varArgs := make([]any, 1)
+					varArgs[0] = p.DefaultValue()
+					actualParams[p.Name()] = varArgs
+				} else {
+					actualParams[p.Name()] = p.DefaultValue()
+				}
+			}
+		}
+	}
+
+	if info.MaxArgs() >= 0 && info.MaxArgs() < len(actualParams) {
+		err = ErrTooManyParams(name, info.MaxArgs(), len(actualParams))
+	}
+	return
+}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // function.go
@@ -14,11 +14,6 @@ type FuncTemplate func(ctx ExprContext, name string, args map[string]any) (resul

 type DeepFuncTemplate func(a, b any) (eq bool, err error)

-func IsFunctor(v any) (ok bool) {
-	_, ok = v.(Functor)
-	return
-}
-
 // ---- Common functor definition
 type BaseFunctor struct {
 	info ExprFunc
@@ -54,17 +49,17 @@ func (functor *BaseFunctor) GetDefinitionContext() ExprContext {
 }

 // ---- Function Parameters
-type paramFlags uint16
+type FuncParamFlags uint16

 const (
-	PfDefault paramFlags = 1 << iota
+	PfDefault FuncParamFlags = 1 << iota
 	PfOptional
 	PfRepeat
 )

 type funcParamInfo struct {
 	name         string
-	flags        paramFlags
+	flags        FuncParamFlags
 	defaultValue any
 }

@@ -72,11 +67,11 @@ func NewFuncParam(name string) ExprFuncParam {
 	return &funcParamInfo{name: name}
 }

-func NewFuncParamFlag(name string, flags paramFlags) ExprFuncParam {
+func NewFuncParamFlag(name string, flags FuncParamFlags) ExprFuncParam {
 	return &funcParamInfo{name: name, flags: flags}
 }

-func NewFuncParamFlagDef(name string, flags paramFlags, defValue any) *funcParamInfo {
+func NewFuncParamFlagDef(name string, flags FuncParamFlags, defValue any) *funcParamInfo {
 	return &funcParamInfo{name: name, flags: flags, defaultValue: defValue}
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // iterator.go
@@ -7,6 +7,7 @@ package kern
 import (
 	// "errors"
 	"fmt"
+	"slices"
 )

