rearrange builtins

builtins/len.go

@@ -0,0 +1,23 @@
+package builtins
+
+import "monkey/object"
+
+import "unicode/utf8"
+
+// Len ...
+func Len(args ...object.Object) object.Object {
+	if len(args) != 1 {
+		return newError("wrong number of arguments. got=%d, want=1",
+			len(args))
+	}
+
+	switch arg := args[0].(type) {
+	case *object.Array:
+		return &object.Integer{Value: int64(len(arg.Elements))}
+	case *object.String:
+		return &object.Integer{Value: int64(utf8.RuneCountInString(arg.Value))}
+	default:
+		return newError("argument to `len` not supported, got %s",
+			args[0].Type())
+	}
+}