[GHC] #15969: Generic1 deriving should use more coercions

[GHC]

#15969: Generic1 deriving should use more coercions
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 I think this problem suggests that `Generic1` got composition the wrong
 way around. Let's take a slightly bigger example:

 {{{#!hs
 data Baz a = Baz (Maybe [Either Int a])
 }}}

 We currently derive

 {{{#!hs
 Rep1 Baz = ... (Maybe :.: ([] :.: Rec1 (Either Int)))
 }}}

 Suppose we did it the other way:

 {{{#!hs
 Rep1 Baz = .... ((Rec1 Maybe :.: []) :.: Either Int)
 }}}

 then we'd have the `Rec1` "exposed" properly:

 {{{#!hs
 from1 (Baz q) = M1 (M1 (M1 (Comp1 (Comp1 (Rec1 q)))))
 }}}

 No fanciness required at all!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15969#comment:1>
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