 // Operator names
@@ -50,3 +51,45 @@ func IsIterator(v any) (ok bool) {
 	_, ok = v.(Iterator)
 	return
 }
+
+type IteratorBase struct {
+	ItemIndex int64
+	ItemCount int64
+	current   any
+}
+
+func (it *IteratorBase) Current() (item any, err error) {
+	return it.current, nil
+}
+
+func (it *IteratorBase) Index() int64 {
+	return it.ItemIndex
+}
+
+func (it *IteratorBase) Count() int64 {
+	return it.ItemCount
+}
+
+func (it *IteratorBase) HasOperation(name string) bool {
+	return slices.Contains([]string{NextName, IndexName, CountName, CurrentName}, name)
+}
+
+func (it *IteratorBase) Clean() (err error) {
+	return
+}
+
+func (it *IteratorBase) Reset() (err error) {
+	it.ItemIndex = -1
+	it.ItemCount = 0
+	it.current = nil
+	return
+}
+
+func (it *IteratorBase) Increment() {
+	it.ItemIndex++
+	it.ItemCount++
+}
+
+func (it *IteratorBase) SetCurrent(v any) {
+	it.current = v
+}
@@ -0,0 +1,273 @@
+// simple-list project slist.go
+package kern
+
+import (
+	"fmt"
+)
+
+type ListNode struct {
+	data any
+	next *ListNode
+}
+
+func (node *ListNode) Next() *ListNode {
+	return node.next
+}
+
+func (self *ListNode) Data() any {
+	return self.data
+}
+
+type LinkedList struct {
+	count uint32
+	first *ListNode
+	last  *ListNode
+}
+
+func NewLinkedList() (list *LinkedList) {
+	list = &LinkedList{0, nil, nil}
+	return
+}
+
+func (ls *LinkedList) Len() uint32 {
+	return ls.count
+}
+
+func (ls *LinkedList) Empty() bool {
+	return ls.count == 0
+}
+
+func (ls *LinkedList) PushFront(data any) *ListNode {
+	ls.first = &ListNode{data, ls.first}
+	if ls.last == nil {
+		ls.last = ls.first
+	}
+	ls.count++
+	return ls.first
+}
+
+func (ls *LinkedList) SeqPushFront(data any) (result *LinkedList) {
+	switch seq := data.(type) {
+	case Iterator:
+		result = ls.IterPushFront(seq)
+	default:
+		ls.PushFront(data)
+		result = ls
+	}
+	return
+}
+
+func (ls *LinkedList) IterPushFront(it Iterator) *LinkedList {
+	for item, err1 := it.Next(); err1 == nil; item, err1 = it.Next() {
+		ls.PushFront(item)
+	}
+	return ls
+}
+
+func (ls *LinkedList) PushBack(data any) (node *ListNode) {
+	node = &ListNode{data, nil}
+	if ls.last != nil {
+		ls.last.next = node
+	}
+	ls.last = node
+	if ls.first == nil {
+		ls.first = node
+	}
+	ls.count++
+	return
+}
+
+func (ls *LinkedList) SeqPushBack(data any) (result *LinkedList) {
+	switch seq := data.(type) {
+	case Iterator:
+		result = ls.IterPushBack(seq)
+	default:
+		ls.PushBack(data)
+		result = ls
+	}
+	return
+}
+
+func (ls *LinkedList) IterPushBack(it Iterator) *LinkedList {
+	for item, err1 := it.Next(); err1 == nil; item, err1 = it.Next() {
+		ls.PushBack(item)
+	}
+	return ls
+}
+
+func (ls *LinkedList) FirstNode() *ListNode {
+	return ls.first
+}
+
+func (ls *LinkedList) First() (data any, err error) {
+	if ls.first != nil {
+		data = ls.first.data
+	} else {
+		err = errorListEmpty()
+	}
+	return
+}
+
+func (ls *LinkedList) LastNode() *ListNode {
+	return ls.last
+}
+
+func (ls *LinkedList) Last() (data interface{}, err error) {
+	if ls.last != nil {
+		data = ls.last.data
+	} else {
+		err = errorListEmpty()
+	}
+	return
+}
+
+func (ls *LinkedList) NodeAt(index uint32) (node *ListNode, err error) {
+	if ls.count == 0 {
+		err = errorListEmpty()
+	} else if index > ls.count {
+		err = errorOutOfRange()
+	} else if index == ls.count-1 {
+		node = ls.last
+	} else {
+		current := ls.first
+		for pos := uint32(0); pos < index; pos++ {
+			current = current.next
+		}
+		node = current
+	}
+	return
+}
+
+func (ls *LinkedList) At(index uint32) (data interface{}, err error) {
+	node, err := ls.NodeAt(index)
+	if err == nil {
+		data = node.data
+	}
+	return
+}
+
+func (ls *LinkedList) Insert(index uint32, data interface{}) *ListNode {
+	var prev *ListNode
+
+	prev = nil
+	current := ls.first
+	for pos := uint32(0); current != nil && pos < index; pos++ {
+		prev = current
+		current = current.next
+	}
+
+	node := &ListNode{data, current}
+
+	if prev != nil {
+		prev.next = node
+	}
+
+	if ls.first == current {
+		ls.first = node
+	}
+	if node.next == nil {
+		ls.last = node
+	}
+	ls.count++
+	return node
+}
+
+func (ls *LinkedList) PushBackStringArray(items []string) {
+	for _, v := range items {
+		ls.PushBack(v)
+	}
+}
+
+// type TraverseOperator func(index uint32, elem interface{}, userData interface{}) (err error)
+
+// func (self *LinkedList) Traverse(op TraverseOperator, user_data interface{}) (err error) {
+// 	index := uint32(0)
+// 	for current := self.first; current != nil; current = current.next {
+// 		err = op(index, current.data, user_data)
+// 		if err != nil {
+// 			break
+// 		}
+// 		index++
+// 	}
+// 	return
+// }
+
+// func (self *LinkedList) Traverse2(observer Observer, abortOnError bool) (err error) {
+// 	index := uint32(0)
+// 	for current := self.first; current != nil; current = current.next {
+// 		err = observer.Observe(current, index)
+// 		if err != nil && abortOnError {
+// 			break
+// 		}
+// 		index++
+// 	}
+// 	return
+// }
+
+type EqualFunc func(current, target any) bool
+
+func (ls *LinkedList) FindFirst(eqFunc EqualFunc, targetData any) (targetNode *ListNode) {
+	for current := ls.first; current != nil && targetNode == nil; current = current.next {
+		if eqFunc(current.data, targetData) {
+			targetNode = current
+		}
+	}
+	return
+}
+
+func (ls *LinkedList) FindNext(eqFunc EqualFunc, startNode *ListNode) (targetNode *ListNode) {
+	if startNode != nil {
+		for current := startNode.next; current != nil && targetNode == nil; current = current.next {
+			if eqFunc(current.data, startNode.Data()) {
+				targetNode = current
+			}
+		}
+	}
+	return
+}
+
+// type DataFeeder func(user_data interface{}) interface{}
+// type NodeObserver func(node *ListNode, index uint32, userData interface{})
+
+// func (self *LinkedList) FeedTail(feeder DataFeeder, feederUserData interface{}, observer NodeObserver, observerUserData interface{}) (count uint32) {
+// 	count = 0
+// 	for item := feeder(feederUserData); item != nil; item = feeder(feederUserData) {
+// 		// fmt.Println("Item", count, item)
+// 		node := self.PushBack(item)
+// 		if observer != nil {
+// 			observer(node, count, observerUserData)
+// 		}
+// 		count++
+// 	}
+// 	return
+// }
+
+// func (self *LinkedList) FeedTail2(feeder Feeder, observer Observer, abortOnError bool) (count uint32, err error) {
+// 	count = 0
+// 	// item := feeder.Next()
+// 	for item, err1 := feeder.Next(); item != nil; item, err1 = feeder.Next() {
+// 		if err1 != nil {
+// 			if err == nil {
+// 				err = err1
+// 			}
+// 			if abortOnError {
+// 				break
+// 			}
+// 		}
+// 		// fmt.Println("Item", count, item)
+// 		node := self.PushBack(item)
+// 		if observer != nil {
+// 			observer.Observe(node, count)
+// 		}
+// 		count++
+// 	}
+// 	return
+// }
+
+func errorListEmpty() error {
+	return fmt.Errorf("List is empty")
+}
+
+func errorOutOfRange() error {
+	return fmt.Errorf("Out of range")
+}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // list-type.go
@@ -50,13 +50,13 @@ func ListFromStrings(stringList []string) (list *ListType) {
 	return
 }

