On Wed, Apr 18, 2007 at 05:10:27PM +0100, Neil Mitchell wrote:
> I have now reduced this bug to a smallish test case:
>
> test :: T1 -> T1
> test = test2
>
> test2 :: C3 (T2 a) a => a -> a
> test2 = undefined
>
> data T1 = C1 deriving Show
> data T2 a = T2 a deriving Show
>
> class C1 a where
> class C2 a where
> class C3 a b where
>
> instance C1 T1 where
> instance C1 a => C1 (T2 a) where
> instance (C1 a, C2 a) => C3 T1 a where
> instance (C3 a (T2 a)) => C2 (T2 a) where
> instance (C2 (T2 a)) => C3 (T2 a) b where
>
> In Hugs I get (overlapping instances and unsafe overlapping instances
> turned on):
>
> *** The type checker has reached the cutoff limit while trying to
> *** determine whether:
> *** C1 T1
> *** can be deduced from:
> *** ()
>
> The explicit instance for this type makes it surprising that it fails
> to find an instance.
The error message is silly, but there really is a loop here:
C3 (T2 T1) T1 <= C2 (T2 T1) <= C3 T1 (T2 T1) <= C2 (T2 T1)
> In GHC with -fglasgow-exts -fallow-undecidable-instances, it works fine.
That's because recent GHCs detect the loop when they get to an identical
constraint, and declare the constraint solved. Simon calls this feature
recursive dictionaries; Martin Sulzmann calls it co-induction. I can't
find where it's documented in the GHC User's Guide, though.
Hugs does not aim to duplicate that behaviour; the bug here is the error
message. Here's a simpler illustration of it:
class C a where
f :: a -> a
instance (Eq a, C a) => C a
test = f True
with -98 says it can't prove Eq Bool, when it should say C Bool.
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