Fixed Clean() and added error message when calling optional missing functions

data-cursor.go

@@ -5,6 +5,7 @@
 package expr
 
 import (
+	"errors"
 	"io"
 )
 
@@ -46,15 +47,19 @@ func (dc *dataCursor) Reset() (err error) {
 			dc.index = -1
 		}
 		exportObjects(dc.ctx, ctx)
+	} else {
+		err = errors.New("no 'reset' function defined in the data-source")
 	}
 	return
 }
 
 func (dc *dataCursor) Clean() (err error) {
-	if dc.resetFunc != nil {
+	if dc.cleanFunc != nil {
 		ctx := cloneContext(dc.ctx)
 		_, err = dc.cleanFunc.Invoke(ctx, cleanName, []any{})
 		exportObjects(dc.ctx, ctx)
+	} else {
+		err = errors.New("no 'clean' function defined in the data-source")
 	}
 	return
 }