-func (dict *ListType) ToString(opt FmtOpt) (s string) {
+func (ls *ListType) ToString(opt FmtOpt) (s string) {
 	indent := GetFormatIndent(opt)
 	flags := GetFormatFlags(opt)

 	var sb strings.Builder
 	sb.WriteByte('[')
-	if len(*dict) > 0 {
+	if len(*ls) > 0 {
 		innerOpt := MakeFormatOptions(flags, indent+1)
 		nest := strings.Repeat("  ", indent+1)

@@ -64,7 +64,7 @@ func (dict *ListType) ToString(opt FmtOpt) (s string) {
 			sb.WriteByte('\n')
 			sb.WriteString(nest)
 		}
-		for i, item := range []any(*dict) {
+		for i, item := range []any(*ls) {
 			if i > 0 {
 				if flags&MultiLine != 0 {
 					sb.WriteString(",\n")
@@ -96,11 +96,11 @@ func (dict *ListType) ToString(opt FmtOpt) (s string) {
 	return
 }

-func (dict *ListType) String() string {
-	return dict.ToString(0)
+func (ls *ListType) String() string {
+	return ls.ToString(0)
 }

-func (dict *ListType) TypeName() string {
+func (ls *ListType) TypeName() string {
 	return "list"
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // term.go
@@ -1,6 +1,6 @@
 //go:build darwin

-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // lib-ext-darwin.go
@@ -1,6 +1,6 @@
 //go:build linux

-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // lib-ext-linux.go
@@ -1,6 +1,6 @@
 //go:build windows

-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // lib-ext-windows.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // list-iterator.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-dict.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-expr.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-func.go
@@ -70,14 +70,14 @@ func evalFuncDef(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		paramList := make([]kern.ExprFuncParam, 0, len(opTerm.Children))
 		for _, param := range opTerm.Children {
 			var defValue any
-			flags := paramFlags(0)
+			flags := kern.FuncParamFlags(0)
 			if len(param.Children) > 0 {
-				flags |= PfDefault
+				flags |= kern.PfDefault
 				if defValue, err = param.Children[0].Compute(ctx); err != nil {
 					return
 				}
 			}
-			info := NewFuncParamFlagDef(param.Source(), flags, defValue)
+			info := kern.NewFuncParamFlagDef(param.Source(), flags, defValue)
 			paramList = append(paramList, info)
 		}
 		v = newExprFunctor(ast, paramList, ctx)
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-iterator.go
@@ -125,7 +125,7 @@ func evalIterator(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		} else {
 			if dictIt, ok := firstChildValue.(*kern.DictType); ok {
 				var args []any
-				if args, err = evalSibling(ctx, opTerm.Children, nil); err == nil {
+				if args, err = evalSiblings(ctx, opTerm.Children, nil); err == nil {
 					v, err = NewDictIterator(dictIt, args)
 				}
 			} else {
@@ -134,24 +134,55 @@ func evalIterator(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		}
 	} else if list, ok := firstChildValue.(*kern.ListType); ok {
 		var args []any
-		if args, err = evalSibling(ctx, opTerm.Children, nil); err == nil {
+		if args, err = evalSiblings(ctx, opTerm.Children, nil); err == nil {
 			v = NewListIterator(list, args)
 		}
 	} else if intVal, ok := firstChildValue.(int64); ok {
 		var args []any
-		if args, err = evalSibling(ctx, opTerm.Children, intVal); err == nil {
+		if args, err = evalSiblings(ctx, opTerm.Children, intVal); err == nil {
 			v, err = NewIntIterator(args)
 		}
+	} else if it, ok := firstChildValue.(kern.Iterator); ok {
+		v, err = NewIterIter(it, ctx, opTerm.Children[1:])
 	} else {
-		var list []any
-		if list, err = evalSibling(ctx, opTerm.Children, firstChildValue); err == nil {
-			v = NewArrayIterator(list)
+		var siblings []any
+		if siblings, err = evalSiblings(ctx, opTerm.Children, firstChildValue); err == nil {
+			v = NewArrayIterator(siblings)
 		}
 	}
 	return
 }

-func evalSibling(ctx kern.ExprContext, terms []*scan.Term, firstChildValue any) (list []any, err error) {
+// func evalIterIter(ctx kern.ExprContext, firstChildValue any, siblings []any) (v any, err error) {
+// 	var op kern.Functor
+// 	var args map[string]any
+
+// 	if it, ok := firstChildValue.(kern.Iterator); ok {
+// 		if len(siblings) > 1 {
+// 			if op, ok = siblings[1].(kern.Functor); ok {
+// 				args = make(map[string]any, len(siblings)-2)
+// 				for i, arg := range siblings[2:] {
+// 					switch a := arg.(type) {
+// 					case *kern.DictType:
+// 						for keyAny, item := range *a {
+// 							if key, ok := keyAny.(string); ok {
+// 								args[key] = item
+// 							}
+// 						}
+// 					default:
+// 						args["arg"+strconv.Itoa(i+1)] = arg
+// 					}
+// 				}
+// 			} else if op == nil {
+// 				return nil, fmt.Errorf("the first sibling parameter must be a functor to be used as operation for the iterator")
+// 			}
+// 		}
+// 		v, err = NewIterIter(it, ctx, op, args)
+// 	}
+// 	return
+// }
+
+func evalSiblings(ctx kern.ExprContext, terms []*scan.Term, firstChildValue any) (list []any, err error) {
 	items := make([]any, 0, len(terms))
 	for i, tree := range terms {
 		var param any
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-list.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-literal.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-selector-case.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-var.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserightChilded.

