headpats
v0.3.0
Published
Pattern matching and tagged unions without extra syntax.
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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]