[GHC] #14391: Make the simplifier independent of the typechecker

26 Oct 2017

      #14391: Make the simplifier independent of the typechecker
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           Reporter:  nomeata        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           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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 I noticed that the simplifier module depends on all of the type checker,
 and HsSyn stuff, and renamer stuff, which I found strange.

 After a little investigation, it seems that the simplifier depends on
 CoreMonad, and that pulls some very few type-checker related things:

 1.
     {{{
     import TcRnMonad        ( initTcForLookup )
     import {-# SOURCE #-} TcSplice ( lookupThName_maybe )
     }}}
     for
     {{{
     thNameToGhcName :: TH.Name -> CoreM (Maybe Name)
     thNameToGhcName th_name = do
         hsc_env <- getHscEnv
         liftIO $ initTcForLookup hsc_env (lookupThName_maybe th_name)
     }}}
     which is not even used in GHC, but only in GHC Plugins, so this could
     probably be moved to a separate module pulled in by GhcPlugins.hs

 2.
     {{{
     import TcEnv            ( lookupGlobal )
     }}}
     for
     {{{
     instance MonadThings CoreM where
         lookupThing name = do { hsc_env <- getHscEnv
                               ; liftIO $ lookupGlobal hsc_env name }
     }}}
     This might be a bit harder to disentangle. But if successful, it would
     probably make building GHC in parallel quite a bit faster. And it just
     seems strange to me that the Core-to-Core code should depend on the
     type checker…

 Simon says:
...
Both of these code paths go through initTcForLookup which is massive
 overkill, as the comment with `TcEnv.lookupGlobal` says.  There's clearly
 a ToDo here to strip off the redundant stuff and do a minimal lookup.
I am optimistically marking this as `newcomer` because it is a refactoring
 task, and a good way of learning a bit about various pieces, with a
 reasonably clear notion of “success”.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14391
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