[GHC] #9112: data families with representational matching

[GHC]

#9112: data families with representational matching
-------------------------------------+------------------------------------
        Reporter:  jwlato            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      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 simonpj):

 The trouble is that you can say
 {{{
 data instance MVector s Int = ...rep1...
 data instance MVector s Age = ...rep2...
 }}}
 so that `MVector` over `Int` has an entirely different representation to
 `MVector` over `Age`. Indeed that is often the very reason that people
 define a newtype in the first place!  For example, if you want `sort` to
 sort into reverse order, can write
 {{{
 import Data.Ord( Down(..) )

 downSort :: Ord a => [a] -> [a]
 downSort xs = coerce (sort (coerce xs :: [Down a]))
 }}}
 We coerce the `[a]` to `[Down a]`, then sort (using `Down`'s ordering),
 then coerce back.

 It's going to be quite confusing `data instance` can sometimes match on a
 newtype, and sometimes not.  And then there are nested cases to worry
 about:
 {{{
 newtype MVector s [Age] = ...
 newtype MVector s [Int] = ...
 }}}
 Moreover, you still (presumably) want `MVector s Age` and `MVector s Int`
 to be distinct types!

 None of this smells good to me.

 But here's an idea.  You want `MVector s Age` and `MVector s Int` But you
 want them to be represented the same way. That's what we use newtypes for.
 So how about this:
 {{{
 newtype instance MVector s Age = MVA (MVector s Int)
 }}}
 Would that help?  For example, this compiles without complaint:
 {{{
 {-# LANGUAGE MultiParamTypeClasses, TypeFamilies,
 GeneralizedNewtypeDeriving #-}
 module T9112 where

 class MVectorClass (v :: * -> * -> *) a where
     basicLength :: v s a -> Int

 data family MVector s a

 data instance MVector s Int = MV -- implementation not important

 newtype Age = Age Int deriving (MVectorClass MVector)

 newtype instance MVector s Age = MV1 (MVector s Int)

 instance MVectorClass MVector Int where
   basicLength x = 0
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9112#comment:2>
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