Package Exports

headpats

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (headpats) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

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]