restructure project

internal/builtins/len.go

@@ -0,0 +1,21 @@
+package builtins
+
+import (
+	"monkey/internal/object"
+	"monkey/internal/typing"
+)
+
+// Len ...
+func Len(args ...object.Object) object.Object {
+	if err := typing.Check(
+		"len", args,
+		typing.ExactArgs(1),
+	); err != nil {
+		return newError(err.Error())
+	}
+
+	if size, ok := args[0].(object.Sizeable); ok {
+		return &object.Integer{Value: int64(size.Len())}
+	}
+	return newError("TypeError: object of type '%s' has no len()", args[0].Type())
+}