The foreign interface consists of the following components: The Foreign Function Interface language specification (which constitutes most of this Chapter, beginning with ). You must use the -fglasgow-exts command-line option to make GHC understand the foreign declarations defined by the FFI. The Foreign module (see ) collects together several interfaces which are useful in specifying foreign language interfaces, including the following: The ForeignObj module (see ), for managing pointers from Haskell into the outside world. The StablePtr module (see ), for managing pointers into Haskell from the outside world. The CTypes module (see ) gives Haskell equivalents for the standard C datatypes, for use in making Haskell bindings to existing C libraries. The CTypesISO module (see ) gives Haskell equivalents for C types defined by the ISO C standard. The Storable library, for primitive marshalling of data types between Haskell and the foreign language. &ffi-body; The following sections also give some hints and tips on the use of the foreign function interface in GHC. foreign export dynamicwith GHC When GHC compiles a module (say M.hs) which uses foreign export dynamic, it generates two additional files, M_stub.c and M_stub.h. GHC will automatically compile M_stub.c to generate M_stub.o at the same time. The C file M_stub.c contains small helper functions used by the code generated for the foreign export dynamic, so it must be linked in to the final program. When linking the program, remember to include M_stub.o in the final link command line, or you'll get link errors for the missing function(s) (this isn't necessary when building your program with ghc --make, as GHC will automatically link in the correct bits). C calls, function headers When generating C (using the -fvia-C directive), one can assist the C compiler in detecting type errors by using the -#include directive () to provide .h files containing function headers. For example, #include "HsFFI.h" void initialiseEFS (HsInt size); HsInt terminateEFS (void); HsForeignObj emptyEFS(void); HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x); HsInt lookupEFS (HsForeignObj a, HsInt i); The types HsInt, HsForeignObj etc. are described in . Note that this approach is only essential for returning floats (or if sizeof(int) != sizeof(int *) on your architecture) but is a Good Thing for anyone who cares about writing solid code. You're crazy not to do it.