[GHC] #13672: Pattern match on LHS of pattern synonym declaration

[GHC]

#13672: Pattern match on LHS of pattern synonym declaration
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Changing `ZipList` to a newtype

 {{{#!hs
 import Data.Bifunctor.Fix
 import Data.Bifunctor.Tannen

 type ZipList = Fix (Tannen Maybe (,))

 pattern Nil :: ZipList a
 pattern Nil = In (Tannen Nothing)

 infixr 5 :::
 pattern (:::) :: a -> ZipList a -> ZipList a
 pattern a:::as = In (Tannen (Just (as, a)))
 }}}

 works fine for the `Nil` pattern synonym (it remains implicitly
 bidirectional)

 {{{#!hs
 newtype ZipList a = ZL (Fix (Tannen Maybe (,)) a)

 pattern Nil :: ZipList a
 pattern Nil = ZL (In (Tannen Nothing))
 }}}

 but `(:::)` needs to be explicitly bidirectional

 {{{#!hs
 infixr 5 :::
 pattern (:::) :: a -> ZipList a -> ZipList a
 pattern a:::as   <- ZL (In (Tannen (Just (ZL -> as, a))))
   where a:::ZL as = ZL (In (Tannen (Just (as, a))))
 }}}

 ----

 I would like to write it as

 {{{#!hs
 pattern (:::) :: a -> ZipList a -> ZipList a
 pattern a:::ZL as = ZL (In (Tannen (Just (as, a))))
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13672>
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