[GHC] #13450: Better parse error for empy character literal

19 Mar 2017

      #13450: Better parse error for empy character literal
-------------------------------------+-------------------------------------
           Reporter:  parsonsmatt    |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Empty character literals give an obscure error message. As an example,
 this code:

 {{{#!hs
 example = foo : bar
   where
     foo = ''
     bar = "asdf"
 }}}

 gives the following error when loaded in GHCi:

 {{{
 err.hs:4:6:
     parse error (possibly incorrect indentation or mismatched brackets)
 }}}

 This has affected me once, as a beginner. I've seen it affect a few other
 beginners, and the error message isn't very helpful to figuring out what's
 wrong.

 It would be nice if a better error message could be reported. If there's a
 parse error on `''`, then GHC could report something like:

 {{{
 err.hs:3:11:
     parse error on `''`.
     Note: Character literals may not be empty.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13450>
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