[GHC] #12483: Improve parse error message on indentation mistake

[GHC]

#12483: Improve parse error message on indentation mistake
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           Reporter:  harendra       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following code fragment:
 {{{#!hs
                    (ExecGhc, args) -> execCompiler "" args
                     -- NOTE: this won't currently work for GHCJS, because
 it doesn't have
                     -- a runghcjs binary. It probably will someday,
 though.
                     (ExecRunGhc, args) ->
 }}}

 The last line is indented by one space but it is hard to notice esp when
 we looking at this along with a screenful of code and there are many
 comments between the two lines. When this code is compiled ghc throws the
 following error:

 {{{
 Main.hs:745:40: error: parse error on input ‘->’
 }}}

 I look at the error and then the code and wonder what is wrong with this
 code, there is nothing noticeably wrong. If ghc can provide some context I
 can easily make out where the problem is. ghc can perhaps tell me how the
 current expression is being parsed e.g. `(ExecRunGhc, args) ->` is being
 considered as a continuation of the expression on the previous line.

 I have run into this many times. The first time it was really frustrating.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12483>
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