Array element matching bug on empty arrays

[oguimbal]

Hi !

Describe the bug

import { match, __, not } from 'ts-pattern';

console.log(match([]) // Logs 👉 first
  .with([__], () => 'first')
  .otherwise(() => 'default'));

console.log(match([]) // Logs 👉 first
  .with([not(__.nullish)], () => 'first')
  .otherwise(() => 'default'));

Even if the first is a bit ambiguous, I would at least expect the second one NOT to print "first"...

Code Sandbox with a minimal reproduction case

https://codesandbox.io/s/icy-glade-3ty9l?file=/src/index.ts

Versions

• TypeScript version: 4.3.4
• ts-pattern version: 3.2.5
• environment: any

[gvergnaud]

I agree this behavior is confusing, but it's expected. [subpattern] patterns are list patterns, matching any list in which all elements match the subpattern, including empty lists. If you want to match empty lists separately a workaround is to add a .with([], handler) clause before your [__] pattern.

In retrospect, I don't think this was a great API choice because it can be confused with an unary tuple pattern (a pattern matching a list with a single element). I've been thinking about introducing a separate list(subpattern) helper that would look like this:

import { match, list, __ } from 'ts-pattern';

match(someList)
   .with([], () => 'empty list')
   .with([__], () => 'list with a single element')
   .with(list(not(null)), () => 'list with n elements')
   .exhaustive();

But this would be a breaking change, which makes it a bit tricky to release.

[oguimbal]

It would be tricky indeed 🤔

That said, it would be more intuitive, IMO.
And it would match Haskells behaviour.

Maybe for ts-pattern v4 🥳 ?

[gvergnaud]

Closing since TS-Pattern v4 has been released and fixes this problem. Release note