I was trying to create a typeclass for an abstract Stack class, and ran
into some problems. The following 'works' fine:
{-# OPTIONS_GHC -XEmptyDataDecls -XFlexibleContexts
-fno-monomorphism-restriction #-}
module Stack where
data Void
class Stack s where
push_ :: s a r -> b -> s b (s a r)
empty :: s () Void
top :: s a (s b r) -> (a, s b r)
first :: s a r -> a
instance Stack (,) where
push_ s a = (a,s)
empty = ((),undefined::Void)
top = id
first = fst
p = flip push_
test0 = top . p 2 . p 3 $ empty
-- But the following doesn't - I get an "Ambiguous type variable `s' in
the contraint `Stack s' arising from the use of `first':
test1 = first . p 2 . p 3 $ empty
-- sure, that makes sense, it somehow needs to know what flavour of
Stack to use even though (or perhaps because) the answer is independent
of it.
-- So I listen to the "probable fix" and add a type signature:
test1 :: Stack (,) => Integer
-- This does not however help at all! The only way I have found of
'fixing' this requires annotating the code itself, which I most
definitely do not want to do because I specifically want the code to be
polymorphic in that way. But GHC 6.8.2 does not want to let me do this.
What are my options?
Jacques
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