Thanks, I think I was just confused... (and other types elsewhere may have been interfearing)... there still seems something not quite right... If I add the following definiton to the test code: instance Test (a -> m b) (m b) where test _ _ = "Third" then I add the following print: print $ test (\_ -> [True]) [True] it says no instance for (t -> [Bool), but if I add a type annotation all is Okay: print $ test ((\_ -> [True]) :: () -> [Bool]) [True] Is this expected behavior? Finally, If I change the definition to: instance Test (a -> m b) z where test _ _ = "Third" it now complains about it overlapping with both of the other definitions... Why does this overlap? Regards, Keean. Simon Peyton-Jones wrote:
| I have been doing some work recently which involves classes with | overlapping instances... for example | | class Test x y where | test :: x -> y | | instance Test (a b) (c b) where | test = | | instance Test (a b) (a b) where | test = | | This gives an overlapping instance error - which cannot be avoided with | -fallow-overlapping-instances. | However - it is fairly obvious that the first case 'a' cannot be unified | with 'c' or it would be a type error, therefore | the cases do not overlap... Is this a bug in ghc, is it easily fixable - | or am I confused?
You are right. They don't overlap. The program below runs fine with GHC 6.0.1, and prints
cam-02-unx:~/tmp$ ghc -fallow-overlapping-instances -fglasgow-exts Foo.hs cam-02-unx:~/tmp$ ./a.out "Second" "First"
Simon
========================= module Main where
class Test x y where test :: x -> y -> String
instance Test (a b) (c b) where test x y = "First"
instance Test (a b) (a b) where test x y = "Second"
main = do { print (test [True] [True]) ; print (test [True] (Just True)) }