Editing FFI complete examples (section)

== When main is in Python ==

=== Using ctypes ===

This is described at the Python wiki page
[https://wiki.python.org/moin/PythonVsHaskell PythonVsHaskell], look for "Using both Python & Haskell with ctypes"

=== Using swig (and a c wrapper) ===

This example is adapted from [http://www.swig.org/tutorial.html The Swig Tutorial]. The factorial function has been modified to call into Haskell.

Foo.hs:

<haskell>
{-# LANGUAGE ForeignFunctionInterface #-}
module Foo where

import Foreign.C.Types

foreign export ccall hs_fact :: CInt -> CInt
hs_fact :: CInt -> CInt
hs_fact n = product [1..n]
</haskell>

example.c:

<pre>
#include "Foo_stub.h"

void py_init(int argc, char *argv[]) {
    hs_init(&argc, &argv);
}

void py_exit() {
    hs_exit();
}

int fact(int n) {
    return hs_fact(n);
}
</pre>

example.i:

<pre>
%module example
%{

/* Put header files here or function declarations like below */
extern int fact(int n);
extern void py_init(int argc, char** argv);
extern void py_exit();
%}

%typemap(in) (int argc, char **argv) {
  /* Check if is a list */
  if (PyList_Check($input)) {
    int i;
    $1 = PyList_Size($input);
    $2 = (char **) malloc(($1+1)*sizeof(char *));
    for (i = 0; i < $1; i++) {
      PyObject *o = PyList_GetItem($input,i);
      if (PyString_Check(o))
    $2[i] = PyString_AsString(PyList_GetItem($input,i));
      else {
    PyErr_SetString(PyExc_TypeError,"list must contain strings");
    free($2);
    return NULL;
      }
    }
    $2[i] = 0;
  } else {
    PyErr_SetString(PyExc_TypeError,"not a list");
    return NULL;
  }
}

%typemap(freearg) (int argc, char **argv) {
  free((char *) $2);
}

extern int fact(int n);
extern void py_init(int argc, char** argv);
extern void py_exit();
</pre>

run.py:

<pre>
import example
import sys
example.py_init(sys.argv)
print example.fact(5)
example.py_exit()
</pre>

To build (you may need to change the include path if you are using a different installation of python):

<pre>
$ ghc -c -dynamic -fPIC Foo.hs
$ swig -python example.i
$ gcc -fpic -c example.c example_wrap.c -I/usr/include/python2.6 -I`ghc --print-libdir`/include
$ ghc -o _example.so -shared -dynamic -fPIC example.o example_wrap.o Foo.o Foo_stub.o -lHSrts-ghc6.12.3
$ python run.py
</pre>