[GHC] #10961: Make it possible to purely use the parser

[GHC]

#10961: Make it possible to purely use the parser
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:  dalaing
            Type:  task              |               Status:  patch
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
  (Parser)                           |
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10143            |  Differential Rev(s):  D2208
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by dalaing):

 I've dealt with the pretty printing of warnings by changing from `messages
 :: Messages` to `messages :: DynFlags -> Messages` in `PState`.  This also
 means there are some changes to pass in `DynFlags` when `getMessages` is
 called.

 This would change back if a resolution of #10143 meant we could pretty
 print warnings without needing `DynFlags`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10961#comment:7>
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