 // operator-assign.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-bitwise.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-bool.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-builtin.go
@@ -37,7 +37,7 @@ func evalBuiltin(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	} else {
 		var moduleSpec any
 		var it kern.Iterator
-		if it, err = NewIterator(childValue); err != nil {
+		if it, err = NewIterator(ctx, childValue, nil); err != nil {
 			return
 		}
 		for moduleSpec, err = it.Next(); err == nil; moduleSpec, err = it.Next() {
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-but.go
@@ -0,0 +1,151 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-cat.go
+package expr
+
+import (
+	"fmt"
+	"io"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
+	"git.portale-stac.it/go-pkg/expr/scan"
+)
+
+//-------- cat term
+
+func newCatTerm(tk *scan.Token) (inst *scan.Term) {
+	return &scan.Term{
+		Tk:       *tk,
+		Children: make([]*scan.Term, 0, 2),
+		Position: scan.PosInfix,
+		Priority: scan.PriIterOp,
+		EvalFunc: evalCat,
+	}
+}
+
+func evalCat(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
+	var leftValue, rightValue any
+	var itLeft, itRight kern.Iterator
+	// var item any
+	var ok bool
+
+	if err = opTerm.CheckOperands(); err != nil {
+		return
+	}
+
+	if leftValue, err = opTerm.Children[0].Compute(ctx); err != nil {
+		return
+	}
+
+	if rightValue, err = opTerm.Children[1].Compute(ctx); err != nil {
+		return
+	}
+
+	if itLeft, ok = leftValue.(kern.Iterator); !ok {
+		if itLeft, err = NewIterator(ctx, leftValue, nil); err != nil {
+			return nil, fmt.Errorf("left operand of JOIN must be an iterable data-source; got %s", kern.TypeName(leftValue))
+		}
+	}
+
+	if itRight, ok = rightValue.(kern.Iterator); !ok {
+		if itRight, err = NewIterator(ctx, rightValue, nil); err != nil {
+			return nil, fmt.Errorf("right operand of JOIN must be an iterable data-source; got %s", kern.TypeName(rightValue))
+		}
+	}
+
+	// values := kern.NewListA()
+	// for _, it := range []kern.Iterator{itLeft, itRight} {
+	// 	for item, err = it.Next(); err == nil; item, err = it.Next() {
+	// 		values.AppendItem(item)
+	// 	}
+	// }
+	// if err == io.EOF {
+	// 	err = nil
+	// }
+	// v = values
+	v = newCatIterator(itLeft, itRight)
+	return
+}
+
+const catIteratorType = "cat"
+
+type catIterator struct {
+	kern.IteratorBase
+	itLeft    kern.Iterator
+	itRight   kern.Iterator
+	itCurrent kern.Iterator
+}
+
+func newCatIterator(itLeft, itRight kern.Iterator) (it *catIterator) {
+	it = &catIterator{
+		IteratorBase: kern.IteratorBase{},
+		itLeft:       itLeft,
+		itRight:      itRight,
+		itCurrent:    itLeft,
+	}
+	it.Reset()
+	return
+}
+
+func (it *catIterator) TypeName() string {
+	return catIteratorType
+}
+
+func (it *catIterator) String() string {
+	return fmt.Sprintf("$(%s %s %s)", it.itLeft, catIteratorType, it.itRight)
+}
+
+func (it *catIterator) Next() (item any, err error) {
+	if it.itCurrent == nil {
+		err = io.EOF
+	} else {
+		if item, err = it.itCurrent.Next(); err == nil {
+			it.Increment()
+		} else if err == io.EOF {
+			if it.itCurrent == it.itLeft {
+				it.itCurrent = it.itRight
+			} else {
+				return
+			}
+			if item, err = it.itCurrent.Next(); err == nil {
+				it.Increment()
+			} else {
+				it.itCurrent = nil
+			}
+		}
+	}
+	it.SetCurrent(item)
+	return
+}
+
+// func (it *catIterator) Reset() (err error) {
+// 	err = it.itLeft.Reset()
+// 	it.IteratorBase.Reset()
+// 	return
+// }
+
+func (it *catIterator) CallOperation(name string, args map[string]any) (v any, err error) {
+	switch name {
+	case kern.NextName:
+		v, err = it.Next()
+	// case kern.ResetName:
+	// 	err = it.Reset()
+	// case kern.CleanName:
+	// 	err = it.Clean()
+	case kern.IndexName:
+		v = it.Index()
+	case kern.CurrentName:
+		v, err = it.Current()
+	case kern.CountName:
+		v = it.Count()
+	default:
+		err = kern.ErrNoOperation(name)
+	}
+	return
+}
+
+// init
+func init() {
+	scan.RegisterTermConstructor(scan.SymKwCat, newCatTerm)
+}
@@ -1,10 +1,12 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-context-value.go
 package expr

 import (
+	"io"
+
 	"git.portale-stac.it/go-pkg/expr/kern"
 	"git.portale-stac.it/go-pkg/expr/scan"
 )
@@ -23,8 +25,8 @@ func newContextTerm(tk *scan.Token) (inst *scan.Term) {

