restructure project

internal/builtins/lower.go

@@ -0,0 +1,21 @@
+package builtins
+
+import (
+	"monkey/internal/object"
+	"monkey/internal/typing"
+	"strings"
+)
+
+// Lower ...
+func Lower(args ...object.Object) object.Object {
+	if err := typing.Check(
+		"lower", args,
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
+	}
+
+	str := args[0].(*object.String)
+	return &object.String{Value: strings.ToLower(str.Value)}
+}