Query regarding GHC handling of overlapping instances.

Keean Schupke k.schupke at imperial.ac.uk
Fri Sep 12 13:35:48 EDT 2003

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?


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
>| -fallow-overlapping-instances.
>| However - it is fairly obvious that the first case 'a' cannot be
>| 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
>cam-02-unx:~/tmp$ ./a.out
>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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