[GHC] #13592: Newtype type class with compiler generated instances

[GHC]

#13592: Newtype type class with compiler generated instances
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 '''Even''' if we remove `type O` from the class the compiler only needs to
 generate the `type instance`s for `O`

 {{{#!hs
 type family O n

 class (Coercible (O n) n, Coercible n (O n)) => Newtype n where
   pack :: O n -> n
   pack = coerce

   unpack :: n -> O n
   unpack = coerce

 instance (Coercible (O n) n, Coercible n (O n)) => Newtype n
 }}}

 if you want to make `Sum` into a newtype, all the compiler has to do is
 generate

 {{{#!hs
 type instance O (Sum a) = a
 }}}

 pretty nifty!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13592#comment:4>
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