On 24 November 2011 16:46, José Pedro Magalhães wrote:

Hi Bas,

On Thu, Nov 24, 2011 at 09:23, Bas van Dijk wrote:

...

Hello,

Now that we have DefaultSignatures, why is it not allowed to have multiple default method implementations, as in:

{-# LANGUAGE DefaultSignatures #-}

class Foo a where    foo :: a    foo = error "foo"

   default foo :: Num a => a    foo = 1

GHC complains: "Conflicting definitions for `foo'"

The following use of multiple default signatures also gives the same error:

class Foo a where    foo :: a

   default foo :: Fractional a => a    foo = 0.5

   default foo :: Num a => a    foo = 1

Couldn't GHC always pick the most specific default method, just as it does with instances when OverlappingInstances is enabled?

As far as I understand, GHC never looks at the context to decide which instance is applicable: http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/type-class-extensions...  Your instances above are duplicates.

Right. The reason I asked is that I'm adding default generic implementations for the 'arbitrary' and 'shrink' methods of the Arbitrary type class of QuickCheck: class Arbitrary a where arbitrary :: Gen a shrink :: a -> [a] shrink _ = [] default arbitrary :: (Generic a, GArbitrary (Rep a)) => Gen a arbitrary = fmap to gArbitrary default shrink :: (Generic a, GArbitrary (Rep a)) => a -> [a] shrink = map to . gShrink . from However the normal default implementation of 'shrink' conflicts with the generic default implementation. So I had to remove it and manually add it to each of the instances that previously implicitly used the default implementation. This is not a big deal though. Bas