Florian Weimer:
I can't figure out why the following code doesn't compile with the October 2n GHC 6.10 beta (-XTypeFamilies -XFlexibleContexts) when the type declaration is not commented out.
It's a bug that the code is accepted *without* the signature, as the signature is ambiguous: http://hackage.haskell.org/trac/ghc/ticket/1897 This is not because the code fails to be type-safe, but because (a) you can't use the function erase_range anyway and (b) that it is accepted without a signature, but not with the signature leads to confusion, as you experienced. BTW, the method 'erase' in your code has the same problem. Manuel
module T where
type family RangeTrait c
class InputRange r where remaining :: r -> Bool advance :: r -> r
class (InputRange (RangeTrait s)) => Sequence s where erase :: RangeTrait s -> IO (RangeTrait s)
-- erase_range :: (Sequence s) => RangeTrait s -> IO (RangeTrait s) erase_range r = if remaining r then do r' <- erase r erase_range r' else return r
GHCi says the type is precisely as specified in the comment. However, when I activate the type declaration, GHC complains:
T.hs:16:22: Couldn't match expected type `RangeTrait s' against inferred type `RangeTrait s2' In the first argument of `erase', namely `r' In a stmt of a 'do' expression: r' <- erase r In the expression: do r' <- erase r erase_range r'
T.hs:17:22: Couldn't match expected type `RangeTrait s1' against inferred type `RangeTrait s2' In the first argument of `erase_range', namely `r'' In the expression: erase_range r' In the expression: do r' <- erase r erase_range r'
Any suggestions? Is this a bug in GHC? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe