Getting Rewrite Rules from ModGuts

[Brandon Allbery]

You might need to enable optimization for RULES to get picked up at all.

On Thu, Apr 11, 2019 at 11:03 PM Bill Hallahan <william.hallahan at yale.edu>
wrote:

> 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
>       #-}
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>


-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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