On 24 November 2011 16:46, José Pedro Magalhães j...@cs.uu.nl wrote:
Hi Bas,
On Thu, Nov 24, 2011 at 09:23, Bas van Dijk v.dijk@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.
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
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