[GHC] #13451: Bind implicit parameter in patterns

[GHC]

#13451: Bind implicit parameter in patterns
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  ImplicitParams                     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Is there any reason why variables for `ImplicitParams` can't be bound in
 patterns, so you can write

 {{{#!hs
 data Exp = Val Int | Op Op Exp Exp

 data Op = Add | Mul

 data Protect where
   Protect :: (?op :: Op) => Protect

 protectOp :: Exp -> Maybe Protect
 protectOp (Op ?op _ _) = Just Protect
 protectOp _            = Nothing
 }}}

 instead of

 {{{#!hs
 protectOp :: Exp -> Maybe Protect
 protectOp (Op op _ _) = Just Protect where ?op = op
 protectOp _           = Nothing
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13451>
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