Simon Marlow
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.
To be more precise, I would say that each orphan instance in a library API must be the only export of a dedicated visible module, and only exported by that module. (The only importers of orphan instances are typically Main modules, or ``almost-Main'' modules.)
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.
I would say that there should the a separate module Data.Function.Functor () exporting only that instance. (And I find it a surprising decision of Haskell98 that something that is so obviously a functor in so obviously only one way must not be a Functor instance...) Wolfram