matchacho

Pattern matching for JavaScript

Read TC39 proposal.

Supported patterns:

• primitive values
• primitive types
• classes
• regular expressions
• test functions
• arrays of patterns
• objects with any of the above

Syntax

Weird

The match function takes a value as the first argument, followed by an even number of arguments, each pair of which is a pattern and a matching result. Optionally, a default value may be provided as the last argument.

match(value,
  pattern, result,
  pattern, result,
  default)

The result and default arguments may be either a value or a function.

Chaining

Alternately, a more conventional chaining syntax may be used:

match(value)
  .when(pattern, result)
  .when(pattern, result)
  .default(default)

default() must be placed last in the chain, event if no default value is provided.

Chaining syntax is more readable and provides better type inference.

Another option is to create an instance:

const cases = match()
  .when(pattern, result)
  .default(default)

cases.match(value)

Usage

npm i matchacho

import { match } from 'matchacho'

Real-world examples

return match(input,
  String, (item: string) => [item],
  Array, input,
  null, () => [])

Same as above with chaining syntax:

return match(input)
  .when(String, (item) => [item])
  .when(Array, input)
  .when(null, () => [])
  .default()

for (const directive of directives)
  mask |= match<number>(directive,
    'private', PRIVATE,
    'public', PUBLIC,
    'no-cache', NO_CACHE,
    0)

return match(Class,
  Role, () => new Role(value, this.discovery.roles),
  Rule, () => new Rule(value, this.create.bind(this)),
  Incept, () => new Incept(value, this.discovery),
  () => new Class(value))

throw match(error.code,
  'NOT_ACCEPTABLE', () => new UnsupportedMediaType(),
  'TYPE_MISMATCH', () => new BadRequest(),
  error)

return match<Buffer>(type,
  'image/heic', async () => await convert({ buffer, format: 'JPEG' }),
  /image\/(?!jpeg$|png$|gif$)/, async () => await sharp(buffer).jpeg().toBuffer(),
  'image/gif', async () => await sharp(buffer).png().toBuffer(),
  buffer)

Examples approved by Ed.

Reference

return match(value,
  // equals
  1, () => 'One!',

  // instanceof
  Readable, (stream) => stream.read(),
  Error, (error) => throw error,

  // test function
  positive, (number) => Math.sqrt(number),
  (x) => x < 0, (number) => Math.sqrt(-number),

  // regular expression
  /(\d+) \+ (\d+)/, (groups) => groups[0] + groups[1],
  /(?<left>\d+) \+ (?<right>\d+)/, (groups) => groups.left + groups.right,

  // object matching
  { statusCode: 200 }, (response) => response.data,

  // array matching
  [0, 1], (numbers) => `${numbers[0]} + ${numbers[1]}`,

  // nested test function
  { statusCode: (x) => x >= 200 && x < 300 }, (response) => response.data,

  // default
  (value) => value
)

function positive (value) {
  return value > 0
}

See tests.