[Haskell-cafe] Accessing and Inspecting StgBindings in Ghci

[Michael Christensen]

Hi all,

I discovered that the issue of trying to use the "dumpSTG" function from 
with ghci was occuring whenever I tried to run it on one particular 
Haskell input file. When I used it on a different file, it allowed me to 
get the list of STG bindings in ghci as desired. I'll be investigating 
why that first file is causing the ghci panic, but other than that, my 
question has been answered/resolved.

Thanks,
Michael

On 02/02/2016 10:30 PM, Michael Christensen wrote:
> Hi all,
>
> I'm currently trying to understand how STG works, and my goal right 
> now is to be able to inspect StgBinding values. I've written a short 
> program, based on the wiki article GHC/As a library 
> <https://wiki.haskell.org/GHC/As_a_library>, like below:
>
> -- Code.hs --
> module Lib (printSTG, dumpSTG) where
>
> import           Control.Monad.Ghc  (lift, runGhcT)
> import           CorePrep           (corePrepPgm)
> import           CoreToStg          (coreToStg)
> import           DynFlags           (defaultFatalMessager, 
> defaultFlushOut)
> import           GHC                hiding (runGhcT)
> import           GHC.Paths          (libdir)
> import           HscMain            (newHscEnv)
> import           HscTypes           (hsc_dflags, typeEnvTyCons)
> import           Outputable         (interppSP, showSDoc)
> import           System.Environment (getArgs)
> import           StgSyn             (StgBinding)
>
> dumpSTG :: String -> IO [StgBinding]
> dumpSTG fileName = defaultErrorHandler defaultFatalMessager 
> defaultFlushOut $
>     runGhcT (Just libdir) $ do
>       sess <- getSession
>       let dflags = hsc_dflags sess
>       setSessionDynFlags dflags
>       cm <- compileToCoreModule fileName
>       let md = cm_module cm
>       ml <- fmap ms_location $ getModSummary $ moduleName md
>       lift $ do
>         cp <- corePrepPgm sess ml <$> cm_binds <*> (typeEnvTyCons . 
> cm_types) $ cm
>         coreToStg dflags md cp
>
> printSTG =
>     getArgs >>= \x -> case x of
>     [] -> putStrLn "usage: Main <file.hs>"
>     (fileName:_) -> do
>       bindings <- dumpSTG fileName
>       str <- runGhcT (Just libdir) $ do
>         dflags <- getSessionDynFlags
>         return $ showSDoc dflags $ interppSP bindings
>       putStrLn str
>
> This works when I compile it to print out a Haskell file in STG format.
>
> However, *my question is*, is there a way so that I can call dumpSTG 
> to get back that list of StgBindings, *from within Ghci*? Whenever I 
> do so within Ghci, i.e.
>
> ghci> :l code.hs
> ghci> bindings <- dumpSTG "fileToTest.hs"
> <interactive>: panic! (the 'impossible' happened)
>   (GHC version 7.10.3 for x86_64-unknown-linux):
>   no package state yet: call GHC.setSessionDynFlags
>
> I know that it's saying I need to set the session dyn flags, but I'm 
> not sure how that can get done from within Ghci, especially because 
> that needs to get done within the GhcMonad. Part of my problem may 
> stem from having a inadequate understanding of monads. Any help or 
> pointers would be greatly appreciated!
>
> Thank you very much,
> Mike
>

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