headpats

Headpats

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.

Examples

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]