Why no multiple default method implementations?

[José Pedro Magalhães]

Hi Bas,

On Thu, Nov 24, 2011 at 09:23, Bas van Dijk <v.dijk.bas at 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
>
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