Custom formatter based on go's fmt package. (#185)

* Custom formatter based on go's fmt package.

* Cleanup

* Cleanup

* Added tengo.MaxStringLen check

* Cleanup

objects/builtin_format.go

@@ -0,0 +1,27 @@
+package objects
+
+func builtinFormat(args ...Object) (Object, error) {
+	numArgs := len(args)
+	if numArgs == 0 {
+		return nil, ErrWrongNumArguments
+	}
+
+	format, ok := args[0].(*String)
+	if !ok {
+		return nil, ErrInvalidArgumentType{
+			Name:     "format",
+			Expected: "string",
+			Found:    args[0].TypeName(),
+		}
+	}
+	if numArgs == 1 {
+		return format, nil // okay to return 'format' directly as String is immutable
+	}
+
+	s, err := Format(format.Value, args[1:]...)
+	if err != nil {
+		return nil, err
+	}
+
+	return &String{Value: s}, nil
+}

objects/builtins.go

@@ -111,4 +111,8 @@ var Builtins = []*BuiltinFunction{
 		Name:  "type_name",
 		Value: builtinTypeName,
 	},
+	{
+		Name:  "format",
+		Value: builtinFormat,
+	},
 }