zkat/pattycake

playground for pattern matching api

pattycake

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!

Install

$ npm install pattycake

Table of Contents

• Example
• API

Example

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)
)

API

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.

Example

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/, () => ...
)