[GHC] #5941: Add compilation stage plugins

[GHC]

#5941: Add compilation stage plugins
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              Reporter:  tibbe       |            Owner:
                  Type:  feature     |           Status:  infoneeded
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:  7.4.1
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rodlogic):

 I find this a desirable feature, so I took a quick look.

 It seems that tcPlugin is currently focused on a pluggable constraint
 solver only:
 {{{#!hs
 data Plugin = Plugin {
     installCoreToDos :: [CommandLineOption] -> [CoreToDo] -> CoreM
 [CoreToDo]
     -- ^ Modify the Core pipeline that will be used for compilation.
     -- This is called as the Core pipeline is built for every module
     -- being compiled, and plugins get the opportunity to modify the
     -- pipeline in a nondeterministic order.
   , tcPlugin :: [CommandLineOption] -> Maybe TcPlugin
     -- ^ An optional typechecker plugin, which may modify the
     -- behaviour of the constraint solver.
   }

 data TcPlugin = forall s. TcPlugin
   { tcPluginInit  :: TcPluginM s
     -- ^ Initialize plugin, when entering type-checker.

   , tcPluginSolve :: s -> TcPluginSolver
     -- ^ Solve some constraints.
     -- TODO: WRITE MORE DETAILS ON HOW THIS WORKS.

   , tcPluginStop  :: s -> TcPluginM ()
    -- ^ Clean up after the plugin, when exiting the type-checker.
   }

 type TcPluginSolver = [Ct]    -- given
                    -> [Ct]    -- derived
                    -> [Ct]    -- wanted
                    -> TcPluginM TcPluginResult
 }}}

 At this point it seems that there are two main venues:
 1. extend TcPlugin to add a hook for processing the final type-checked
 module (and widening the scope of {{{tcPluginInit}}} and
 {{{tcPluginSolve}}}), or
 2. Add another plugin for AST hooks (ParsedModule, RenamedModule, and
 TypecheckedModule).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5941#comment:4>
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