Hi,

I was trying to write a FFI wrapper for my Haskell program which manipulates
ByteString. But I am unable to compile/link it.

Here is the toy program.

{-# LANGUAGE ForeignFunctionInterface #-}

module B where

import Foreign.C.Types
import Foreign.C.String
import qualified Data.ByteString as BS

rev :: BS.ByteString -> BS.ByteString
rev bstr = BS.reverse bstr

rev_hs :: CString -> IO CString
rev_hs cstr =
    do { bstr <- BS.packCString cstr
       ; let bstr' = rev bstr
       ; cstr' <- newCString (show bstr')
       ; return cstr'
       }

foreign export ccall rev_hs :: CString -> IO CString

And here is the C counter-part.

#include "B_stub.h"
#include <stdio.h>

int main(int argc, char *argv[]) {
char *str;
hs_init(&argc, &argv);

str = rev_hs("it works.");
printf("Rev: %s\n", str);

hs_exit();
return 0;
}

Compiling B.hs alone seems fine, but errors popped up when I was trying to compile/link it with C.

$ ghc -c -O B.hs

$ ghc -optc-O test_b.c B.o B_stub.o -o test_b
Undefined symbols:
"___stginit_bytestringzm0zi9zi1zi4_DataziByteString_", referenced from:
      ___stginit_Lib_ in B.o
"_bytestringzm0zi9zi1zi4_DataziByteString_zdwreverse_info", referenced from:
      _s19w_info in B.o
"_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_info", referenced from:
      _s19v_info in B.o
"_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_closure", referenced from:
      _Lib_zdwa_srt in B.o
"_bytestringzm0zi9zi1zi4_DataziByteString_zdwa4_info", referenced from:
      _Lib_zdwa_info in B.o
"_bytestringzm0zi9zi1zi4_DataziByteString_reverse_info", referenced from:
      _Lib_rev_info in B.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

If I replace ByteString with the ordinary String, the above programs can be compiled and linked.

Can someone tell me what I did wrong here?

-Kenny