Changing ap arguments order for consistency with map and chain, and for better types support

[rpominov]

The current order of arguments (if we count this as an argument) in ap is problematic. I see two problems about it.

It's inconsistent with map and chain

map :: (this: A, f: A => B) => MyType<B>
chain :: (this: A, f: A => MyType<B>) => MyType<B>
ap :: (this: MyType<A => B>, x: MyType<A>) => MyType<B>

// sorry about made up types lang, I hope it's clear what I mean here

In map and chain value goes first and function goes second, in ap the other way around.

It's impossible to type with Flow and probably other type systems

... or at least hard, and I didn't figured out how to do it.

// @flow

// With current arguments order we have to use `any`, and therefore loose type info
class MyType<A> {
  ap(other: MyType<any>): MyType<any>
}

// With reversed order it's straightforward
class MyType<A> {
  ap<B>(other: MyType<(x: A) => B>): MyType<B>
}

I understand this is a braking change that will probably break a lot of code. But I just want to open this discussion, and hear what others think. Maybe this is a pain point for many people, and it would be reasonable to make the change.

[SimonRichardson]

It's been mentioned before, I'll dig out the references, but I'm in favour as well.

[SimonRichardson]

#50 (comment)

[rpominov]

Ah, right. #50 is specifically about this problem. I remember it was mentioned before, but thought that it was mentioned briefly in an unrelated issue. I guess I created a duplicate then, sorry. I'll close this one.

[SimonRichardson]

Let's do it, open a PR.

[rpominov]

OK, I'll open one tomorrow.