[GHC] #11194: Frontend plugins

[GHC]

#11194: Frontend plugins
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           Reporter:  ezyang         |             Owner:  ezyang
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  GHC API        |           Version:  7.11
           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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 A frontend plugin adds a new major mode to GHC, like `--make`, `--abi-
 hash`, etc.  This will be useful for GHC API clients, because GHC's
 frontend `ghc/Main.hs` handles a lot of plumbing (flag parsing, setting up
 the session) which otherwise is painful to reimplement in GHC API (worse
 yet, it rapidly changes between GHC versions).  For example, suppose
 someone wants to reimplement `--make` using the Shake library.  They would
 define a frontend plugin which implements the `--shake` major mode.

 Here's how it will work:

 * A frontend plugin is a module which exports a single identifier
 `frontendPlugin` of type `GhcPlugins.FrontendPlugin`.  This data structure
 has the following form:
 {{{
 type FrontendPluginAction
   = [CommandLineOption] -> [(String, Maybe Phase)] -> GHC ()
 data FrontendPlugin = FrontendPlugin {
     frontendPluginAction :: FrontendPluginAction,
     -- possibly more fields
   }

 defaultFrontendPlugin :: FrontendPluginAction -> FrontendPlugin
 }}}
 * There is a new major mode, `--frontend Foo.Plugin`, which loads and runs
 your plugin. You can pass extra flags to the plugin using `-ffrontend-
 opt`.

 We reuse the dynamic loading facilities for normal plugin loading.

 This should be relatively easy to backport.

 Possible design flex points:

 1. Here, you only load the plugin that you actually want to run. You could
 also imagine loading a plugin adding a new major mode flag.
 2. An alternative to having a full-on plugin architecture is to lib-ify
 most of `ghc/Main.hs`, so that other clients can use it but with different
 sets of options, etc.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11194>
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