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Pattern matching in Typescript.

Pattern Matching is a declarative much more powerful and less verbose alternative to imperatives "if/else" conditions.
A definition can be found inside Scala Documentation

“Pattern matching tests whether a given value (or sequence of values) has the shape defined by a pattern, and, if it does, binds the variables in the pattern to the corresponding components of the value (or sequence of values).”

In Functional Programming languages, there're built-in keywords for Pattern Matching. Typescript though is one language that works very well with Functional Programming but lacks this feature.
This package aims to bring Pattern Matching feature to Typescript through Discriminated Union Types / Algebraic Data Types.

Index

Installation
Usage

Match
- Match Option
- Match Default
MatchW

License

Installation

yarn

yarn add pattern-matching-ts

npm

npm install --save pattern-matching-ts

Usage

MatchW

Option MatchW

import * as M from 'pattern-matching-ts/lib/match'
import { pipe } from 'fp-ts/lib/function'
import * as O from 'fp-ts/lib/Option'

const optionMatching = (o: unknown) =>
  pipe(
    o,
    M.matchW('_tag')({
      Some: ({ value }) => 'Something: ' + value,
      None: () => 'Nothing',
      _: () => 'Default'
    })
  )

assert.deepStrictEqual(optionMatching(O.some('data')), 'Something: data')
assert.deepStrictEqual(optionMatching(O.none), 'Nothing')
assert.deepStrictEqual(optionMatching((undefined as unknown) as O.None), 'Default')

Either MatchW

import * as M from 'pattern-matching-ts/lib/match'
import { pipe } from 'fp-ts/lib/function'
import * as E from 'fp-ts/lib/Either'

type RGB = Record<'r' | 'g' | 'b', number>
const either = (maybeRgb: E.Either<string, RGB>) =>
  pipe(
    maybeRgb,
    M.matchW('_tag')({
      Left: ({ left }) => 'Error: ' + left,
      Right: ({ right: { r, g, b } }) => `Red: ${r} | Green: ${g} | Blue: ${b}`
    })
  )

assert.deepStrictEqual(either(E.right({ r: 255, g: 255, b: 0 })), 'Red: 255 | Green: 255 | Blue: 0')

Default MatchW

import * as M from 'pattern-matching-ts/lib/match'
import { pipe } from 'fp-ts/lib/function'

interface ServerResponse<Code extends string | number> {
  readonly code: Code
}

interface Response<Body> {
  readonly response: {
    readonly body: Body
  }
}

interface Success extends ServerResponse<200>, Response<ReadonlyArray<string>> {}

interface NotFoundError extends ServerResponse<404> {}

interface ServerError extends ServerResponse<500> {
  readonly detail: string
}

type Responses = Success | NotFoundError | ServerError

const matchResponse = (response: Responses) =>
  pipe(
    response,
    M.matchW('code')({
      500: ({ detail }) => ({ message: 'Internal server error', detail }),
      404: () => ({ message: 'The page cannot be found!' }),
      200: ({ response }) => response.body,
      _: () => 'Unexpected response'
    })
  )

assert.deepStrictEqual(either(E.right({ r: 255, g: 255, b: 0 })), 'Red: 255 | Green: 255 | Blue: 0')
assert.deepStrictEqual(matchResponse({ code: 200, response: { body: ['data'] } }), ['data'])
assert.deepStrictEqual(matchResponse({ code: 500, detail: 'Cannot connect to the database' }), {
  message: 'Internal server error',
  detail: 'Cannot connect to the database'
})
assert.deepStrictEqual(matchResponse({ code: 404 }), { message: 'The page cannot be found!' })

Match

Option Match

import * as M from 'pattern-matching-ts/lib/match'
import * as O from 'fp-ts/lib/Option'

const optionMatching = M.match<O.Option<string>, string>({
  Some: (x) => `Something: ${x.value}`,
  None: () => 'Nothing'
})

assert.deepStrictEqual(optionMatching(O.some('data')), 'Something: data')
assert.deepStrictEqual(optionMatching(O.none), 'Nothing')

Default Match

import * as M from 'pattern-matching-ts/lib/match'

interface ChangeColor<T = number> {
  readonly _tag: 'ChangeColor'
  readonly value: {
    readonly r: T
    readonly g: T
    readonly b: T
  }
}
interface Move<T = number> {
  readonly _tag: 'Move'
  readonly value: {
    readonly x: T
    readonly y: T
  }
}
interface Write {
  readonly _tag: 'Write'
  readonly value: {
    readonly text: string
  }
}

type Cases = ChangeColor<number> | Move | Write
const matchMessage = M.match<Cases, string>({
  ChangeColor: ({ value: { r, g, b } }) => `Change the color to Red: ${r} | Green: ${g} | Blue: ${b}`,
  Move: ({ value: { x, y } }) => `Move in the x direction: ${x} and in the y direction: ${y}`,
  Write: ({ value: { text } }) => `Text message: ${text}`,
  _: () => 'Default message'
})

const ChangeColor = ({ r, g, b }: ChangeColor<number>['value']): ChangeColor<number> => ({
  _tag: 'ChangeColor',
  value: { r, g, b }
})

const Move = ({ x, y }: Move['value']): Move => ({
  _tag: 'Move',
  value: { x, y }
})

const Write = ({ text }: Write['value']): Write => ({
  _tag: 'Write',
  value: { text }
})

assert.deepStrictEqual(
  matchMessage(Move({ x: 500, y: 100 })),
  'Move in the x direction: 500 and in the y direction: 100'
)

assert.deepStrictEqual(
  matchMessage(ChangeColor({ r: 12, g: 20, b: 30 })),
  'Change the color to Red: 12 | Green: 20 | Blue: 30'
)

assert.deepStrictEqual(matchMessage(Write({ text: 'my message' })), 'Text message: my message')

Blog posts that introduces the API.

👉 Pattern Matching in Typescript

👉 Pipeable Pattern Matching in Typescript

MIT

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