To build this, do the following:
Make the C++ test library (the make install step installs it to
/usr/local/c++-ffi-example which you can clean up when you're done):
cd c++-lib
make
sudo make install
Make the Haskell library and test program:
cd ..
cabal configure
cabal build
Run the test program (setting up the shared library load path to pick up the C++ library):
export LD_LIBRARY_PATH=/usr/local/c++-ffi-example/lib:$LD_LIBRARY_PATH
./dist/build/tst-prog/tst-prog