Re: [GHC] #8165: Use GeneralizedNewtypeDeriving to automatically create associated type families

28 Aug 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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