Pattern matching and tagged unions in JavaScript without new syntax.
See the guide on how to use Headpats!
Refer to the documentation for specific information on pattern matching and built-in features.
Setup
const pat = require('headpats');
const { $, $$, _, is, rest } = pat;
Stringly Typed
function doOperation(operation, a, b) {
return pat
.case('add', () => a b)
.case('minus', () => a - b)
.case(_, () => NaN)(operation);
}
doOperation('add', 1, 2)
→ 3
doOperation('minus', 1, 2)
→ -1
doOperation('something', 1, 2)
→ NaN
Safe Traversal
const o = { x: { y: { z: 10 } } };
pat.match({ x: { y: { z: $.z } } }, o)
→ { z: 10 }
pat.match({ x: { y: { what: $.what } } }, o)
→ null
Result Matching
const ok = Symbol('ok');
const err = Symbol('err');
function divide(a, b) {
if (b === 0) {
return [err, null];
}
return [ok, a / b];
}
pat.match([ok, $.num], divide(10, 2));
→ { num: 5 }
Option Type
const Option = pat.union('Option', 'Some', 'None');
const { Some, None } = Option;
const double = pat
.case($$(Some, $.x), ({ x }) => new Some(x * 2))
.case($$(None, _), () => new None());
double(new Some(5))
→ Some { x: 10 }
double(new None())
→ None {}
double(100)
→ Error
Recursive Map
const map = pat
.clause([], _, () => [])
.clause([$.x, [rest, $.xs]], $.f, ({ x, xs, f }) => [f(x)].concat(map(xs, f)));
map([1, 2, 3, 4], x => x * 2)
→ [2, 4, 6, 8]