[GHC] #12917: Location info for error message with multiple source locations

[GHC]

#12917: Location info for error message with multiple source locations
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        Reporter:  gracjan           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gracjan):

 This:

 {{{
 MonadFailErrors.hs:44:5: error:
     • No instance for (MonadFail (ST s))
         arising from a do statement
         with the failable pattern ‘Just x’
     • In a stmt of a 'do' block: Just x <- undefined
       In the expression:
         do { Just x <- undefined;
              undefined }
       In an equation for ‘st’:
           st
             = do { Just x <- undefined;
                    undefined }
 }}}

 Could be better described with:

 {{{
 MonadFailErrors.hs:44:5-11: No instance for (MonadFail (ST s)) arising
 from a failable patter in a do statement
 }}}

 And count on Emacs to highlight relevant source code part, that is `Just
 x`.

 We could try to be more mouthful:

 {{{
 MonadFailErrors.hs:44:5-11: No instance for (MonadFail (ST s)) arising
 from a failable patter in a do statement
 MonadFailErrors.hs:41:4: that begins here
 }}}

 but in this case this is overkill. In other cases it might be useful.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12917#comment:1>
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