instance visibility

kahl at kahl at
Fri Sep 26 21:25:44 EDT 2008

Simon Marlow <marlowsd at> wrote:

 >   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...)


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