Match with in JS DSL

match-with-example.js

import match, { $, $$$ } from 'match-with';


// l is a list

const length = match(l).with(
  // $() is a single-item named placeholder
  // $$$() is a rest-collecting named placeholder
  // .when(predicate) adds a guard predicate
  clause([$('head'), $$$('tail')]).when(({ tail }) => tail.length > 0)).then(({ head, tail }) => 1 + tail.length),
  // $ is a single-item anonymous placeholder
  // $$$ is a rest-collecting anonymous placeholder
  clause([$]).then(() => 1),
  clause([]).then(() => 0), 
);