 func evalContextValue(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	var childValue any
-
 	var sourceCtx kern.ExprContext
+
 	if len(opTerm.Children) == 0 {
 		sourceCtx = ctx
 	} else if opTerm.Children[0].Symbol() == scan.SymVariable && opTerm.Children[0].Source() == "global" {
@@ -33,6 +35,8 @@ func evalContextValue(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error
 		if dc, ok := childValue.(*dataCursor); ok {
 			sourceCtx = dc.ctx
 		}
+	} else {
+		return
 	}

 	if sourceCtx != nil {
@@ -53,6 +57,20 @@ func evalContextValue(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error
 			}
 			v = d
 		}
+	} else if childValue != nil {
+		if it, ok := childValue.(kern.Iterator); ok {
+			var item any
+			values := kern.NewListA()
+			for item, err = it.Next(); err == nil; item, err = it.Next() {
+				values.AppendItem(item)
+			}
+			if err == io.EOF {
+				err = nil
+				v = values
+			}
+		} else {
+			err = opTerm.ErrIncompatiblePrefixPostfixType(childValue)
+		}
 	} else {
 		err = opTerm.ErrIncompatiblePrefixPostfixType(childValue)
 	}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-ctrl.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-default.go
@@ -111,7 +111,7 @@ func evalAssignDefault(ctx kern.ExprContext, opTerm *scan.Term) (v any, err erro
 		if functor, ok := rightValue.(kern.Functor); ok {
 			//ctx.RegisterFunc(leftTerm.source(), functor, 0, -1)
 			ctx.RegisterFunc(leftTerm.Source(), functor, kern.TypeAny, []kern.ExprFuncParam{
-				NewFuncParamFlag(kern.ParamValue, PfDefault|PfRepeat),
+				kern.NewFuncParamFlag(kern.ParamValue, kern.PfDefault|kern.PfRepeat),
 			})
 		} else {
 			v = rightValue
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-digest.go
@@ -37,7 +37,7 @@ func evalDigest(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if it, err = NewIterator(leftValue); err != nil {
+	if it, err = NewIterator(ctx, leftValue, nil); err != nil {
 		return nil, fmt.Errorf("left operand of DIGEST must be an iterable data-source; got %s", kern.TypeName(leftValue))
 	}

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-dot.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-fact.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-filter.go
@@ -6,7 +6,6 @@ package expr

 import (
 	"fmt"
-	"io"

 	"git.portale-stac.it/go-pkg/expr/kern"
 	"git.portale-stac.it/go-pkg/expr/scan"
@@ -25,9 +24,11 @@ func newFilterTerm(tk *scan.Token) (inst *scan.Term) {
 }

 func evalFilter(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
-	var leftValue, rightValue any
+	// var leftValue, rightValue any
+	var leftValue any
 	var it kern.Iterator
-	var item any
+	// var item any
+	var ok bool

 	if err = opTerm.CheckOperands(); err != nil {
 		return
@@ -37,35 +38,126 @@ func evalFilter(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if it, err = NewIterator(leftValue); err != nil {
-		return nil, fmt.Errorf("left operand of FILTER must be an iterable data-source; got %s", kern.TypeName(leftValue))
+	if it, ok = leftValue.(kern.Iterator); !ok {
+		if it, err = NewIterator(ctx, leftValue, nil); err != nil {
+			return nil, fmt.Errorf("left operand of FILTER must be an iterable data-source; got %s", kern.TypeName(leftValue))
+		}
 	}

