[GHC] #14993: QuantifiedConstraints and principal types

[GHC]

#14993: QuantifiedConstraints and principal types
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

Old description:

> 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..

New description:

 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 principal 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#comment:5>
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