[GHC] #10498: "if ... then \case -> else ..." causes a "missing else clause" error

[GHC]

#10498: "if ... then \case -> else ..." causes a "missing else clause" error
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        Reporter:  dramforever       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.3
       Component:  Compiler          |                 Version:  7.10.1
  (Parser)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Changes (by thomie):

 * keywords:  newcomer =>


Comment:

 I'm inclined to just the delete the parser changes from Phab:D201. They
 were supposed to make error messages better, but instead they make them
 worse.

 Here's an example from the testsuite (parser/should_fail/readFail020):
 {{{
 f = let x = 42 } in x
 -- before:
 parse error on input ‘}’
 -- after:
 parse error in let binding: missing required 'in'
 }}}

 Here's another:
 {{{
 f = if module then True else False
 -- before:
 parse error on input ‘module’
 -- after:
 parse error in if statement: naked if statement
 }}}



 For that last one, the parser code looks like this:
 {{{
 | 'if' error      {% hintIf (getLoc $1) "naked if statement" }
 | 'if' exp optSemi error                              {% ... something ...
 }
 | 'if' exp optSemi 'then' error                       {% ... something ...
 }
 | 'if' exp optSemi 'then' exp optSemi error           {% ... something ...
 }
 | 'if' exp optSemi 'then' exp optSemi 'else' error    {% ... something ...
 }
 }}}

 What this means is that a lexical error in the condition of an if-
 expression, that we don't handle gracefully in the parser, will be
 reported as "naked if statement".

 The example from the description has the same cause.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10498#comment:6>
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