-	values := kern.NewListA()
-	for item, err = it.Next(); err == nil; item, err = it.Next() {
-		ctx.SetVar("_", item)
+	// values := kern.NewListA()
+	// for item, err = it.Next(); err == nil; item, err = it.Next() {
+	// 	ctx.SetVar("_", item)
+	// 	ctx.SetVar("__", it.Index())
+	// 	ctx.SetVar("_#", it.Count())
+	// 	if rightValue, err = opTerm.Children[1].Compute(ctx); err == nil {
+	// 		if success, valid := kern.ToBool(rightValue); valid {
+	// 			if success {
+	// 				values.AppendItem(item)
+	// 			}
+	// 		} else {
+	// 			err = fmt.Errorf("filter expression must return a boolean or a castable to boolean, got %v [%T]", rightValue, rightValue)
+	// 		}
+	// 	}
+	// 	ctx.DeleteVar("_#")
+	// 	ctx.DeleteVar("__")
+	// 	ctx.DeleteVar("_")
+	// 	if err != nil {
+	// 		break
+	// 	}
+	// }
+	// if err == io.EOF {
+	// 	err = nil
+	// }
+	// v = values
+
+	v = newFilerIterator(ctx, it, opTerm.Children[1])
+	return
+}
+
+const filterIteratorType = "filter"
+
+type filterIterator struct {
+	kern.IteratorBase
+	ctx   kern.ExprContext
+	itSrc kern.Iterator
+	expr  *scan.Term
+}
+
+func newFilerIterator(ctx kern.ExprContext, itSrc kern.Iterator, filterExpr *scan.Term) (it *filterIterator) {
+	it = &filterIterator{
+		IteratorBase: kern.IteratorBase{},
+		ctx:          ctx,
+		itSrc:        itSrc,
+		expr:         filterExpr,
+	}
+	it.IteratorBase.Reset()
+	return
+}
+
+func (it *filterIterator) TypeName() string {
+	return filterIteratorType
+}
+
+func (it *filterIterator) String() string {
+	return fmt.Sprintf("$(%s %s bool-expr)", it.itSrc, filterIteratorType)
+}
+
+func (it *filterIterator) Next() (item any, err error) {
+	var attempt, result any
+
+	ctx := it.ctx
+
+	for attempt, err = it.itSrc.Next(); err == nil; attempt, err = it.itSrc.Next() {
+		ctx.SetVar("_", attempt)
 		ctx.SetVar("__", it.Index())
 		ctx.SetVar("_#", it.Count())
-		if rightValue, err = opTerm.Children[1].Compute(ctx); err == nil {
-			if success, valid := kern.ToBool(rightValue); valid {
-				if success {
-					values.AppendItem(item)
-				}
-			} else {
-				err = fmt.Errorf("filter expression must return a boolean or a castable to boolean, got %v [%T]", rightValue, rightValue)
-			}
-		}
+		result, err = it.expr.Compute(ctx)
 		ctx.DeleteVar("_#")
 		ctx.DeleteVar("__")
 		ctx.DeleteVar("_")
+
+		if err == nil {
+			if success, valid := kern.ToBool(result); valid {
+				if success {
+					item = attempt
+					break
+				}
+			} else {
+				err = fmt.Errorf("filter expression must return a boolean or a castable to boolean, got %v [%T]", result, result)
+			}
+		}
 		if err != nil {
 			break
 		}
 	}
-	if err == io.EOF {
-		err = nil
+
+	it.SetCurrent(item)
+	return
+}
+
+// func (it *filterIterator) Reset() (err error) {
+// 	err = it.itLeft.Reset()
+// 	it.IteratorBase.Reset()
+// 	return
+// }
+
+func (it *filterIterator) CallOperation(name string, args map[string]any) (v any, err error) {
+	switch name {
+	case kern.NextName:
+		v, err = it.Next()
+	// case kern.ResetName:
+	// 	err = it.Reset()
+	// case kern.CleanName:
+	// 	err = it.Clean()
+	case kern.IndexName:
+		v = it.Index()
+	case kern.CurrentName:
+		v, err = it.Current()
+	case kern.CountName:
+		v = it.Count()
+	default:
+		err = kern.ErrNoOperation(name)
 	}
-	v = values
 	return
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operand-fraction.go
@@ -31,6 +31,7 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	var item any
 	var sKey string
 	var keyByIndex bool
+	var ok bool

 	if err = opTerm.CheckOperands(); err != nil {
 		return
@@ -40,8 +41,10 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if it, err = NewIterator(leftValue); err != nil {
-		return nil, fmt.Errorf("left operand of MAP must be an iterable data-source; got %s", kern.TypeName(leftValue))
+	if it, ok = leftValue.(kern.Iterator); !ok {
+		if it, err = NewIterator(ctx, leftValue, nil); err != nil {
+			return nil, fmt.Errorf("left operand of MAP must be an iterable data-source; got %s", kern.TypeName(leftValue))
+		}
 	}

 	rightTk := opTerm.Children[1].Tk
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-in.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-include.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-index.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-insert.go
@@ -31,6 +31,41 @@ func newAppendTerm(tk *scan.Token) (inst *scan.Term) {
 	}
 }

+func prependToList(ctx kern.ExprContext, opTerm *scan.Term, leftValue, rightValue any) (result any, err error) {
+	if list, ok := rightValue.(*kern.ListType); ok {
+		var it kern.Iterator
+		if it, ok = leftValue.(kern.Iterator); !ok {
+			if it, err = NewIterator(ctx, leftValue, nil); err != nil {
+				return
+			}
+		}
+
+		ls := kern.NewLinkedList()
+		ls.SeqPushBack(it)
+
+		newList := kern.ListType(nil)
+		if newSize := len(*list) + int(ls.Len()); newSize > cap(*list) {
+			newList = make([]any, 0, newSize)
+			for node := ls.FirstNode(); node != nil; node = node.Next() {
+				newList = append(newList, node.Data())
+			}
+		}
+		for _, item := range *list {
+			newList = append(newList, item)
+		}
+		result = &newList
+
+		// ***** EVENTUALMENTE ABILITARE LA MODIFICA DELLA VARIABILE
+		// ***** CON UN OPERATORE SPECIFICO
+		// if opTerm.Children[1].Symbol() == scan.SymVariable {
+		// 	ctx.UnsafeSetVar(opTerm.Children[1].Source(), result)
+		// }
+	} else {
+		err = opTerm.ErrIncompatibleTypes(leftValue, rightValue)
+	}
+	return
+}
+
 func evalPrepend(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	var leftValue, rightValue any

