Difference between revisions of "Calling Haskell from C"

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(Delete lines in C file which prevents linking in GHC 8.6.5 (apparently not needed anymore))
 
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#ifdef __GLASGOW_HASKELL__
 
#ifdef __GLASGOW_HASKELL__
 
#include "Safe_stub.h"
 
#include "Safe_stub.h"
extern void __stginit_Safe(void);
 
 
#endif
 
#endif
 
#include <stdio.h>
 
#include <stdio.h>
Line 37: Line 36:
 
int i;
 
int i;
 
hs_init(&argc, &argv);
 
hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
 
hs_add_root(__stginit_Safe);
 
#endif
 
 
 
 
i = fibonacci_hs(42);
 
i = fibonacci_hs(42);
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the Haskell objects on the command line:
 
the Haskell objects on the command line:
   
  +
If GHC version < 7.2
 
$ ghc -optc-O test.c Safe.o Safe_stub.o -o test
 
$ ghc -optc-O test.c Safe.o Safe_stub.o -o test
 
  +
else
(Alternatively, fewer files to enumerate: <code>ghc --make -no-hs-main -optc-O test.c Safe -o test</code>)
+
$ ghc --make -no-hs-main -optc-O test.c Safe -o test
   
 
Then run your C code:
 
Then run your C code:

Latest revision as of 17:08, 27 June 2019

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"
#endif
#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 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:

If GHC version < 7.2

$ ghc -optc-O test.c Safe.o Safe_stub.o -o test

else

$ ghc --make -no-hs-main -optc-O test.c Safe -o test

Then run your C code:

$ ./test 
Fibonacci: 267914296

And that's it.