[GHC] #14362: Allow: Coercing (a:~:b) to (b:~:a)

[GHC]

#14362: Allow: Coercing (a:~:b) to (b:~:a)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  roles
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):

 A user defined type with "no represented value arguments" could generate
 an `Underlying` type instance

 {{{#!hs
 type Cat obj = obj -> obj -> Type

 data LessThanEq :: Cat Nat where
   LessThanEq :: a <= b => LessThanEq a b

 data GreaterThanEq :: Cat Nat where
   GreaterThanEq :: a >= b => GreaterThanEq a b

 type instance Underlying (LessThanEq    a b) = a <= b
 type instance Underlying (GreaterThanEq a b) = a => b
 }}}

 So depending how smart our constraint solver is, we can coerce

 {{{#!hs
 coerce :: LessThanEq a b -> LessThanEq a (10 + b)

 coerce :: LessThanEq a b -> GreaterThanEq b a
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14362#comment:4>
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