fp-ts

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🚀 Feature request

Example:

type UserId = '1' | '2' | '3';
declare const users: Record<UserId, User>;
const filteredUsers = pipe(users, record.filter(...));

Current Behavior

Above example produces Record<string, User>

Desired Behavior

Above example produces Record<UserId, User>

Suggested Solution

Add these signatures:

<K extends string, A, B extends A>(refinement: Refinement<A, B>): (fa: Record<K, A>) => Record<K, B>
<K extends string, A>(predicate: Predicate<A>): (fa: Record<K, A>) => Record<K, A>

Who does this impact? Who is this for?

Useful with advanced type usage - limited set of keys, or for example in our case using branded strings for marking entity IDs.

Your environment

Software	Version(s)
fp-ts	2.7.0
TypeScript	4.4.3

If you are filtering (unlike mapping) you can get less entries, returning Record<UserId, User> is not safe

If you are filtering (unlike mapping) you can get less entries, returning Record<UserId, User> is not safe

Yep that is technically true, but Typescript is a compromise between usability and correctness (and so is fp-ts - current signature is not safe too). And in this particular case, I still think my suggestion would improve the correctness - Record<string, something> will seem to contain all possible strings, while with my suggestion it will at least limit to previously known keys.

And from pragmatic point of view, I think many people use Record<K, V> where you should theoretically have Partial<Record<K, V>> just to keep things simple, and current implementation would force them to use type assertions which are even more unsafe or implement other workarounds when they need to filter a record.