[GHC] #11350: Allow visible type application in patterns

[GHC]

#11350: Allow visible type application in patterns
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11385            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Have only referenced this in passing: Allow type application for methods

 {{{#!hs
 class Semigroup a where
   (<>) @a :: a -> a -> a

 class Semigroup a => Monoid a where
   mempty @a :: a
 }}}

 and their instances with some #12363

 {{{#!hs
 instance (Semigroup a, Semigroup b) => Semigroup (a, b) where
   (<>) @(a, b) :: (a, b) -> (a, b) -> (a, b)
   (a1, b1) <> @(a, b) (a2, b2) = (a1 <> @a a2, b1 <> @b b2)

 instance (Monoid a, Monoid b) => Monoid (a, b) where
   mempty @(a, b) :: (a, b)
   mempty @(a, b) = (mempty @a, mempty @b)

 instance Semigroup b => Semigroup (a -> b) where
   (<>) @(a -> b) :: (a -> b) -> (a -> b) -> (a -> b)
   (f <> @(a -> b) g) x = f x <> @b g x
 -- ^ not recommended style

 class Monoid b => Monoid (a -> b) where
   mempty @(a -> b) :: a -> b
   mempty @(a -> b) _ = mempty @b
 }}}

 The type variables should probably match the order of application a
 regular expression context.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11350#comment:12>
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