Flow: Array Class does not check on Interface

[jgrund]

Consider the following code (flow 0.27):

interface Functor<A> {
  map<B>(fn:(a:A) => B):Functor<B>;
}

declare function map<T,R>(fn:(x:T) => R, xs:Functor<T>):Functor<R>;

map(x => x + 1, [1, 2, 3]);

Running this outputs:

[No file]:0
inconsistent use of library definitions
module. *** Recursion limit exceeded ***
module


Found 1 error

Given that Array should structurally implement Functor, I would expect this to pass.

[samwgoldman]

Uh oh! This should never happen—something is causing a loop in the type checker. Will investigate ASAP. Thanks a lot for the report!

[jgrund]

The above can be simplified to:

// @flow

interface Functor<A> {
  map<B>(fn:(a:A) => B):Functor<B>;
}

const a = [1];
(a:Functor<number>);

To get flow to pass, we can do:

// @flow

interface Functor<A> {
  map<B>(fn:(a:A) => B):Array<B>;
}

const a = [1];
(a:Functor<number>);

That seems to imply that it doesn't match well on the return of map being Functor.

[jgrund]

Another way of writing the interface:

// @flow


interface Functor<A> {
  map<B, F:Functor<B>>(fn: (a:A) => B): F
}

const a = [1];
(a:Functor<number>);

This outputs: *** Recursion limit exceeded *** but also outputs the following error:

inconsistent use of library definitions
201:     map<U>(callbackfn: (value: T, index: number, array: Array<T>) => U, thisArg?: any): Array<U>;
                                                                                             ^^^^^^^^ array type. This type is incompatible with. See lib: /private/tmp/flow/flowlib_34da7a76/core.js:201
201:     map<U>(callbackfn: (value: T, index: number, array: Array<T>) => U, thisArg?: any): Array<U>;
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ some incompatible instantiation of `F`. See lib: /private/tmp/flow/flowlib_34da7a76/core.js:201

[johnsonw]

Nice catch @jgrund. Support for functors is something we definitely need in our code base, we use flow heavily in our project.