Question

I want to create a haskell interpreter that I can use from C++ on linux.

I have a file FFIInterpreter.hs which implements the interpreter in haskell and exports the functions via FFI to C++.

module FFIInterpreter where

import Language.Haskell.Interpreter

import Data.IORef
import Foreign.StablePtr
import Foreign.C.Types
import Foreign.C.String
import Control.Monad
import Foreign.Marshal.Alloc

type Session = Interpreter ()
type Context = StablePtr (IORef Session)

foreign export ccall createContext :: CString -> IO Context
createContext :: CString -> IO Context
createContext name = join ((liftM doCreateContext) (peekCString name))
  where
    doCreateContext :: ModuleName -> IO Context
    doCreateContext name 
      = do let session = newModule name 
           _ <- runInterpreter session
           liftIO $ newStablePtr =<< newIORef session

newModule :: ModuleName -> Session
newModule name = loadModules [name] >> setTopLevelModules [name]

foreign export ccall freeContext :: Context -> IO ()
freeContext :: Context -> IO ()
freeContext = freeStablePtr

foreign export ccall runExpr :: Context -> CString -> IO CString
runExpr :: Context -> CString -> IO CString
runExpr env input = join ((liftM newCString) (join (((liftM liftM) doRunExpr) env (peekCString input))))
  where
    doRunExpr :: Context -> String -> IO String
    doRunExpr env input
      = do env_value <- deRefStablePtr env
           tcs_value <- readIORef env_value
           result    <- runInterpreter (tcs_value >> eval input)
           return $ either show id result

foreign export ccall freeString :: CString -> IO ()
freeString :: CString -> IO ()
freeString = Foreign.Marshal.Alloc.free

When I compile the whole project with ghc, everything works fine. I use the following command:

ghc -no-hs-main FFIInterpreter.hs main.cpp -lstdc++

But the haskell module is only a small piece of the C++ project and I don’t want the whole project to depend on ghc.

So I want to build a dynamic library with ghc and then link it to the project using g++.

$ ghc -shared -fPIC FFIInterpreter.hs module_init.c -lstdc++
[1 of 1] Compiling FFIInterpreter   ( FFIInterpreter.hs, FFIInterpreter.o )
Linking a.out ...
/usr/bin/ld: /usr/lib/haskell-packages/ghc/lib/hint-0.3.3.2/ghc-7.0.3/libHShint-0.3.3.2.a(Interpreter.o): relocation R_X86_64_32S against `.data' can not be used when making a shared object; recompile with -fPIC
/usr/lib/haskell-packages/ghc/lib/hint-0.3.3.2/ghc-7.0.3/libHShint-0.3.3.2.a: could not read symbols: Bad value
collect2: ld gab 1 als Ende-Status zurück

So I added the -dynamic keyword, but that also doesn’t work:

$ ghc -dynamic -shared -fPIC FFIInterpreter.hs librarymain.cpp -lstdc++
FFIInterpreter.hs:3:8:
    Could not find module `Language.Haskell.Interpreter':
      Perhaps you haven't installed the "dyn" libraries for package `hint-0.3.3.2'?
      Use -v to see a list of the files searched for.

I searched my system for Interpreter.dyn_hi but didn’t find it. Is there a way to get it?
I also tried to install hint manually, but this also doesn’t deliver the Interpreter.dyn_hi file.

Answer 1

You have to install the library (and all it depends on) with the --enable-shared flag (using cabal-install) to get the .dyn_hi and .dyn_o files. You may consider setting that option in your ~/.cabal/config file.

Perhaps the easiest way is to uncomment the shared: XXX line in ~/.cabal/config, set the option to True and

cabal install –reinstall world

For safety, run that with the --dry-run option first to detect problems early. If the --dry-run output looks reasonable, go ahead and reinstall – it will take a while, though.