On Thu, Nov 24, 2011 at 09:23, Bas van Dijk <v.dijk.bas@gmail.com> 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.html#instance-overlap

Your instances above are duplicates.

Cheers,

Pedro

Regards,

Bas

_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users