Infer properly Error of a Result

[Nainterceptor]

Hello,

I have three functions : Child1, Child2 and Parent.
All of them are returning Result.
In Parent, I want to type the Result with errors coming from Child1 and Child2.
I'm able to write intermediary types like Child1Errors and Child2Errors then combining them in Parent like Result<_, Child1Errors | Child2Errors>.

It isn't possible to infer errors from a function ? I've tried to do type InferError<T> = T extends Result<any, infer E> ? E : never; but it lead to a exhaustive branch malfunction using ts-pattern…

Any idea ?

[zoontek]

@Nainterceptor Could you create an exemple in the Typescript playground? That's not super clear.

[Nainterceptor]

https://stackblitz.com/edit/m21xpf?file=index.ts
-> Here, types are OK

https://stackblitz.com/edit/m21xpf-ket3t3?file=index.ts
-> Here, I've an issue with my type InferError<T> = T extends Result<any, infer E> ? E : never;

I think that it's coming from the never part, but I don't know how to replace that.

error: TS2322 [ERROR]: Type 'Error<null, Child1Error>' is not assignable to type 'Result<null, never>'.
  Type 'Error<null, Child1Error>' is not assignable to type 'Error<null, never>'.
    Type 'Child1Error' is not assignable to type 'never'.
    return result1;
    ~~~~~~
    at file:///Users/·/scratch_6.ts:30:5

TS2322 [ERROR]: Type 'Error<null, Child2Error>' is not assignable to type 'Result<null, never>'.
  Type 'Error<null, Child2Error>' is not assignable to type 'Error<null, never>'.
    Type 'Child2Error' is not assignable to type 'never'.
    return result2;
    ~~~~~~
    at file:///Users/·/scratch_6.ts:35:5

TS2345 [ERROR]: Argument of type '{ readonly tag: "Error"; readonly error: Chainable<GuardP<unknown, Child2Error>>; }' is not assignable to parameter of type 'PatternMatcher<never>'.
  Property '[matcher]' is missing in type '{ readonly tag: "Error"; readonly error: Chainable<GuardP<unknown, Child2Error>>; }' but required in type 'Matcher<never, unknown, any, any, unknown>'.
    Result.P.Error(P.instanceOf(Child2Error)),
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    at file:///Users/·/scratch_6.ts:48:5

    '[matcher]' is declared here.
        [matcher](): MatcherProtocol<input, narrowed, matcherType, selections, excluded>;
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        at file:///Users/·/ts-pattern/5.3.1/dist/types/Pattern.d.ts:24:5

Found 3 errors.

[zoontek]

@Nainterceptor That's normal, because in your example, you have this:

type InferError<T> = T extends Result<any, infer E> ? E : never;

And this:

function Parent(): Result<
  null,
  InferError<typeof Child1> | InferError<typeof Child2>
>

But typeof Child1 and typeof Child2 are not of type Result, but of type () => Result<null, Child1Error> and () => Result<null, Child2Error>.

To fix it, you can ever either use ReturnType:

function Parent(): Result<
  null,
  InferError<ReturnType<typeof Child1> | ReturnType<typeof Child2>>
>

Or change your InferError (and set a constraint on the generic to have nice reporting in case you pass something else):

type InferError<T extends (...args: any[]) => Result<any, any>> = T extends (
  ...args: any[]
) => Result<any, infer E>
  ? E
  : never;

function Parent(): Result<null, InferError<typeof Child1 | typeof Child2>>

Full example: https://stackblitz.com/edit/m21xpf-clasia?file=index.ts

[Nainterceptor]

Oh. Thanks, I've missed that.