New interface to Typer: the function TypeName() returns a more readable type name

formatter.go

@@ -4,6 +4,8 @@
 // formatter.go
 package expr
 
+import "fmt"
+
 type FmtOpt uint16
 
 const (
@@ -18,3 +20,16 @@ const (
 type Formatter interface {
 	ToString(options FmtOpt) string
 }
+
+type Typer interface {
+	TypeName() string
+}
+
+func getTypeName(v any) (name string) {
+	if typer, ok := v.(Typer); ok {
+		name = typer.TypeName()
+	} else {
+		name = fmt.Sprintf("%T", v)
+	}
+	return
+}

operand-dict.go

@@ -90,6 +90,14 @@ func (dict *DictType) ToString(opt FmtOpt) string {
 	return sb.String()
 }
 
+func (dict *DictType) String() string {
+	return dict.ToString(0)
+}
+
+func (dict *DictType) TypeName() string {
+	return "dict"
+}
+
 func (dict *DictType) hasKey(target any) (ok bool) {
 	for key := range *dict {
 		if ok = reflect.DeepEqual(key, target); ok {

operand-list.go

@@ -62,6 +62,10 @@ func (ls *ListType) String() string {
 	return ls.ToString(0)
 }
 
+func (ls *ListType) TypeName() string {
+	return "list"
+}
+
 func (list *ListType) indexDeepCmp(target any) (index int) {
 	index = -1
 	for i, item := range *list {

term.go

@@ -161,11 +161,18 @@ func (self *term) toInt(computedValue any, valueDescription string) (i int, err
 }
 
 func (self *term) errIncompatibleTypes(leftValue, rightValue any) error {
+	leftType := getTypeName(leftValue)
+	rightType := getTypeName(rightValue)
 	return self.tk.Errorf(
-		"left operand '%v' [%T] and right operand '%v' [%T] are not compatible with operator %q",
+		"left operand '%v' [%s] and right operand '%v' [%s] are not compatible with operator %q",
-		leftValue, leftValue,
+		leftValue, leftType,
-		rightValue, rightValue,
+		rightValue, rightType,
 		self.source())
+	// return self.tk.Errorf(
+	// 	"left operand '%v' [%T] and right operand '%v' [%T] are not compatible with operator %q",
+	// 	leftValue, leftValue,
+	// 	rightValue, rightValue,
+	// 	self.source())
 }
 
 func (self *term) errIncompatibleType(value any) error {