Bound polymorphism misbehaves with external type annotations

[Gozala]

Here is an example code thad should but does not produce errors:

/* @flow */

export type Model = {value:number}
export type F <model:Model> = (state:model) => model

export const f:F = (model) => model

f({})

And equivalent with inline type annotations that does error as expected:

/* @flow */

export type Model = {value:number}

export const f = <model:Model> (state:model):model => state

f({})

[samwgoldman]

The type F is parameterized, but when you used it to annotate the type of f, you didn't supply type parameters. In this case, Flow acts as if you had written F<any>. This is why you don't see an error.

Really, these two code snippets are not equivalent. To be equivalent to the latter snippet, you should define F like so: type F = <model:Model>(state:model) => model.

[Gozala]

@samwgoldman thanks for explanation.

The type F is parameterized, but when you used it to annotate the type of f, you didn't supply type parameters. In this case, Flow acts as if you had written F. This is why you don't see an error.

That behavior seems pretty odd to me because not providing type annotations drops constraint that was defined on the F type which makes me wonder how that is useful ?

In fact I do think that having both type f <x> = () => x and type f = <x> () => x is just going to cause confusion especially if they will have different meanings.