| 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)) }