pattycake is a little playground being used to prototype concepts surrounding the TC39 pattern matching proposal. It's not a spec, it's not a standard, and it doesn't represent the actual look and feel of the JS feature. But it'll help figure out what that could actually be!

$ npm install pattycake

• Example
• API

import match, {$} from 'pattycake'

const res = await fetch(jsonService)
const val = match (res) (
  {
    status: 200,
    headers: {'Content-Length': $}
  }, ({
    headers: {'Content-Length', s}}
  ) => `size is ${s}`,
  {status: 404}, () => 'JSON not found',
  $({status: $}, ({status}) => status >= 400), () => throw new RequestError(res)
)

This documentation described the sugared version of the match expression. The API exported by pattycake is similar, but uses functions and different syntax for the same underlying concepts.

To convert a sugary match to a pattycake match:

1. Replace the main {} pair with ()
2. Separate match clauses and bodies into matcher expressions and a fat arrow function, using the parameter list for the fat arrow for destructuring.
3. Replace any variable clauses in the match side with match.$.
4. If using guards, convert the guard to a function and pass it as the last argument to match.$. If you weren't already using match.$ for a certain clause (because it wasn't necessary), wrap that clause with match.$ and pass the guard function as the second argument.
5. If using ...rests with array or object matchers, replace the ...rest with $.rest and destructure the array in the fat arrow body.

match (x) {
  {a: 1, b} => ...,
  [1, 2, ...etc] => ...,
  1 => ...,
  'string' => ...,
  true => ...,
  null => ...,
  /regexhere/ => ...
}

// Converts to...
const $ = match.$
match (x) (
  {a: 1, b: $}, ({b}) => ...,
  [1, 2, $.rest], ([a, b, ...etc]) => ...,
  1, () => ...,
  'string', () => ...,
  true, () => ...,
  null, () => ...,
  /regexhere/, () => ...
)