@@ -38,13 +73,39 @@ func evalPrepend(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if kern.IsList(rightValue) {
-		list, _ := rightValue.(*kern.ListType)
-		newList := append(kern.ListType{leftValue}, *list...)
-		v = &newList
-		if opTerm.Children[1].Symbol() == scan.SymVariable {
-			ctx.UnsafeSetVar(opTerm.Children[1].Source(), v)
+	v, err = prependToList(ctx, opTerm, leftValue, rightValue)
+	return
+}
+
+func appendToList(ctx kern.ExprContext, opTerm *scan.Term, leftValue, rightValue any) (result any, err error) {
+	if list, ok := leftValue.(*kern.ListType); ok {
+		var it kern.Iterator
+		if it, ok = rightValue.(kern.Iterator); !ok {
+			if it, err = NewIterator(ctx, rightValue, nil); err != nil {
+				return
+			}
 		}
+
+		ls := kern.NewLinkedList()
+		ls.SeqPushBack(it)
+
+		newList := *list
+		if newSize := len(*list) + int(ls.Len()); newSize > cap(*list) {
+			newList = make([]any, 0, newSize)
+			for _, item := range *list {
+				newList = append(newList, item)
+			}
+		}
+		for node := ls.FirstNode(); node != nil; node = node.Next() {
+			newList = append(newList, node.Data())
+		}
+		result = &newList
+
+		// ***** EVENTUALMENTE ABILITARE LA MODIFICA DELLA VARIABILE
+		// ***** CON UN OPERATORE SPECIFICO
+		// if opTerm.Children[0].Symbol() == scan.SymVariable {
+		// 	ctx.UnsafeSetVar(opTerm.Children[0].Source(), result)
+		// }
 	} else {
 		err = opTerm.ErrIncompatibleTypes(leftValue, rightValue)
 	}
@@ -58,16 +119,8 @@ func evalAppend(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if kern.IsList(leftValue) {
-		list, _ := leftValue.(*kern.ListType)
-		newList := append(*list, rightValue)
-		v = &newList
-		if opTerm.Children[0].Symbol() == scan.SymVariable {
-			ctx.UnsafeSetVar(opTerm.Children[0].Source(), v)
-		}
-	} else {
-		err = opTerm.ErrIncompatibleTypes(leftValue, rightValue)
-	}
+	v, err = appendToList(ctx, opTerm, leftValue, rightValue)
+
 	return
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-iter-value.go
@@ -1,68 +0,0 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
-// All rights reserved.
-
-// operator-join.go
-package expr
-
-import (
-	"fmt"
-	"io"
-
-	"git.portale-stac.it/go-pkg/expr/kern"
-	"git.portale-stac.it/go-pkg/expr/scan"
-)
-
-//-------- join term
-
-func newJoinTerm(tk *scan.Token) (inst *scan.Term) {
-	return &scan.Term{
-		Tk:       *tk,
-		Children: make([]*scan.Term, 0, 2),
-		Position: scan.PosInfix,
-		Priority: scan.PriIterOp,
-		EvalFunc: evalJoin,
-	}
-}
-
-func evalJoin(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
-	var leftValue, rightValue any
-	var itLeft, itRight kern.Iterator
-	var item any
-
-	if err = opTerm.CheckOperands(); err != nil {
-		return
-	}
-
-	if leftValue, err = opTerm.Children[0].Compute(ctx); err != nil {
-		return
-	}
-
-	if rightValue, err = opTerm.Children[1].Compute(ctx); err != nil {
-		return
-	}
-
-	if itLeft, err = NewIterator(leftValue); err != nil {
-		return nil, fmt.Errorf("left operand of JOIN must be an iterable data-source; got %s", kern.TypeName(leftValue))
-	}
-
-	if itRight, err = NewIterator(rightValue); err != nil {
-		return nil, fmt.Errorf("right operand of JOIN must be an iterable data-source; got %s", kern.TypeName(rightValue))
-	}
-
-	values := kern.NewListA()
-	for _, it := range []kern.Iterator{itLeft, itRight} {
-		for item, err = it.Next(); err == nil; item, err = it.Next() {
-			values.AppendItem(item)
-		}
-	}
-	if err == io.EOF {
-		err = nil
-	}
-	v = values
-	return
-}
-
-// init
-func init() {
-	scan.RegisterTermConstructor(scan.SymKwJoin, newJoinTerm)
-}
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-length.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-map.go
@@ -6,7 +6,6 @@ package expr

 import (
 	"fmt"
-	"io"

 	"git.portale-stac.it/go-pkg/expr/kern"
 	"git.portale-stac.it/go-pkg/expr/scan"
@@ -25,9 +24,11 @@ func newMapTerm(tk *scan.Token) (inst *scan.Term) {
 }

 func evalMap(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
-	var leftValue, rightValue any
+	// var leftValue, rightValue any
+	var leftValue any
 	var it kern.Iterator
-	var item any
+	// var item any
+	var ok bool

 	if err = opTerm.CheckOperands(); err != nil {
 		return
@@ -37,31 +38,34 @@ func evalMap(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	if it, err = NewIterator(leftValue); err != nil {
-		return nil, fmt.Errorf("left operand of MAP must be an iterable data-source; got %s", kern.TypeName(leftValue))
+	if it, ok = leftValue.(kern.Iterator); !ok {
+		if it, err = NewIterator(ctx, leftValue, nil); err != nil {
+			return nil, fmt.Errorf("left operand of MAP must be an iterable data-source; got %s", kern.TypeName(leftValue))
+		}
 	}

