Proposal: refactor Arrow class

[Ross Paterson]

On Wed, Jan 09, 2013 at 07:57:02AM +0000, Edward Kmett wrote:
> It lets you lift core `cast`'s out over the functorial argument,
> which isn't something I can do from outside of the class. If I tried
> to write something where the end user hands me an arbitrary Functor (or
> Profunctor) and I unsafeCoerce to cast, this would expose unsafeCoerce
> to the end user. 
>
> The implementation trick is to place these extra methods in the class
> but hidden in an explicitly Unsafe module and with default definitions
> that are correct but slow.
>
> Then the provider of the functor-like class can explicitly import that
> module, and implement the methods, and mark his module Trustworthy. He
> hasn't exposed unsafeCoerce to the end user, they have to import an
> explicitly Unsafe module to get access to it, incurring the obligation
> themselves to provide something that is operationally id or a cast.
>
> This enables you to have the efficient implementation but guarded by an
> explicitly Unsafe module so the end user has to import that to get the
> efficient functionality, but you can discharge your obligations locally.
>
> Similarly you can discharge the obligation about the representation
> of the operation you are passing at the use site. This means that you
> can reason about these separately.

You could achieve the same effects by splitting off (.#) and (#.) into
a subclass, couldn't you?  It would just mean more instance declarations.