Effect-TS/match

Functional pattern matching with the full power of TypeScript

Functional pattern matching with the full power of Typescript.

Getting started

To install from npm:

npm install @effect/match

Once you have installed the library, you can import the necessary types and functions from the @effect/match module.

import * as Match from "@effect/match"

Defining a Matcher

To define a Matcher from a given type, you can use the type constructor function.

You can then use the when, not & tag combinators to specify the patterns to match against.

For example:

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const match = pipe(
  Match.type<{ a: number } | { b: string }>(),
  Match.when({ a: Match.number }, (_) => _.a),
  Match.when({ b: Match.string }, (_) => _.b),
  Match.exhaustive,
)

console.log(match({ a: 0 })) // 0
console.log(match({ b: "hello" })) // "hello"

You can also create a Matcher from a value using the value constructor function.

For example:

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const result = pipe(
  Match.value({ name: "John", age: 30 }),
  Match.when(
    { name: "John" },
    (user) => `${user.name} is ${user.age} years old`,
  ),
  Match.orElse(() => "Oh, not John"),
)

console.log(result) // "John is 30 years old"

Types of patterns

Predicates

Values can be tested against arbitrary functions.

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const match = pipe(
  Match.type<{ age: number }>(),
  Match.when({ age: (age) => age >= 5 }, (user) => `Age: ${user.age}`),
  Match.orElse((user) => `${user.age} is too young`),
)

console.log(match({ age: 5 })) // "Age: 5"
console.log(match({ age: 4 })) // "4 is too young"

not patterns

not lets you match on everything but a specific value.

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const match = pipe(
  Match.type<string | number>(),
  Match.not("hi", (_) => "a"),
  Match.orElse(() => "b"),
)

console.log(match("hello")) // "a"
console.log(match("hi")) // "b"

tag patterns

Matches against the tag in a Discriminated Union

import * as Match from "@effect/match"
import * as E from "@effect/data/Either"
import { pipe } from "@effect/data/Function"

// type Either<L, R> = { _tag: "Right", right: R } | { _tag: "Left", left: L }
const match = pipe(
  Match.type<E.Either<string, number>>(),
  Match.tag("Right", (_) => _.right),
  Match.tag("Left", (_) => _.left),
  Match.exhaustive,
)

console.log(match(E.right(123))) // 123

Evaluating a Matcher

option

A Matcher that might match a value. Returns an Option.

import * as Match from "@effect/match"
import * as E from "@effect/data/Either"
import { pipe } from "@effect/data/Function"

// type Either<L, R> = { _tag: "Right", right: R } | { _tag: "Left", left: L }
// type Option<T> = { _tag: "Some", value: T } | { _tag: "None" }
const result = pipe(
  Match.value(E.right(0)),
  Match.when({ _tag: "Right" }, (_) => _.right),
  Match.option,
)

console.log(result) // { _tag: "Some", value: 0 }

exhaustive

A Matcher that marks the end of the matching process and checks if all possible matches were made. Returns the match (for Match.value) or the evaluation function (for Match.type).

import * as Match from "@effect/match"
import * as E from "@effect/data/Either"
import { pipe } from "@effect/data/Function"

// type Either<L, R> = { _tag: "Right", right: R } | { _tag: "Left", left: L }
const result = pipe(
  Match.value(E.right(0)),
  Match.when({ _tag: "Right" }, (_) => _.right),
  Match.exhaustive, // TypeError! { _tag: "left", left: never } is not assignable to never
)

orElse

A Matcher that marks the end of the matcher and allows to provide a fallback value if no patterns match. Returns the match (for Match.value) or the evaluation function (for Match.type).

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const match = pipe(
  Match.type<string | number>(),
  Match.when("hi", (_) => "hello"),
  Match.orElse(() => "I literally do not understand"),
)

console.log(match("hello")) // "I literally do not understand"
console.log(match("hi")) // "hello"

either

A Matcher that might match a value. Returns an Either in the shape of Either<NoMatchResult, MatchResult>.

import * as Match from "@effect/match"
import { pipe } from "@effect/data/Function"

const match = pipe(
  Match.type<string>(),
  Match.when("hi", (_) => "hello"),
  Match.either,
)

// type Either<L, R> = { _tag: "Right", right: R } | { _tag: "Left", left: L }
console.log(match("hi")) // { _tag: "Right", value: "hello" }
console.log(match("shigidigi")) // { _tag: "Left", value: "shigidigi" }

License

The MIT License (MIT)