Undefinable

A Dart library for representing values that can be explicitly "undefined" or "defined" (including null). This is especially useful for APIs like copyWith, where you need to distinguish between "do not change this field" and "set this field to null or a new value".

Motivation

In Dart, optional parameters default to null, which makes it impossible to distinguish between "no change" and "set to null" in patterns like copyWith. Undefinable<T> solves this by providing an explicit undefined state.

Features

• Explicitly represent undefined and defined values
• Convenient API for checking state and extracting values
• Extension for easy conversion
• Helper functions for construction

Usage

Basic Usage

import 'package:undefinable/undefinable.dart';

void main() {
  final undef = Undefinable<String>.undefined();
  final defined = Undefinable<String>.defined('Hello');
  final definedNull = Undefinable<String?>.defined(null);

  print(undef.isUndefined); // true
  print(defined.isDefined); // true
  print(defined.value); // Hello
  print(definedNull.isDefined); // true
  print(definedNull.value); // null
}

Using with copyWith

class Person {
  final String? name;
  final int? age;

  Person({this.name, this.age});

  Person copyWith({
    Undefinable<String?> name = const Undefinable.undefined(),
    Undefinable<int?> age = const Undefinable.undefined(),
  }) {
    return Person(
      name: name.isUndefined ? this.name : name.value,
      age: age.isUndefined ? this.age : age.value,
    );
  }
}

final person = Person(name: 'Alice', age: 30);

final updated = person.copyWith(
  name: Undefinable<String?>.defined(null), // set name to null
  age: Undefinable<int?>.undefined(), // don't change age
);

print(updated.name); // null
print(updated.age); // 30

Extension Usage

final value = 'Hello'.toUndefinable<String>();
print(value.isDefined); // true
print(value.value); // Hello

Helper Functions

final undef = createUndefined<int>();
final defined = createDefined<int>(42);

API Reference

Undefinable

• Undefinable.defined(T value) – create a defined value (can be null)
• Undefinable.undefined() – create an undefined value
• isUndefined – true if value is undefined
• isDefined – true if value is defined
• isNull – true if value is defined and null
• value – get the value (throws if undefined)
• valueOrNull – get the value or null if undefined
• valueOr(fallback) – get the value or fallback if undefined
• valueOrFn(fn) – get the value or result of fn if undefined

Extensions

• toUndefinable<T>() – convert any object to Undefinable

Helper Functions

• createUndefined<T>() – create an undefined value
• createDefined<T>(T value) – create a defined value