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 and matchAny extensions and extension methods for dart builtin types.

Setup

Add the following to your pubspec.yaml:

dependencies:
  match: ^0.2.0
dev_dependencies:
  build_runner:
  match_generator: ^0.2.0

If you are using the @match annotation run:

pub run build_runner build

Class match

Similar to sealed classes in Kotlin (discriminated unions) a match and matchAny 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});
}

match and matchAny extension methods 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),
  );
}

final e = Add(
  e1: Add(e1: Value(value: 10), e2: Value(value: 20)),
  e2: Value(value: 20),
);

expect(eval(e), 50);

final v = Value(value: 20);
final result = v.matchAny(add: (a) => 1, any: () => 2);
expect(result, 2);

The match method takes an named function argument per subclass. matchAny is like match but also takes a named function argument any 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.

The match and matchAny method's named arguments are annotated with @required to allow dartanalyzer to give warnings when cases are missing.

Enum match

An enum can also be annotated with @match to generate a match and matchAny extension methods:

//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,
  blue: () => 3,
);
expect(result, 1);

final result = r.matchAny(
  green: () => 1,
  any: () => 2,
);
expect(result, 2);

Dart builtin types match

For Dart builtin types the match extension methods works a bit differently. Here we have a DSL for match cases that allows for advanced value matching. Lets look at an example where we match an integer:

import 'package:match/match.dart';

final x = 10;
final y = true;
final result = x.match({
  gt(100): () => 1,
  eq(10) | eq(20): () => 2,
  range(30, 40): () => 3,
  any: () => 3,
});

expect(result, 2);

Each case is expressed using a tiny DSL that allows for building more complex matching. The DSL consists of functions that matches the value of x and operators that combines the functions for more complicated matching. If a case matches the values of x the corresponding function is run and the result returned. If no cases matches, an exception will be thrown at runtime.

We have the following general combining operators for the DSL:

  • e1 | e2 the "or" operator matches x if either e1 or e2 matches x.
  • e1 & e2 the "and" operator matches x if both e1 and e2 matches x.

Additionally we have the "guard" operators > and >>:

import 'package:match/match.dart';

final x = 10;
final y = true;
final result = x.match({
  eq(10) > !y : () => 1,
  eq(10) > y : () => 2,
  eq(10) >> () => !y : () => 3,
  eq(10) >> () => y : () => 4,
});

expect(result, 2);
  • e1 > bool where the right hand side of > is a boolean expression that needs to return true for x to match the case.
  • e1 >> bool Function() where the right hand side of >> is a boolean function that needs to return true for the x to match the case.

String.match

import 'package:match/match.dart';

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

expect(result, 1);

Supported match functions:

  • eq(s) matches if x == s
  • any matches any values of x

num.match (double/int)

import 'package:match/match.dart';

final x = 10;
final result = x.match({
  gt(100): () => 1,
  eq(10) | eq(20): () => 2,
  range(30, 40): () => 3,
  any: () => 3,
});

expect(result, 2);

Supported match functions:

  • eq(n) matches if x == n
  • lt(n) matches if x < n
  • gt(n) matches if x > n
  • lte(n) matches if x <= n
  • gte(n) matches if x >= n
  • range(from, to) matches if x >= from && x <= to
  • any matches any values of x

Libraries

match
match_extensions