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
- MatchW Option
- MatchW Either
- MatchW Default

---

- License

Installation

yarn

``sh
yarn add pattern-matching-ts
`

npm

`sh
npm install --save pattern-matching-ts
`

Usage

_MatchW_

> ### Option MatchW

`ts
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

`ts
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) =>
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

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

interface ServerResponse {
readonly code: Code
}

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

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

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

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

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

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

> ### Default Match

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

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

type Cases = ChangeColor | Move | Write
const matchMessage = M.match({
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['value']): ChangeColor => ({
_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')
``

Here's a blog post that introduces the API. 👉
Pattern Matching in Typescript

MIT