[GHC] #11367: [Regression] Pattern synonyms

[GHC]

#11367: [Regression] Pattern synonyms
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           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Regression. This worked in 7.10.2:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ViewPatterns #-}

 pattern A :: Int -> String
 pattern A n <- (read -> n) where
         A 0 = "hi"
         A 1 = "bye"
 }}}

 Removing the final clause works in GHC head but given the same code it
 claims the clause is empty:

 {{{
 % ghci -ignore-dot-ghci /tmp/tmp.t0h0pMgwWb.hs
 GHCi, version 8.1.20160105: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tmp.t0h0pMgwWb.hs, interpreted
 )

 /tmp/tmp.t0h0pMgwWb.hs:4:9: error:
     pattern synonym 'where' clause cannot be empty
     In the pattern synonym declaration for:  A
 Failed, modules loaded: none.
 Prelude>
 }}}

 The where clause is certainly not empty — ironically seems to be caused by
 my very own #10426  ([https://phabricator.haskell.org/D1665 D1665]) :--)
 hoist by my own ticket as we say:

 {{{#!hs
  ; when (length matches /= 1) (wrongNumberErr loc)
 }}}

 Personally a trailing `where` is quite alright and handy when quickly
 checking if a declaration is otherwise OK. It works for data/newtype
 declarations as well as type classes. A workaround is to pattern match in
 other ways:

 {{{#!hs
 pattern A n <- ... where
         A = \case
           0 -> "hi"
           1 -> "bye"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11367>
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