[GHC] #12515: Pattern synonyms with non-conid/consym names give poor error messages

22 Aug 2016

      #12515: Pattern synonyms with non-conid/consym names give poor error messages
-------------------------------------+-------------------------------------
           Reporter:  pkmx           |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 -- Correct: pattern P = ()
 pattern p = ()
 }}}

 gives a rather unhelpful error message:

 {{{
     parse error on input ‘=’
     Perhaps you need a 'let' in a 'do' block?
     e.g. 'let x = 5' instead of 'x = 5'
 }}}

 Ideally, it should hint that the name of pattern should follow constructor
 identifier rules.

 ----

 In the same vein, the following:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 -- Correct: pattern (:+) = ()
 pattern (+) = ()
 }}}

 currently gives:

 {{{
     error: parse error on input ‘+’
 }}}

 It should hint that the operator should begin with a {{{:}}}.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12515
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler