Generic type level scheme in typescript

Asked Jul 19 '18 at 14:43

Active Jul 19 '18 at 14:43

Viewed 36 times

I want to create a tagged scheme of input and output types for function, so that having a tag passed into a function it would infer the input and output types.

Here is an example:

type Aa = {
    tag: 'a',
    a: 2
}

type Ab = {
   tag: 'b',
   a: 3
}

type TInputMap = {
    'a': Aa,
    'b': Ab
}

type TResultMap = {
    'a': string,
    'b': number
}

type A = Ab | Aa;

const fn = <TAction extends keyof TInputMap>(type: TAction, action: TInputMap[TAction]): TResultMap[TAction] => { 
    if (type === 'a') {
      action.a // expected to be 2, but inferred as 2 | 3
    }
}

Is it at all possible? Maybe using conditional types?

Thanks.

asked Jul 19 '18 at 14:43

Yuriy Yazlovytskyy

Typescript will not currently narrow the type of for a generic type parameter. – Titian Cernicova-Dragomir Jul 19 '18 at 14:55
Thank you for replying. @TitianCernicova-Dragomir And there are no workarounds whatsoever for now? – Yuriy Yazlovytskyy Jul 19 '18 at 15:20
Use type assertions to the typeyou expect. `let aAction = action as Aa` – Titian Cernicova-Dragomir Jul 19 '18 at 15:44
@TitianCernicova-Dragomir got it, thanks. Hopefully this will get fixed at some point. – Yuriy Yazlovytskyy Jul 19 '18 at 16:43

Generic type level scheme in typescript

0 Answers0