Re: [Haskell-cafe] Accessing and Inspecting StgBindings in Ghci

3 Feb 2016

      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 "
    (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