diff --git a/ast.go b/ast.go
index e562b76..8620720 100644
--- a/ast.go
+++ b/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 {
diff --git a/expr.go b/expr.go
index a102d1b..730e149 100644
--- a/expr.go
+++ b/expr.go
@@ -6,6 +6,7 @@ package expr
 
 // ----Expression interface
 type Expr interface {
+	Typer
 	Eval(ctx ExprContext) (result any, err error)
 	String() string
 }