Getting Rewrite Rules from ModGuts

11 Apr 2019

      Hi,

I'm trying to use the GHC API (8.2.2) to extract the rewrite rules from a module, but have run into some difficulties.  I've written the following code (all code is also attached as files):

    module Main where

    import GHC
    import GHC.Paths
    import HscTypes

    main :: IO ()
    main = do
        loadProj "Test.hs"

    loadProj :: FilePath -> IO ()
    loadProj src = do
        modgutss <- runGhc (Just libdir) $ do
            flags <- getSessionDynFlags
            _ <- setSessionDynFlags flags

            target <- guessTarget src Nothing
            _ <- setTargets [target]
            _ <- load LoadAllTargets

            mod_graph <- getModuleGraph
            parsed_mods <- mapM parseModule mod_graph
            typed_mods <- mapM typecheckModule parsed_mods
            desug_mods <- mapM desugarModule typed_mods

            return $ map coreModule desug_mods

        let rules = map (\mg -> ( moduleNameString . moduleName $ mg_module mg
                                , length $ mg_rules mg)
                        ) modgutss
        print $ rules

        return ()

It loads Test.hs (see below my signature) which contains a single rewrite rule.  Then, it obtains the ModGuts, and prints a tuple of the name of the module, and the length of the rule list.  I would expect this to be ("Test", 1), but, in fact, what gets printed is ("Test", 0).  For some reason, the ModGuts does not have the rewrite rule from the file.

I'm guessing I'm misunderstanding something about how the GHC API works.  If anyone has any advice about how to fix this code to load rewrite rules (or even where to begin looking) I would appreciate it!

Thanks,
Bill Hallahan

Test.hs:
    module Test where

    import Prelude hiding (map)

    map :: (a -> b) -> [a] -> [b]
    map f (x:xs) = f x:map f xs
    map _ [] = []
    {-# NOINLINE [0] map #-}

    {-# RULES
     "map/map"   forall f g xs . map f (map g xs) = map (f . g) xs
      #-}