[GHC] #8165: Use GeneralizedNewtypeDeriving to automatically create associated type families
GHC
ghc-devs at haskell.org
Wed Aug 28 00:54:29 UTC 2013
#8165: Use GeneralizedNewtypeDeriving to automatically create associated type
families
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Reporter: MikeIzbicki | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.3
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: None/Unknown | Difficulty: Unknown
Test Case: | Blocked By:
Blocking: | Related Tickets:
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Comment (by MikeIzbicki):
In all the cases where I've wanted to use this, the type is polymorphic.
Here's a real example of when I've wanted to use this:
{{{
class HasRing a where
type Ring a
instance HasRing [a] where type Ring [a] = a
instance Hasring (Vector a) where type Ring (Vector a) = a
newtype L2Norm a = L2Norm a
deriving HasRing
newtype L1Norm a = L1Norm a
deriving HasRing
}}}
The deriving clauses would be equivalent to:
{{{
instance HasRing a => HasRing (L2Norm a) where
type Ring (L2Norm a) = Ring a
instance HasRing a => HasRing (L1Norm a) where
type Ring (L1Norm a) = Ring a
}}}
This might become problematic if there are multiple type parameters
though.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8165#comment:3>
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