when_extension 0.1.2

Dart library with when annotation for generating custom when extensions and extension methods for dart builtin types

when_extension

when extension methods for Dart. Inspired by Kotlin's when expression.

This library contains @when annotation for generating custom when extensions and extension methods for dart builtin types.

Class when

Similar to sealed classes in Kotlin (discriminated unions) a when extension method for a single level class hierarchy can be generated using the @when annotation. All classes must be defined in the same file.

//types.dart:
import 'package:when/when.dart';

part 'types.g.dart';

@when
abstract class Expr {}

class Value implements Expr {
  int value;
  Value({this.value});
}

class Add implements Expr {
  Expr e1;
  Expr e2;
  Add({this.e1, this.e2});
}

A when extension method on Expr function will be generated in the types.g.dart file. And can be used as follows:

int eval(Expr expr) {
  return expr.when(
    value: (v) => v.value,
    add: (a) => eval(a.e1) + eval(a.e2),
    any: () => 0,
  );
}

The method takes an optional named function argument per subclass and an any named argument that will called if none of the provided arguments matches. If any is not provided and no argument matches an exception will be thrown at runtime.

Enum when

An enum can also be annotated with @when to generate a when extension method:

//types.dart:
import 'package:when/when.dart';

part 'types.g.dart';

@when
enum Color {
  red,
  green,
  blue,
}

final r = Color.red;
final result = r.when(
  red: () => 1,
  green: () => 2,
  any: () => 3,
);
//result: 1

String.when

import 'package:when/when.dart';

final s = 'aaa';
final result = s.when(
  {
    'aaa': () => 1,
    'bbb': () => 2,
  },
  any: () => 3,
);
//result: 1

Setup

Add the following to your pubspec.yaml:

dependencies:
  when: ^0.1.0
dev_dependencies:
  build_runner:
  when_generator: ^0.1.0

If you are using the @when annotation run:

pub run build_runner build