What is it for? Yes, you would know that only A and B are Public, but you have no way of telling that to the compiler. I usually prefer something like that: class Public x where blah :: ... isAB :: forall y. (A -> y) -> (B -> y) -> x -> y Both solutions, however, allow the user to declare some new instances when GeneralizedNewtypeDeriving is enabled. On 17 Jul 2009, at 19:38, Conor McBride wrote:
Friends
Is closing a class this easy?
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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 = ...
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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
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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
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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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