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
Jacques Carette wrote:
-- 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?
If my guess is correct (sorry if it's not), you want the code to be polymorhic so that you don't have to write the shape of the stack twice. Then the way out is to annotate the type of 'empty': test1 = first . p 2 . p 3 $ (empty :: ((), Void))
If you want to defer the choice of 's' you've to make it appear in the type
signature of test1, so you've to introduce an artificial parameter even if
we're interested only in its type. e.g.:
data Proxy (s :: * -> * -> *) -- useful because we can't have an argument
of type 's' directly, since it's higher-kinded,
-- and to document that we're using a
phantom argument
proxy :: Proxy s
proxy = undefined
test1 :: Stack s => Proxy s -> Integer
test1 pr = first . p 2 . p 3 $ empty `asTypeOf` toStack pr
where toStack :: Proxy s -> s a b
testTuple = test1 (proxy :: Proxy (,))
enabling LANGUAGE ScopedTypeVars you can rewrite test1 in a more direct
fashion:
test1 :: forall s. Stack s => Proxy s -> Integer
test1 _ = fist . p 2 . p 3 $ (empty :: s () Void)
On Mon, Nov 24, 2008 at 5:09 AM, Jacques Carette
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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
participants (3)
-
Andrea Vezzosi -
Gleb Alexeyev -
Jacques Carette