instance visibility

[Duncan Coutts]

On Fri, 2008-09-26 at 21:51 +0100, Simon Marlow wrote:

> You will argue that they have control over what they import, and can 
> thereby control whether they export the instance.  This is certainly 
> true, but it ignores the need for module abstraction: the need to be 
> able to change the implementation of a module without changing its API. 
>   We must have the property that the imports of a module do not affect 
> its API - and the only way to have this property is to avoid orphan 
> instances in library APIs.
> 
> (hmm, I finally feel like I've explained this clearly.  I hope it comes 
> across that way.)

Yes, I think that's a clear explanation and one I think is quite
convincing.

> There are basically only two sensible choices for the Functor instance 
> for (->):
> 
>   (a) don't define one at all
>   (b) define one in Control.Monad, and give up on Haskell 98 compliance
> 
> The current situation, namely
> 
>   (c) define it as an orphan, and give up on module abstraction
> 
> is not a sensible choice.

Right.

No orphan instances in libraries.

Duncan