Calling Haskell from C
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It is not uncommon to want to call a Haskell function from C code. Here's how to do that.
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
Note the foreign export. When GHC sees this, it will generate stubs for C, to help it work out the Haskell types.
And call it from C:
#include <HsFFI.h>
#ifdef __GLASGOW_HASKELL__
#include "Safe_stub.h"
extern void __stginit_Safe(void);
#endif
#include <stdio.h>
int main(int argc, char *argv[]) {
int i;
hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
hs_add_root(__stginit_Safe);
#endif
i = fibonacci_hs(42);
printf("Fibonacci: %d\n", i);
hs_exit();
return 0;
}
Now, first compile the Haskell file:
$ ghc -c -O Safe.hs
Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, 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 Safe.o Safe_stub.o -o test
(Alternatively, fewer files to enumerate:
$ ghc --make -no-hs-main -optc-O test.c Safe -o test
)
Then run your C code:
$ ./test Fibonacci: 267914296
And that's it.