[GHC] #13026: RFC functions for sums and products

[GHC]

#13026: RFC functions for sums and products
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              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):

 Some other

 {{{#!hs
 type f ~> g = forall x. f x -> g x

 (****) :: f ~> f' -> g ~> g' -> Product f g ~> Product f' g'
 (f **** g) (fa `Pair` ga) = f fa `Pair` g ga

 first' :: f ~> f' -> Product f g ~> Product f' g
 first' = (**** id)

 second' :: g ~> g' -> Product f g ~> Product f g'
 second' = (id ****)

 (++++) :: f ~> f' -> g ~> g' -> Sum f g ~> Sum f' g'
 f ++++ g = \case
   InL fa -> InL (f fa)
   InR ga -> InR (g ga)

 left' :: f ~> f' -> Sum f g ~> Sum f' g
 left' = (++++ id)

 right' :: g ~> g' -> Sum f g ~> Sum f g'
 right' = (id ++++)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13026#comment:2>
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