no type inference for generic function variable assignment ?

[connectdotz]

TypeScript Version: 3.3.1

Search Terms: generic function assignment, generic type infer

Code

interface A<T> {
    foo: T;
}

const a: A<number> = { foo: 13 };

let getA: <T>() => A<T> = () => a; // <== error here

Expected behavior:
no compile error

Actual behavior:
compile error:

Type 'A<number>' is not assignable to type 'A<T>'.
  Type 'number' is not assignable to type 'T'.

Playground Link: here

Related Issues:

#17574
#26871

[jack-williams]

This is the right behaviour. The generic parameter T is rigid inside the definition of the function so it won’t pick up inferences. It’s better to think of T as a fixed, but unknown type; you can’t just assign a number to it.

If you want type inference you should leave type annotation off the declaration of the function.

[connectdotz]

the idea is to let getA() to take any function so long as it's compliant to the generic interface. If I leave out the signature of getA(), it basically became "fixed" after the first assignment:

interface A<T> {
    foo: T;
}

const a: A<number> = { foo: 13 };

//let getA: <T>() => A<T> = () => a; 
let getA = () => a; 

const b: A<string> = { foo: "13" };
getA = () => b; // <== now error here

playground

this is a common pattern that a system assigned a default initial value/logic but later refined with actual implementation that are not known at compile time other than the generic interface they are conformed... maybe there are other ways to accomplish this?

[Nathan-Fenner]

You want an existential type. You've written that getA has a universal (generic) type.

The easiest way would be to make getA: () => A<unknown>, because that's all that it could be used as. If the caller doesn't know what type they'll get out, then unknown is the best you can do (in most circumstances- if the parameter is used contravariantly you'd want never instead, I guess).

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.