crash caused by generic visitor (?)

[Ranjit Jhala]

Hi all, 

my apologies. Looks like the issue (and a fix!) is described here

  http://mistuke.wordpress.com/category/vsx/

Thanks,

Ranjit.

On May 13, 2011, at 4:34 PM, Ranjit Jhala wrote:

> Hi all,
> 
> I'm trying to extract the set of identifiers that are read in given 
> source file.  To this end, I wrote the following code (full source at end.)
> 
> ------------------------------------------------------------------------
> main 
>  = do fname   <- (!! 0) `fmap` getArgs
>       tcm     <- loadTypecheckedSource fname
>       putStrLn $ showPpr tcm           -- this works fine
>       putStrLn $ showPpr $ allIds tcm  -- this causes the crash
>       return ()
> 
> allIds ::  Data a => a -> [Id]
> allIds = listify (\x -> case (x :: Id) of _ -> True)
> ------------------------------------------------------------------------
> 
> and where:
> 
> 	loadTypecheckedSource ::  FilePath -> IO TypecheckedSource
> 
> unfortunately, when I compile and run it, I get the dreaded:
> 
> 	Bug: Bug: panic! (the 'impossible' happened)
>  	(GHC version 7.0.3 for i386-unknown-linux):
> 		placeHolderNames
> 
> 	Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
> 
> Turns out that the problem is when the file contains a type annotation.
> That is, 
> 
> 	./Bug Test00.hs
> 
> crashes, when Test00.hs is:
> 
> 	module Test where
> 	
> 	x :: Int
> 	x = 0
> 
> but does not crash when the file is:
> 
> 	module Test where
> 	
> 	x = 0
> 
> Can anyone tell me why listify chokes in the latter case? (And how one might 
> get around the problem?) I include the full source below (compiled with: ghc --make Bug, using ghc 7.0.3)
> 
> Thanks!
> 
> Ranjit.
> 
> ------------------------------------------------------------------------------------------------------
> import GHC 
> import Outputable
> import DynFlags (defaultDynFlags)
> import GHC.Paths (libdir)
> 
> import System.Environment (getArgs)
> import Control.Monad
> import qualified Data.List as L
> import Data.Data
> import Data.Generics.Schemes (listify)
> 
> main 
>  = do fname   <- (!! 0) `fmap` getArgs
>       tcm     <- loadTypecheckedSource fname
>       putStrLn $ showPpr tcm           -- this works fine
>       putStrLn $ showPpr $ allIds tcm  -- this causes the crash
>       return ()
> 
> allIds ::  Data a => a -> [Id]
> allIds = listify (\x -> case (x :: Id) of _ -> True)
> 
> loadTypecheckedSource ::  FilePath -> IO TypecheckedSource
> loadTypecheckedSource fname 
>  = defaultErrorHandler defaultDynFlags $ 
>      runGhc (Just libdir) $ do
>        df  <- getSessionDynFlags
>        setSessionDynFlags df
>        tgt <- guessTarget fname Nothing
>        setTargets [tgt]
>        load LoadAllTargets 
>        res <- load LoadAllTargets
>        if failed res
>          then pprPanic "Load Failed!!" (text "AAARGH!")
>          else tm_typechecked_source `fmap` getTypecheckedModule fname 
> 
> findModSummary ::  GhcMonad m => FilePath -> m ModSummary
> findModSummary fname 
>  = do msums <- depanal [] False
>       case L.find ((fname ==) . ms_hspp_file) msums of 
>         Just msum -> return msum
>         Nothing   -> pprPanic "ModuleName Lookup Failed!!" (text "AARGHC!")
> 
> getTypecheckedModule :: GhcMonad m => FilePath -> m TypecheckedModule
> getTypecheckedModule = findModSummary >=> parseModule >=> typecheckModule
> 
> 
>