Hi Bas,
On Thu, Nov 24, 2011 at 09:23, Bas van Dijk
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. Cheers, Pedro
Regards,
Bas
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