match #

match extension methods for Dart. Inspired by pattern matching from functional programming languages and Kotlin's when expression.

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

Class match #

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

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

part 'types.g.dart';

@match
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 match 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.match(
    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 match #

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

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

part 'types.g.dart';

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

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

String.match #

import 'package:match/match.dart';

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

Setup #

Add the following to your pubspec.yaml:

dependencies:
  match: ^0.1.0
dev_dependencies:
  build_runner:
  match_generator: ^0.1.0

If you are using the @match annotation run:

pub run build_runner build