[GHC] #9980: TcS monad is too heavy

[GHC]

#9980: TcS monad is too heavy
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              Reporter:  simonpj     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 The typehcecker's constraint-solving monad, `TcS`, is simply built on top
 of `TcM`, but it doesn't use most of `TcM`'s facilities.  That was fine
 while `TcS` was only called from the typechecker, but now (as part of
 [wiki:PatternMatchCheck fixing pattern-match overlap checking]) we invoke
 it from the desugarer.

 It seems quite wrong to construct a vast, and unnecessary `TcM` context
 just to invoke `TcS`.

 Better: make `TcS` into its own monad, with its own `TcSLclEnv`, built on
 `IOEnv` like the others.

 Main objection: the plugins mechanism exposes `unsafeTcPluginTcM`, which
 would become unavailable.  But it it used?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9980>
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