[GHC] #14993: QuantifiedConstraints and principal types

[GHC]

#14993: QuantifiedConstraints and principal types
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints, wipT2893    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This is not a bug but I am trying to understand the ~~implication~~
 significance of `-XQuantifiedConstraints`.

 [https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/jfp-
 outsidein.pdf OutsideIn(X)] (section **2.4**) tells us `test` actually has
 a principle type when we have implication constraints)

 {{{#!hs
 data T :: Type -> Type where
   T1 :: Int -> T Bool
   T2 ::        T a

 test :: (a~Bool => b~Bool) => T a -> b -> b
 test (T1 n) _ = n > 0
 test T2     r = r
 }}}

 that subsumes

 {{{#!hs
 test1 :: T a -> a -> a
 test1 = test

 test2 :: T a -> Bool -> Bool
 test2 = test
 }}}

 Now that we have `-XQuantifiedConstraints` does this change the story at
 all, if not you can close this..

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14993>
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