A foreign function interface wrapper
====================================

This example demonstrates how to compile an executable that uses the
FFI to export a Haskell function to C.  This is currently highly
inelegant, as we really ought to do this automatically.
Unfortunately, the dependency-generation of ghc apparently ignores
these files.  :(

The C source
------------

Our C example is a simple function that accepts a function returning
an int and prints out its value.

file: tests/foreign-wrapper/printit.c
    #include <stdio.h>
    
    void printit(int (*x)()) { printf("It is %d\n", x()); }

The Haskell source
------------------

We call this C function giving it a very simple

file: tests/foreign-wrapper/test.hs
    module Main where
    
    import Foreign.C.Types ( CInt )
    import Foreign.Ptr ( FunPtr )
    
    foreign import ccall "wrapper" wrap :: IO CInt
                                        -> IO (FunPtr (IO CInt))
    foreign import ccall "printit" printit
          :: FunPtr (IO CInt) -> IO ()
    
    main = do fp <- wrap (return 137)
              printit fp

The Setup.hs file
-----------------

Here's the ``Setup.hs`` file.  As you can see, we have to manually

file: tests/foreign-wrapper/Setup.hs
    import Distribution.Franchise
    
    main = build [] (ghcFlags ["-fffi"]) $
           executable "test" "test.hs" ["printit.c","test_stub.o"]

A simple test script
--------------------

file: tests/foreign-wrapper/foreign-wrapper.sh
....set -ev
    runghc Setup.hs build
...../test
    ./test | grep 137
\end{file}