[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:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 This ''should'' make the code in
 [https://ghc.haskell.org/trac/ghc/ticket/8583#comment:7 Richard's comment]
 compile:

 {{{#!hs
 headOfList :: forall a. [a] -> Maybe a
 headOfList (Nil  @[] @a)     = Nothing @a
 headOfList (Cons @[] @a x _) = Just x
 }}}

 The rest of his comment applies to, this is not for dynamically branching
 on types although that could certainly be added as a feature!

 {{{#!hs
 doubleWhenInt :: forall a. Typeable a => a -> a
 doubleWhenInt a =
   case eqT @Int @a of
     Just Refl -> a + a
     Nothing   -> a

 -- Becomes…
 doubleWhenInt :: forall a. Typeable a => a -> a
 doubleWhenInt @Int n = n + n
 doubleWhenInt @_   a = a
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11350#comment:2>
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