minimal example of building a DLL with GHC Haskell and calling it

hsdll.def

EXPORTS
  fmaInt
  hs_init
  hs_exit

hsdll.hs

{-# LANGUAGE ForeignFunctionInterface #-}

module HsDll where

import Foreign.C.Types

foreign export ccall fmaInt :: CInt -> CInt -> CInt -> CInt

fmaInt :: CInt -> CInt -> CInt -> CInt
fmaInt x y z = x * y + z

Makefile

# there is probably a way to get this from "stack path"
INCLUDE=C:\Users\Derrick\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.4\lib\include

all: hsdll.dll hsdll_stub.h usehsdll.exe

%.dll %_stub.h: %.hs %.def
	stack ghc -- $^ --make -O2 -shared -static -o $@

%.exe: %.c hsdll_stub.h hsdll.dll
	gcc -O2 -I$(INCLUDE) -Wall -Wextra -std=c11 -pedantic $^ hsdll.dll -o $@

clean:
	-rm *.dll *.exe

usehsdll.c

#include <stdio.h>
#include "hsdll_stub.h"

int main(void)
{
    hs_init(0, 0);
    printf("fmaInt(3, 4, 5) = %d\n", fmaInt(3, 4, 5));
    hs_exit();
    return 0;
}