[GHC] #13018: TH-spliced pattern synonym declaration fails to typecheck

[GHC]

#13018: TH-spliced pattern synonym declaration fails to typecheck
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I made these functions monadic not too long ago. Looking back through, it
 seems the only `RnM` action taken is `addErrAt` from `mixedVarsErr` (and
 the related `xoptM` call). It wouldn't be hard to augment the
 `FreeKiTyVars` structure to include a list of errors generated. Then, the
 whole lot of these functions could be made pure. This would seem to be an
 improvement -- especially in your case when you are not interested in
 error messages.

 Alternatively, I've recently run into the need to have functions be used
 in both `TcM` and `DsM`, but the only monadic action, again, was error
 generation. It might be reasonable to have a `ErrMonad` class with
 instances for both `TcM` and `DsM`; then these `extractXXX` functions
 could be generalized over that class.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13018#comment:11>
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