Query regarding GHC handling of overlapping instances.

[Simon Peyton-Jones]

| 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)) }