[GHC] #15765: Make the "extract" functions in RnTypes pure

[GHC]

#15765: Make the "extract" functions in RnTypes pure
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  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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 Once upon a time, the `extract` functions at the bottom of RnTypes were
 pure. Then, along came `-XTypeInType`, which needed to do a check in these
 functions for users mixing type variables with kind variables. Now,
 however, with `-XTypeInType` gone again, we no longer do this check. Thus,
 there is no reason to keep these functions monadic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15765>
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