mail:
> Is there a way to write some of the functions in Haskell and then use
> them in my C code via some kind of interface?
Using C just for IO is a bit weird -- perhaps you could illustrate
the kind of IO you're doing? Learning how to do IO in Haskell is
a much safer solution that linking the Haskell runtime into your
C program.
That said, this is done by using 'foreign export' declarations
in your Haskell code, then linking the compiled Haskell objects
into your C code, as follows:
We define the fibonacci function in Haskell:
{-# LANGUAGE ForeignFunctionInterface #-}
module Safe where
import Foreign.C.Types
fibonacci :: Int -> Int
fibonacci n = fibs !! n
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
foreign export ccall fibonacci_hs :: CInt -> CInt
And call it from C:
#include "A_stub.h"
#include <stdio.h>
int main(int argc, char *argv[]) {
int i;
hs_init(&argc, &argv);
i = fibonacci_hs(42);
printf("Fibonacci: %d\n", i);
hs_exit();
return 0;
}
Now, first compile the Haskell file:
$ ghc -c -O A.hs
Which creates some *.c and *.h headers, which you import into
your C program. Now compile your C code with ghc (!), passing
the Haskell objects on the command line:
$ ghc -optc-O test.c A.o A_stub.o -o test
How run your C code:
$ ./test
Fibonacci: 267914296
And that's it.
-- Don
P.S. Its easier to learn how to do IO in Haskell :)
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