data-cursor.go: the inizialization of the current item is done in the Next() method.

This allows the application of the filter and map operator to the first item too.

data-cursor.go

@@ -5,13 +5,13 @@
 package expr
 
 import (
-	"errors"
 	"io"
 )
 
 type dataCursor struct {
 	ds        map[string]Functor
 	ctx       ExprContext
+	initState bool // true if no item has prodiced yet (this replace di initial Next() call in the contructor)
 	index     int
 	count     int
 	current   any
@@ -24,18 +24,19 @@ type dataCursor struct {
 
 func NewDataCursor(ctx ExprContext, ds map[string]Functor, resource any) (dc *dataCursor) {
 	dc = &dataCursor{
-		ds:       ds,
-		index:    -1,
-		count:    0,
-		current:  nil,
-		lastErr:  nil,
-		resource: resource,
-		ctx:      ctx.Clone(),
-		nextFunc: ds[NextName],
+		ds:        ds,
+		initState: true,
+		index:     -1,
+		count:     0,
+		current:   nil,
+		lastErr:   nil,
+		resource:  resource,
+		ctx:       ctx.Clone(),
+		nextFunc:  ds[NextName],
 		cleanFunc: ds[CleanName],
 		resetFunc: ds[ResetName],
 	}
-	dc.Next()
+	//dc.Next()
 	return
 }
 
@@ -107,12 +108,14 @@ func (dc *dataCursor) Reset() (success bool, err error) {
 	if dc.resetFunc != nil {
 		if dc.resource != nil {
 			ctx := cloneContext(dc.ctx)
-			if _, err = dc.resetFunc.Invoke(ctx, ResetName, []any{dc.resource}); err == nil {
-				dc.index = -1
-				dc.count = 0
-			}
+			_, err = dc.resetFunc.Invoke(ctx, ResetName, []any{dc.resource})
 			exportObjects(dc.ctx, ctx)
-			dc.Next()
+			dc.index = -1
+			dc.count = 0
+			dc.initState = true
+			dc.current = nil
+			dc.lastErr = nil
+			//dc.Next()
 		} else {
 			err = errInvalidDataSource()
 		}
@@ -128,19 +131,21 @@ func (dc *dataCursor) Clean() (success bool, err error) {
 		if dc.resource != nil {
 			ctx := cloneContext(dc.ctx)
 			_, err = dc.cleanFunc.Invoke(ctx, CleanName, []any{dc.resource})
-			dc.resource = nil
+			//			dc.resource = nil
 			exportObjects(dc.ctx, ctx)
 		} else {
 			err = errInvalidDataSource()
 		}
 	} else {
-		err = errors.New("no 'clean' function defined in the data-source")
+		err = errNoOperation(CleanName)
 	}
 	success = err == nil
 	return
 }
 
 func (dc *dataCursor) Current() (item any, err error) { // must return io.EOF at the last item
+	dc.init()
+
 	if dc.current != nil {
 		item = dc.current
 	} else {
@@ -167,8 +172,17 @@ func (dc *dataCursor) mapItem(mapper Functor, item any) (mappedItem any, err err
 	return
 }
 
+func (dc *dataCursor) init() {
+	if dc.initState {
+		dc.initState = false
+		dc.Next()
+	}
+}
+
 func (dc *dataCursor) Next() (current any, err error) { // must return io.EOF after the last item
-	if err = dc.lastErr; err != nil {
+	if dc.initState {
+		dc.init()
+	} else if err = dc.lastErr; err != nil {
 		return
 	}
 	current = dc.current
@@ -214,7 +228,6 @@ func (dc *dataCursor) Index() int {
 	return dc.index - 1
 }
 
-
 func (dc *dataCursor) Count() int {
 	return dc.count
 }