Expr embeds Typer and ast implements it

ast.go

@@ -19,6 +19,10 @@ func NewAst() *ast {
 	return &ast{}
 }
 
+func (expr *ast) TypeName() string {
+	return "Expression"
+}
+
 func (expr *ast) ToForest() {
 	if expr.root != nil {
 		if expr.forest == nil {