Typing Promise returning function with Result

[kee-oth]

Hi!

I'm looking for help with how best to type a function like this:

const getArtist
  = (identifier) => fetchArtist (`/artists/${identifier}`)
      .then ((response: GetArtistResponse) => Result.ok (response.data))
      .catch (() => Result.err (ErrorMessage.ArtistNotFound))

I'd like to do something compact like this but TypeScript doesn't like it:

const getArtist
  : (_:string) =>  Promise<Result<Artist, ErrorMessage>>
  = (identifier) => fetchArtist (`/artists/${identifier}`)
    .then ((response: GetArtistResponse) => Result.ok (response.data))
    .catch (() => Result.err (ErrorMessage.ArtistNotFound))

TypeScript is happy with this though:

const getArtist
  : (_:string) =>  Promise<Ok<Artist> | Err<ErrorMessage>>
  = (identifier) => fetchArtist (`/artists/${identifier}`)
    .then ((response: GetArtistResponse) => Result.ok (response.data))
    .catch (() => Result.err (ErrorMessage.ArtistNotFound))

Is this just the way it has to be?

Thanks!

[chriskrycho]

Hi @kee-oth, thanks for the ask. You will need to provide the type for the initial parameter somehow—this isn’t a True Myth limitation but a TypeScript limitation. Unlike languages like F♯, OCaml, Elm, Haskell, etc., TypeScript does not do Hindley-Milner inference or any other kind of full program inference; any function parameter which is not explicitly typed in some way is implicitly typed as any. (I say “in some way” because callbacks can get their types from the context where they’re used, but the definition of the site they are passed to has to be typed well for that to work.) The best thing you can do is something like your final version or simply providing the type explicitly inline.

A couple other notes:

1. It's idiomatic to type it as Result<T, E> rather than Ok<T> | Err<E>.
2. Assuming fetchArtist returns GetArtistResponse when successful, you *don’*t need the type annotation on that callback.

Net, I would write this like this:

const getArtist = (identifier: string): Promise<Result<Artist, ErrorMessage>> =>
  fetchArtist(`/artists/${identifier}`)
    .then((response) => Result.ok(response.data))
    .catch(() => Result.err(ErrorMessage.ArtistNotFound))

[franklin-huang]

I'm facing the same issue and the suggested solution is not quite working for me tho. Here is the screen shot of my code:

However, with try/catch, the error is gone:

Do I miss anything?

[franklin-huang]

Somehow, if we remove .catch from the promise, the error is gone .

[chriskrycho]

Ah, right. This is a limitation of TypeScript's inference capabilities, unfortunately. If you produce a single expression, it can infer the full type correctly; otherwise, you have to explicitly specify the full type. There are two ways to do this:

1. Write Result.ok<OkType, ErrType>(theOkValue) and Result.err<OkType, ErrType>(theErrValue) when creating a Result.
2. Make it be a single expression, as in this playground, which has the two-argument .then() callback instead of separate .then() and .catch() callbacks.

The reason removing the .catch() works is that it drops into (2).

[franklin-huang]

Thanks @chriskrycho
We end up using the first approach. That would work.