Call Haskell from Python, mini project after https://wiki.python.org/moin/PythonVsHaskell

README.md

Haskell Python Interface

implemented after:

https://wiki.python.org/moin/PythonVsHaskell

Geometry.hs

{-# LANGUAGE ForeignFunctionInterface #-}
module Geometry where
 
import Foreign.C.Types
import Foreign.C.String
 
add :: Num a => a -> a -> a
add x y = x + y


f1 :: CInt -> IO CInt
f1 x = do
    return (42 + x)
 
f2 :: CFloat -> IO CFloat
f2 x = do
    return (10.0 + x)

f3 :: CFloat -> IO CFloat
f3 x = do
    return (add 10.0 x)
    
f4 :: CString -> IO CString
f4 s = do
    w <- peekCString s
    newCString (w  ++ " world!")

foreign export ccall
    f1 :: CInt -> IO CInt

foreign export ccall
    f2 :: CFloat -> IO CFloat

foreign export ccall
    f3 :: CFloat -> IO CFloat

foreign export ccall
    f4 :: CString -> IO CString

makefile

HSRTS_LIB=HSrts-ghc8.2.2
MODULE_NAME=Geometry

all: build_lib clean_objects test_lib
build_lib:
	stack exec -- ghc -O2 -dynamic -shared -fPIC --make -no-hs-main \
		-optl '-shared' '-l$(HSRTS_LIB)' -optc '-DMODULE=$(MODULE_NAME)' \
		-o Geometry.so Geometry.hs module_init.c
clean_objects:
	rm *.o
	rm *.hi
	rm *_stub.h
test_lib:
	python run_lib.py
clean:
	rm *.so

module_init.c

#define CAT(a,b) XCAT(a,b)
#define XCAT(a,b) a ## b
#define STR(a) XSTR(a)
#define XSTR(a) #a

#include <HsFFI.h>

extern void CAT (__stginit_, MODULE) (void);

static void library_init (void) __attribute__ ((constructor));
static void
library_init (void)
{
  /* This seems to be a no-op, but it makes the GHCRTS envvar work. */
  static char *argv[] = { STR (MODULE) ".so", 0 }, **argv_ = argv;
  static int argc = 1;

  hs_init (&argc, &argv_);
  hs_add_root (CAT (__stginit_, MODULE));
}

static void library_exit (void) __attribute__ ((destructor));
static void
library_exit (void)
{
  hs_exit ();
}

run_lib.py

from ctypes import *

lib = cdll.LoadLibrary('./Geometry.so')

funcs = {
#    name   restype    argtypes    input  expected value
    'f1':   (c_int,    [c_int],    (10,   52)),
    'f2':   (c_float,  [c_float],  (10.0, 20.0)),
    'f3':   (c_float,  [c_float],  (11.0, 21.0)),
    'f4':   (c_char_p, [c_char_p], ("hello", "hello world!")),
}

for func in funcs:
    f = getattr(lib, func)
    f.restype, f.argtypes, test = funcs[func]
    input, expected = test
    assert f(input) == expected
    print('{0}({1}) == {2}'.format(func, input, expected))