-	values := kern.NewListA()
-	for item, err = it.Next(); err == nil; item, err = it.Next() {
-		ctx.SetVar("_", item)
-		ctx.SetVar("__", it.Index())
-		ctx.SetVar("_#", it.Count())
-		if rightValue, err = opTerm.Children[1].Compute(ctx); err == nil {
-			values.AppendItem(rightValue)
-		}
-		ctx.DeleteVar("_#")
-		ctx.DeleteVar("__")
-		ctx.DeleteVar("_")
-		if err != nil {
-			break
-		}
-	}
-	if err == io.EOF {
-		err = nil
-	}
-	if err == nil {
-		v = values
-	}
+	// values := kern.NewListA()
+	// for item, err = it.Next(); err == nil; item, err = it.Next() {
+	// 	ctx.SetVar("_", item)
+	// 	ctx.SetVar("__", it.Index())
+	// 	ctx.SetVar("_#", it.Count())
+	// 	if rightValue, err = opTerm.Children[1].Compute(ctx); err == nil {
+	// 		values.AppendItem(rightValue)
+	// 	}
+	// 	ctx.DeleteVar("_#")
+	// 	ctx.DeleteVar("__")
+	// 	ctx.DeleteVar("_")
+	// 	if err != nil {
+	// 		break
+	// 	}
+	// }
+	// if err == io.EOF {
+	// 	err = nil
+	// }
+	// if err == nil {
+	// 	v = values
+	// }
+	v, err = NewIterIter(it, ctx, opTerm.Children[1:])
 	return
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-plugin.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-post-inc-dec.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-pre-inc-dec.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-prod.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-range.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-rel.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-selector.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-shift.go
@@ -39,7 +39,9 @@ func evalRightShift(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error)
 		return
 	}

-	v, err = bitRightShift(opTerm, leftValue, rightValue)
+	if v, err = bitRightShift(opTerm, leftValue, rightValue); err != nil {
+		v, err = prependToList(ctx, opTerm, leftValue, rightValue)
+	}
 	return
 }

@@ -71,7 +73,9 @@ func evalLeftShift(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		return
 	}

-	v, err = bitLeftShift(opTerm, leftValue, rightValue)
+	if v, err = bitLeftShift(opTerm, leftValue, rightValue); err != nil {
+		v, err = appendToList(ctx, opTerm, leftValue, rightValue)
+	}
 	return
 }

@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-sign.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-sum.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // operator-unset.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // parser.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // plugin.go
@@ -100,7 +100,7 @@ func importPluginFromSearchPath(ctx kern.ExprContext, name any) (count int, err
 	var it kern.Iterator
 	dirList := buildSearchDirList(ctx, "plugin", ENV_EXPR_PLUGIN_PATH)
 	count = 0
-	if it, err = NewIterator(name); err != nil {
+	if it, err = NewIterator(ctx, name, nil); err != nil {
 		return
 	}
 	for moduleSpec, err = it.Next(); err == nil; moduleSpec, err = it.Next() {
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // ast.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // expr project scanner.go
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // Symbol.go
@@ -138,6 +138,7 @@ func init() {
 		SymKwMap:     {"map", SymClassOperator, PosInfix},
 		SymKwFilter:  {"filter", SymClassOperator, PosInfix},
 		SymKwDigest:  {"digest", SymClassOperator, PosInfix},
+		SymKwCat:     {"cat", SymClassOperator, PosInfix},
 		SymKwJoin:    {"join", SymClassOperator, PosInfix},
 		SymKwGroupBy: {"groupby", SymClassOperator, PosInfix},
 		SymKwFunc:    {"func(", SymClassDeclaration, PosPrefix},
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // Symbol.go
@@ -122,6 +122,7 @@ const (
 	SymKwMap
 	SymKwFilter
 	SymKwDigest
+	SymKwCat
 	SymKwGroupBy
 	SymKwJoin
 	SymKwNil
@@ -141,6 +142,7 @@ func init() {
 		"IN":      SymKwIn,
 		"INCLUDE": SymKwInclude,
 		"MAP":     SymKwMap,
+		"CAT":     SymKwCat,
 		"FILTER":  SymKwFilter,
 		"NOT":     SymKwNot,
 		"OR":      SymKwOr,
@@ -1,4 +1,4 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
 // All rights reserved.

 // t_scanner_test.go
--- a/Show More
+++ b/Show More
Author	SHA1	Message	Date
camoroso	e6844ad1e8	alias operators: '<<' same as '<+', '>>' same as '+>'. Insert and append operation optimized with linked lists	2026-05-16 17:18:23 +02:00
camoroso	3a4e217891	kern/fraction-type.go: changed some syntax	2026-05-13 08:01:18 +02:00
camoroso	0f293fdbbc	restructured some test files	2026-05-11 14:01:22 +02:00
camoroso	45faea7620	map, filter and cat return an iterator	2026-05-09 17:42:04 +02:00
camoroso	e5a61b5638	iter-ops raised above assign; 'cat' operator returns an iterator; $$ operator supports iterators producing a list of items	2026-05-09 12:15:59 +02:00
camoroso	5285b61320	'join' operator renamed as 'cat'	2026-05-09 10:34:46 +02:00
camoroso	5950630cf7	Doc: added more details and new functions and operators (iterator operators)	2026-05-08 15:17:24 +02:00
camoroso	78e431f2b9	forgotten changes to builtin-os-file-iter.go	2026-05-08 11:07:53 +02:00
camoroso	c7dce8288f	file iterators refactored	2026-05-08 11:03:16 +02:00
camoroso	c10053253c	os file builtins refactored with the package 'file'	2026-05-08 10:16:03 +02:00
camoroso	a3c7cf2efa	iter-iter.go: check ctx against nil on creation	2026-05-08 10:14:07 +02:00
camoroso	dfa1491093	kern/common-errors.go: new error function ErrFuncInvalidArg()	2026-05-08 10:10:28 +02:00
camoroso	99c1adc434	copyright header updated to 2026	2026-05-06 09:27:42 +02:00
camoroso	5585b496fb	iter-iter: changed item operation from function and args to a list of expressions	2026-05-06 04:04:08 +02:00
camoroso	acd4f8487d	new iter-iter iterator and kern.func-info module	2026-05-05 20:38:30 +02:00