.with and Pattern.not error since 4.1.3

[adrienharnay]

Describe the bug

Since 4.1.3, when using Pattern.not, the following branches will act as if Pattern.not was in fact Pattern.any. If narrowing in following branches cannot be done, then I would expect the contrary (no narrowing on Pattern.not usages).

Code Sandbox with a minimal reproduction case
https://codesandbox.io/s/typescript-playground-export-forked-48dyon?file=/index.ts

Versions

• TypeScript version: 4.9.4
• ts-pattern version: 4.1.3
• environment: Node 18.11.0

Thanks in advance!

[gvergnaud]

Thanks definitely a bug, I'll take a look shortly

[gvergnaud]

I had a look and it turns out it's incredibly difficult to solve without breaking exhaustive checking for not patterns. It would be much simpler if TypeScript had Negated Types but it unfortunately doesn't.

In the meantime, the best workaround is to use P.not after other cases:

import { match, Pattern } from "ts-pattern";

type Input = { type: "video"; seconds: number };

let input = ({ type: "user", name: "Gabriel" } as unknown) as Input;

const output = match(input)
  .with({ type: "video", seconds: 10 }, () => "video of 10 seconds.")
  // Swapped these 👆👇
  .with(
    { type: "video", seconds: Pattern.not(10) },
    () => "not video of 10 seconds."
  )
  .otherwise(() => "something else");

[adrienharnay]

Ah... That's a shame. The replication is a very simple example of my real code, which looks like this:

match(object)
.with({ __typename: 'Something', conflicts: Pattern.not([]) }, () => 0)
.with({ __typename: 'Something', updated_at: Pattern.string }, () => 1)
...

Any idea how to replace Pattern.not([])?

[gvergnaud]

Yes, you could use P.when(xs => xs.length > 0)

[adrienharnay]

Ah yes, of course! Thank you. Maybe it would make sense to update the docs for Pattern.not to mention this breaks narrowing in following branches?

[gvergnaud]

Hey, I found a way to fix this issue in the end. It's not perfect but it solve your use case, see details in the release note: https://github.com/gvergnaud/ts-pattern/releases/tag/v4.1.4

[jasikpark]

thx for fixing this, sucks that negated types can't be used but it's cool to see this library develop -- I hope for the day we get rust-esque exhaustive match { } value blocks in TS / JS