Conditional callback argument's type

[waxen solstice]

Hi!
I try to fix an issue where I want the type of the callback's first argument depends on another type.
Here a very simple example.
Do you have an idea?

[high tusk]

oltodo#1801

Playground Link

Preview:ts function foo<T extends boolean>( bool: T, callback: ( value: T extends true ? string : number ) => void ) { if (bool) { callback("123") } else { callback(123) } }

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[waxen solstice]

On the two lines with callback I get the error

Argument of type 'string' is not assignable to parameter of type 'T extends true ? string : number'.

Conditional callback argument's type

[worthy crypt]

I think you have to type cast the argument, I'm not sure if there's a cleaner way to do it

[high tusk]

therealjohan#0

Playground Link

Preview:ts function foo<const T extends boolean>( bool: T, callback: (value: T extends true ? string : number) => void ) { if (bool) { callback("123" as T extends true ? string : number); } else { callback(123 as T extends true ? string : number); } ...

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[abstract night]

you could set it up like this so the parameters form a discriminated union that you can narrow:

[high tusk]

mkantor#0

Playground Link

Preview:```ts
type FooArgs =
| [bool: true, callback: (value: string) => void]
| [bool: false, callback: (value: number) => void]

function foo(...[bool, callback]: FooArgs)
...```

[abstract night]

you could alternatively use overload signatures, but those aren't type-checked as thoroughly as this setup is

[waxen solstice]

Thanks a lot,
using discriminated union works fine!