the dataCursor struct has been moved fro operand-iterator.go to the new file data-cursor.go

data-cursor.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// data-cursors.go
+package expr
+
+import (
+	"io"
+)
+
+const (
+	initName    = "init"
+	nextName    = "next"
+	currentName = "current"
+)
+
+type dataCursor struct {
+	ds          map[any]*term
+	ctx         ExprContext
+	index       int
+	resource    any
+	nextFunc    Functor
+	currentFunc Functor
+}
+
+func (dc *dataCursor) String() string {
+	return "$(...)"
+}
+
+func (dc *dataCursor) Current() (item any, err error) { // must return io.EOF at the last item
+	if item, err = dc.currentFunc.Invoke(dc.ctx, currentName, []any{}); err == nil && item == nil {
+		err = io.EOF
+	}
+	return
+}
+
+func (dc *dataCursor) Next() (item any, err error) { // must return io.EOF after the last item
+	if item, err = dc.nextFunc.Invoke(dc.ctx, nextName, []any{}); err == nil {
+		if item == nil {
+			err = io.EOF
+		} else {
+			dc.index++
+		}
+	}
+	return
+}
+
+func (dc *dataCursor) Index() int {
+	return dc.index
+}