I've used a similar approach for a while, for instance in
http://comonad.com/haskell/type-int/src/Data/Type/Boolean.hs
http://comonad.com/haskell/type-int/src/Data/Type/Boolean.hs
But I think your approach is cleaner than mine, because it doesn't need my
seemingly superfluous closure term or fundep.
-Edward Kmett
On Fri, Jul 17, 2009 at 11:38 AM, Conor McBride
Friends
Is closing a class this easy?
--------------------------------------
module Moo ( Public(..) ) where
class Private x => Public x where blah :: ...
class Private x where
instance Private A where instance Public A where blah = ...
instance Private B where instance Public B where blah = ...
--------------------------------------
Modules importing Moo get Public and its instances, but cannot add new ones: any such instances must be accompanied by Private instances, and Private is out of scope.
Does this work? If not, why not? If so, is this well known?
It seems to be just what I need for a job I have in mind. I want a class with nothing but hypothetical instances. It seems like I could write
--------------------------------------
module Noo ( Public(..) , public ) where
class Private x => Public x where blah :: ... blah = ...
class Private x where
public :: (forall x. Public x => x -> y) -> y public f = f Pike
data Pike = Pike instance Private Pike instance Public Pike
--------------------------------------
But if I don't tell 'em Pike, I've ensured that blah can only be used in the argument to public.
Or is there a hole?
Cures youriously
Conor
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