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)) }
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