On Saturday 03 July 2010 2:11:37 pm David Menendez wrote:
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
class C a b c | a -> b, a -> c where op :: a -> b -> c
instance C Bool a a where op _ = id
main = putStrLn $ op True "done"
In this case, I've (arbitrarily) chosen the Bool instance to be a no-op and pass through the types. Because the dependent types are part of the declaration header I can use type variables for them.
That's really weird. In particular, I can add this line to your code without problems:
foo = putStrLn $ if op True True then "done" else "."
but GHC complains about this one:
bar = putStrLn $ if op True True then op True "done" else "."
fd.hs:14:0: Couldn't match expected type `Bool' against inferred type `[Char]' When using functional dependencies to combine C Bool [Char] String, arising from a use of `op' at fd.hs:14:38-51 C Bool Bool Bool, arising from a use of `op' at fd.hs:14:20-31 When generalising the type(s) for `bar'
On the other hand, this is fine, but only with a type signature:
baz :: a -> a baz = op True
I don't think this is an intended feature of functional dependencies.
Indeed. That instance declaration doesn't really make sense, and should probably be rejected. The functional dependencies on C say that b and c are dependent on a, so for any particular a, there should be exactly one b and one c such that C a b c is an instance. Then the instance declares infinitely many instances C Bool a a. This is a violation of the fundep. Based on your error message, it looks like it ends up treating the instance as the first concrete 'a' it comes across, but who knows? -- Dan