GhcPlugin-writing and "finding things"

[Simon Peyton Jones]

Philip

You are right: there are some missing pieces.

* First you need to ask where your plugin's special library module "Foo" is in the file system. This is what findImportedModule is for, and it seems quite reasonable.  However, it (or some variant) should be available to you in CoreM.

* Next, suppose you special library module defines a special type "T".  You need to get its Name.  For this you a CoreM variant of IfaceEnv.lookupOrig.  The function CoreMonad.getOrigNameCache is far too low level and should be killed.  Instead, CoreMonad should expose 
	lookupOrig :: Module -> OccName -> CoreM Name
It should be an easy function to write, using IfaceEnv.lookupOrig; maybe a tiny bit of refactoring.

* Next you want to get from T's Name to T's TyCon. Here CoreMonad is fine: it offers 
	lookupThing :: Name -> CoreM TyThing
This function calls TcEnv.tcLookupGlobal, which will automatically load Foo.hi if need be.

So your code should look like

    foo_mod <- findImportedModule "Foo"
    t_name  <- lookupOrig foo_mod (mkTcOcc "T")
    t_tycon <- lookupThing t_name

corresponding to these three steps.  I suspect that the error cases of findImported module should be dealt with via exceptions in CoreM, to de-clutter the code.


Some of the above suggests a bit of cleaning up of the CoreM API. Would someone like to undertake that?  I can advise, but I don't want to lead.

Simon


| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of p.k.f.holzenspies at utwente.nl
| Sent: 23 July 2014 17:07
| To: glasgow-haskell-users at haskell.org
| Subject: GhcPlugin-writing and "finding things"
| 
| Dear GHC-ers,
| 
| I'm working on a plugin for GHC that should help compile the library
| with which this plugin is to ship. What this plugin does is traverse
| the CoreProgram(s) to find things of types defined in my library and
| optimizes them. I have worked out how to "find" things, but I was
| wondering whether the API could be improved for plugin-writers.
| 
| For the sake of argument, I have the following:
|  - module Foo: library for users to import, containing functions, ADTs
| etc
|  - module Foo.Plugin: GhcPlugin that compiles out all uses of things in
| Foo
| 
| > module Foo where
| >
| > data Foo x = Foo x
| >
| > runFoo :: Foo x -> x
| > runFoo (Foo x) = x
| 
| 
| This example is trivial and I imagine GHC will have no trouble
| eliminating most cases of this, but imagine more complex stuff. Now, if
| I want to traverse the CoreProgram in my plugin, I need to find
| occurrences of these, so somewhere there's stuff like:
| 
| > pass tcFoo _ _ (NonRec b expr)
| >   | varType b `containsTyConAnywhere` tcFoo
| >     = {- clever stuff to compile out Foo -}
| 
| My problem is "getting" tcFoo in this example. Below is how I do it
| now. Maybe I'm being thick, or maybe there's just no simpler way. This
| is my 'plugin' function in Foo.Plugin:
| 
| > plugin = Plugin $ \opts todo -> do
| >  hsc <- getHscEnv
| >  dfs <- getDynFlags
| >  fr  <- liftIO $ findImportedModule hsc (mkModuleName "Foo") Nothing
| > mod <- case fr of
| >    Found ml m -> return m
| >    _ -> panic "Failed to (unambiguously) find 'Foo' (using
| findImportedModule)"
| >  onc <- getOrigNameCache
| >  let nms = lookupWithDefaultModuleEnv nms (panic "No names defined
| for module 'Foo'") mod
| >      find_ d occ fnd nm
| >        = maybe
| >            (fail $ "Failed to find " ++ d ++ " '" ++ nm ++ "'")
| >            fnd
| >            (lookupOccEnv nms $ occ nm)
| >      tcFind = find_ "TyCon"   mkTcOcc   lookupTyCon
| >      dcFind = find_ "DataCon" mkDataOcc lookupDataCon
| >      idFind = find_ "Id"      mkVarOcc  lookupId
| >  tcFoo    <- tcFind "Foo"
| >  dcFoo    <- dcFind "Foo"
| >  idRunFoo <- idFind "runFoo"
| >  return $ CoreDoPluginPass "Foo optimisation" (pass tcFoo dcFoo
| > idRunFoo) : todo
| 
| I have the following questions:
| 
|   1) Is this a/the right way to "find" those things in the plugin?
|   2) There seems to be a lot to gain with quasi-quoting a la Template
| Haskell for people writing plugins to go with a library that they
| wrote. Can such QQ be done? Has it been considered?
|   3) Is findImportedModule the right function to find my starting point
| to begin with?
|   4) What is the 'Maybe FastString' argument in findImportedModule for?
| I've been trying to put in the FSs of PackageIDs, but they make the
| lookup fail. This (dumb) example really made me nervous:
| 
| >  fr  <- liftIO $ findImportedModule hsc (mkModuleName "Foo") Nothing
| > mod <- case fr of
| >    Found ml m -> do
| >      fr' <- liftIO $ findImportedModule hsc (moduleName m)
| > (packageIdFS $ packageId m)
| 
| Here, fr' should always be a "Found ml' m'" such that ml == ml' and m
| == m', but... it consistently results in NotFound{} for me. Also, I
| find this especially round-about. Shouldn't Paths_Foo.hs (the Cabal-
| generated file) maybe contain variables for every module in the
| package? In my case it would thus contain some "modFoo :: Module"
| 
| Comments and suggestions more than welcome!
| 
| Regards,
| Philip
| 
| 
| 
| 
| 
| 
| 
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