On Fri, Jul 17, 2009 at 11:38 AM, Conor McBride
<conor@strictlypositive.org> wrote:
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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