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

[GHC]

#10961: Make it possible to purely use the parser
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           Reporter:  mpickering     |             Owner:
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Parser)                           |
           Keywords:  newcomer       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It is claimed that the parser is pure but one of the arguments is
 `DynFlags`. The only safe way to construct a copy of `DynFlags` requires
 IO so it is not possible to use the parser purely.

 I propose to introduce a new datatype `ParserOptions` which contains the
 options used by the parser and a function `DynFlags -> ParserOptions` so
 that it is possible to purely construct a `ParserOptions` and use the
 parser purely. This would be useful for external users of the parser.

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