Declaring generic type of implicitly called method

[chpoit]

This is a feature request. I am unaware of any recent similar requests
For typing purposes and simple code, it would be good to be able to declare generics of implicitly called methods.

TypeScript Version: Any under 2.3.x at time of writing

Code Example
Take the following method

myImplicitlyCalledMethod = <T>(someVar: any): T => {
    let b:T;
    //Code that converts someVar to T or create an object with type T from someVar
    return b;
}

Actual behavior:
Right now when you call a method with a generic type implicitly, you cannot declare the type directly.
The following code is what I have to do to get the type, along with an implicit call of the method.

myCallingMethodWithoutTyping = (someVar:any) => this.myImplicitlyCalledMethod;
myCallingMethodWithTyping = (someVar:any) => this.myImplicitlyCalledMethod<number>(someVar);

Expected behavior:
This is what I would like to be able to do :

myCallingMethodWithoutTyping = (someVar:any) => this.myImplicitlyCalledMethod<number>;

The code however throws an exception while compiling as it expects you to pass the parameter when you declare the type of the generic.

Language feature checklist

• Syntactic: Allow declaring of generics in implicitly called methods
• Semantic: Should not change the core logic of typescript, or anything.
• Emit: Emitted code would not change, it would simply be to help with declaring types with less code.
• Compatibility: Code using this wouldn't be backwards compatible.
• Other:
  • Performance: This might affect performance depending on how it's implemented.
  • Tooling: Would actually help with inferred typing

[aluanhaddad]

Definitely interesting. Do you see this extending to other kinds of values?

[chpoit]

I'm unsure what you mean by value, by my main usecase would be to use through Rxjs observables in angular to reduce the amount of what I consider to be useless code, for examble being a simple api call going from :

this.http.get(url, options)
    .map((res)=>this.extractApiResponse<someType>(res))
    .catch(this.handleError);

to :

this.http.get(url, options)
    .map(this.extractApiResponse<someType>)
    .catch(this.handleError);

Which I personally find better, and would allow me to do as much with less code. Granted it's not much code, but when you write a slight variation of this 2-5 times a day it gets tedious.
I guess to some extent it's laziness, but I also find the second example would allow for less "useless" code.

[billba]

The expected behavior I personally am looking for is around functions, not methods, but same idea applies:

const myCallingFunctionWithoutTyping = myImplicitlyCalledFunction<any>;
const myCallingFunctionWithTyping = myImplicitlyCalledFunction<number>;

In my head I sometimes try to replace const with something resembling a type alias, because of course there's no reason we need to create two variables whose whole purpose in life is to narrow the typing of an existing function:

alias myCallingFunctionWithoutTyping = myImplicitlyCalledFunction<any>;
alias myCallingFunctionWithTyping = myImplicitlyCalledFunction<number>;

But in the end I'd be happy with just making them variables, because it solves the problem.

[davetayls]

This seems to have gone quiet for a while. Is there any news as to whether this is planned for being implemented?

Thanks

[RyanCavanaugh]

There is a lot to be worked out here before it'd be actionable. Functions can be overloaded, and generics can have defaults, so it's not entirely clear what f<T> means for all possible definitions of f

[pelotom]

As far as overloading, it seems like f<T> should mean that the set of overloads is filtered to just those for which a single type parameter of type T is acceptable, and then the parameter is removed and all instances of it replaced with T. So an overloaded function becomes a less overloaded, more specialized overloaded function.

[maciejw]

there was an issue about generic namespaces #19728 but it was closed in favor of this one, so I have interesting case I think. namespaces are interesting in context of generics, because namespaces can hold types, like type aliases. but I thing that what I want to achieve could be awkward with namespaces basically I want o have a type "constructor", or template, that is in types realm not values realm.

namespace X<T> {
  type Y = Record<T, string>
  type Z = Record<T, boolean>
}

this also is interesting

type X<T> = {
  type Y = Record<T, string>
  type Z = Record<T, boolean>
}

then I could do

type XX = X<'foo'>

and then later I could use

function f(y: XX.Y, z: XX.Z ){}

what do you think about it?

[RyanCavanaugh]

I think this would reasonably be subsumed by #17574 so doesn't need to exist as a one-off feature that'd be almost the same amount of work for a lot less value

[typescript-bot]

This issue has been marked as "Too Complex" and has seen no recent activity. It has been automatically closed for house-keeping purposes.