awaited operator doesn't work with type guards #37512

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1. Ryan Cavanaugh (RyanCavanaugh)

   added

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   SuggestionAn idea for TypeScriptAn idea for TypeScript

   on Mar 22, 2020
2. falsandtru commented on Mar 23, 2020

   

   falsandtru (falsandtru)

   on Mar 23, 2020

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   Probably we have to replace isPromiseLike with isAwaited.

             if (isPromiseLike(value)) {
               internal.status = {
                 state: State.resolved,
                 result: value,
               };
               value.then(
                 value => {
                   assert(internal.status.state === State.resolved);
                   internal.status = {
                     state: State.fulfilled,
                     result: value,
                   };
                   resume(internal);
                 },
                 reason => {
                   assert(internal.status.state === State.resolved);
                   internal.status = {
                     state: State.rejected,
                     result: reason,
                   };
                   resume(internal);
                 });
             }
             else {
               internal.status = {
                 state: State.fulfilled,
                 result: value!, // error
               };
               resume(internal);
             }

   Type 'NonNullable<T>' is not assignable to type 'awaited T'.
     Type 'T' is not assignable to type 'awaited T'.ts(2322)
   

   https://github.com/falsandtru/spica/blob/v0.0.343/src/promise.ts
3. rbuckton commented on Mar 24, 2020

   

   Ron Buckton (rbuckton)

   on Mar 24, 2020

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   The problem is that awaited unknown is just reduced to unknown, so you are asserting that value is unknown. The awaited type does work in type guards:

   

   Playground link: https://www.typescriptlang.org/play/index.html?ts=3.9.0-dev.20200322#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXywGcBBAdyiwxGAB4AVAPgAoA3KCZEALnjvgB94UcpWq8AlDzYcERISKrBeAbgBQKdNjxJ6zAB48+g4RUUT4Ab1XwbBRPCZEyp6kz3jxl67Z961PgF9VAKA
4. rbuckton commented on Mar 24, 2020

   

   Ron Buckton (rbuckton)

   on Mar 24, 2020

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   Let me know if that addresses your concerns so that I can close this issue.
5. falsandtru commented on Mar 24, 2020

   

   falsandtru (falsandtru)

   on Mar 24, 2020

   Last edited by falsandtru

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   Basically type arguments are not used in type guards. Can you remove type arguments to accept all types? Actually, your function doesn't accept T | awaited T | PromiseLike<T>.

   Ideally, since T can be considered as thenable when T is not awaited T, isAwaited function should work as follows. Here isAwaited function proves or ensures that T is just a union of awaited T and PromiseLike<T>.

   declare function isAwaited(value: any): value is awaited any; // or awaited typeof value
   function f<T>(x: T | awaited T | PromiseLike<T>) {
       if (isAwaited(x)) {
           x; // awaited T
       }
       else {
           x; // PromiseLike<T>
       }
   }
6. nmain commented on Mar 24, 2020

   

   Nicholai Main (nmain)

   on Mar 24, 2020

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   Just to make sure I'm understanding this right, would this be a reasonable implementation of isAwaited? No matter what T is, awaited T is never PromiseLike ("thennable"), so value is awaited unknown just means that value is not PromiseLike.

   function isAwaited(value: any): value is awaited unknown {
       return !value || typeof value.then !== "function";
   }
7. rbuckton commented on Mar 24, 2020

   

   Ron Buckton (rbuckton)

   on Mar 24, 2020

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   awaited is only preserved when the type argument is generic, so awaited unknown is just unknown. Similar to awaited any is just any. It's only preserved at higher order.
8. Ron Buckton (rbuckton)

   closed this as completedin #37610on Mar 26, 2020