Flow: Type definition of a "callable object"

[gabro]

Hi, I'm writing a definition file for a library. The libraries defines a sort of DSL like:

const get = () => 42;
const obj = () => ({ get: get });
obj.get = get;

which allows the following syntax

obj().get() // 42
obj.get() // 42

Now, I'm trying to model this with flow using a union type

type Resource = {
  get: () => number;
};

declare module 'foo' {

  declare class Foo {
    f: Resource | () => Resource;
  }

  declare var exports: Class<Foo>;
}

But clearly it doesn't typecheck

// @flow

import Foo from 'foo';

const f = new Foo().f;
const x: number = f().get();
const y: number = f.get();

foo.js:6
  6: const x: number = f().get();
                       ^^^ function call. Callable signature not found in
  6: const x: number = f().get();
                       ^ Resource

foo.js:6
  6: const x: number = f().get();
                           ^^^ property `get`. Property not found in
  6: const x: number = f().get();
                       ^^^ Resource

foo.js:7
  7: const y: string = f.get();
                         ^^^ property `get`. Property not found in
  7: const y: string = f.get();
                       ^ Resource

So the question is: how can I express the type of a "callable object"? It's essentially a function with some well-specified properties on it.

[mroch]

there are some docs here, but I don't think they're very clear: https://flowtype.org/docs/quick-reference.html#callable-objects

here's a potentially better example:

type Foo = {
  (): Foo,
  get: () => number
}

function bar(x: Foo) {
  (x().get(): number); // ok
  (x.get(): number); // ok
  (x.foo(): number); // error
}

[gabro]

Face meets palm...

I totally missed that documentation page, thank you @mroch!

[lohithac]

https://flowtype.org/docs/quick-reference.html#callable-objects now redirects to a place which is irrelevant. Can someone point out the current correct location for that link please?

[saadq]

It looks like the new version of the docs doesn't have callable objects mentioned anywhere.

[kevinSuttle]

https://flow.org/en/docs/types/functions/#toc-callable-objects