Extracting STG of Haskell Source

Question

I am trying to extract the STG representation of a Haskell source as a String via Outputable, but it looks like coreToStgArgs is panicing with the following dump:

user@machine ~/Desktop/hue $ runhaskell test.hs 
[foo :: forall a. Num a => a -> a
 [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType] =
     \r srt:SRT:[] [$dNum a1] + $dNum a1 a1;,
 bar :: Int -> Int
 [GblId,test.hs: test.hs: panic! (the 'impossible' happened)
  (GHC version 7.10.3 for x86_64-unknown-linux):
    coreToStgArgs I# 3

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Here is the file FooBar.hs that I want to extract:

module FooBar where

foo a = a + a

bar :: Int -> Int
bar b = b + 3

Here is the source of test.hs that I used:

import CoreToStg
import GHC
import GHC.Paths
import Outputable
import StgSyn

mkDynFlags :: IO DynFlags
mkDynFlags = runGhc (Just libdir) getSessionDynFlags

mkSTG :: FilePath -> FilePath -> IO [StgBinding]
mkSTG proj src = do
    dflags  <- mkDynFlags
    ghc_core <- runGhc (Just libdir) $ do
        setSessionDynFlags (dflags {importPaths = [proj]})
        compileToCoreSimplified src
        -- compileToCoreModule src
    coreToStg dflags (cm_module ghc_core) (cm_binds ghc_core)

mkIOStr :: (Outputable a) => a -> IO String
mkIOStr obj = do
    dflags <- mkDynFlags
    let ppr_str = showPpr dflags obj
    return ppr_str

main :: IO ()
main = do
    let proj = "/home/user/Desktop/hue"
    let src  = proj ++ "/FooBar.hs"
    res <- mkIOStr =<< mkSTG proj src
    putStrLn res

It looks like someone several years before me has run into a similar problem:

https://ghc.haskell.org/trac/ghc/ticket/7159

However, I have no idea what has happened since. I am also not sure if this is the correct way to go about extracting the STG of an arbitrary Haskell source, so if there are better alternatives that work, I would like to hear about them.

EDIT: STG translation appears successful for the following program where bar b = b + 3 is changed to bar b = 3:

module FooBar where

foo a = a + a

bar :: Int -> Int
bar b = 3

In fact, at first glance, things appear to work if the induced Core Haskell does not force primitive operations to be performed. For instance bar b = 3 + 9 fails.

Answer 1

Many thanks to melpomene for pointing out something I missed in the documentation.

Here is the modified source of the test.hs that works:

import CorePrep
import CoreToStg
import GHC
import GHC.Paths
import GhcMonad
import HscTypes
import Outputable
import StgSyn
import System.IO

mkSTG :: FilePath -> FilePath -> IO [StgBinding]
mkSTG proj src = runGhc (Just libdir) $ do
        env    <- getSession
        dflags <- getSessionDynFlags
        setSessionDynFlags (dflags {importPaths = [proj]})
        target <- guessTarget src Nothing
        setTargets [target]
        load LoadAllTargets

        mod_graph <- getModuleGraph
        let mod_sum = head mod_graph  -- This is bad practice
        pmod <- parseModule mod_sum
        tmod <- typecheckModule pmod
        dmod <- desugarModule tmod
        let guts  = coreModule dmod
        let loc   = ms_location mod_sum
        let binds = mg_binds guts
        let tcs   = mg_tcs guts
        prep <- liftIO $ corePrepPgm env loc binds tcs
        liftIO $ coreToStg dflags (mg_module guts) prep

mkIOStr :: (Outputable a) => a -> IO String
mkIOStr obj = do
    dflags <- runGhc (Just libdir) getSessionDynFlags
    let ppr_str = showPpr dflags obj
    return ppr_str

main :: IO ()
main = do
    let proj = "/home/celery/Desktop/hue"
    let src  = proj ++ "/FooBar.hs"
    res <- mkIOStr =<< mkSTG proj src
    putStrLn res

I am not sure what the best way to recover a ModSummary (and hence the ModuleName) from a Target is, but I do vaguely remember it being the first element of the ModuleGraph, which is defined as type ModuleGraph = [ModSummary].

The type signature for corePrepPgm is also different between GHC 7 and 8:

https://downloads.haskell.org/~ghc/7.10.1/docs/html/libraries/ghc-7.10.1/CorePrep.html

https://downloads.haskell.org/~ghc/latest/docs/html/libraries/ghc-8.0.1/CorePrep.html

Suggestions for improvement are welcome :)

EDIT: I have found instances of counter examples to this -- the head of a ModuleGraph is not always the target. My current workaround is to see if any ModSummary within the ModuleGraph contains a location which matches that of the initial source file location.