ts-pattycake

Zero-runtime pattern matching for ts-pattern.

You get to have your cake (pattern matching), and eat it too.

About

ts-pattern is a great library that brings the ergonomics of pattern matching from languages like Rust and OCaml to Typescript, but at the cost of being orders of magnitude slower.

patsy compiles ts-pattern's match() expressions into an optimized chain of if statements to completely eliminate that cost. In our initial benchmarks, it outperforms ts-pattern by 24-30x.

In essence, patsy converts a ts-pattern match() expression like this:

let html = match(result)
  .with(
    { type: 'error', error: { foo: [1, 2] }, nice: '' },
    () => '<p>Oups! An error occured</p>',
  )
  .with({ type: 'ok', data: { type: 'text' } }, function (data) {
    return '<p>420</p>';
  })
  .with(
    { type: 'ok', data: { type: 'img', src: 'hi' } },
    (src) => `<img src=${src} />`,
  )
  .otherwise(() => 'idk bro');

Into this:

let html;
out: {
  if (
    result.type === 'error' &&
    Array.isArray(result.error.foo) &&
    result.error.foo.length >= 2 &&
    result.error.foo[0] === 1 &&
    result.error.foo[1] === 2
  ) {
    html = '<p>Oups! An error occured</p>';
    break out;
  }
  if (result.type === 'ok' && result.data.type === 'text') {
    let data = result;
    html = '<p>420</p>';
    break out;
  }
  if (
    result.type === 'ok' &&
    result.data.type === 'img' &&
    result.data.src === 'hi'
  ) {
    let src = result;
    html = `<img src=${src} />`;
    break out;
  }
  html = 'idk bro';
  break out;
}

Feature parity with ts-pattern

• [x] Literal patterns
  • [x] string
  • [x] number
  • [x] booleans
  • [x] bigint
  • [x] undefined
  • [x] null
  • [x] NaN
• [x] Object patterns
• [x] Array/tuples patterns
• [ ] .when()
• [ ] Wildcards patterns
  • [ ] P._
  • [ ] P.string
  • [ ] P.number
• [ ] Special matcher functions
  • [ ] P.not
  • [ ] P.when
  • [ ] P.select

Notes

Fallback / compatibility with ts-pattern

If patsy is unable to optimize a match() expression, it will fallback to using ts-pattern. This is enabled right now because we don't support the full feature set of ts-pattern.

Inlining handlers

One performance problem of ts-pattern's are handler functions:

match(foo)
  .with({ foo: 'bar', () => /* this is a handler function */)
  .with({ foo: 'baz', () => /* another one */)

Function calls usually have an overhead, and a lot of the time these handlers are small little functions (e.g. (result) => result + 1) which can be much faster if just directly inlined in the code.

Additionally, a match() with many branches means creating a lot of function objects in the runtime.

The JIT-compiler and optimizer in JS engines can do inlining of functions, but in general with JIT you need to run your code several times or it to determine what to optimize.

So when possible, patsy will try to inline function expression (anonymous functions / arrow functions) handlers directly into the code if it is small.

IIFEs

When possible, patsy will try to generate a block of code (like in the example above). But there are times where this is not possible without breaking the semantics of source code.

Roadmap

• Support full feature set of ts-pattern
• Further optimizations