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Calling Haskell from C

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(fix filenames; one more standard #include; update to a GHC need (hs_add_root); another compile command)
(Remove unneeded code indentation)
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<haskell>
<haskell>
-
{-# LANGUAGE ForeignFunctionInterface #-}
+
{-# LANGUAGE ForeignFunctionInterface #-}
-
module Safe where
+
module Safe where
-
import Foreign.C.Types
+
import Foreign.C.Types
-
fibonacci :: Int -> Int
+
fibonacci :: Int -> Int
-
fibonacci n = fibs !! n
+
fibonacci n = fibs !! n
-
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
+
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
-
fibonacci_hs :: CInt -> CInt
+
fibonacci_hs :: CInt -> CInt
-
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
+
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
-
foreign export ccall fibonacci_hs :: CInt -> CInt
+
foreign export ccall fibonacci_hs :: CInt -> CInt
</haskell>
</haskell>
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And call it from C:
And call it from C:
-
<haskell>
+
#include <HsFFI.h>
-
#include <HsFFI.h>
+
#ifdef __GLASGOW_HASKELL__
-
#ifdef __GLASGOW_HASKELL__
+
#include "Safe_stub.h"
-
#include "Safe_stub.h"
+
extern void __stginit_Safe(void);
-
extern void __stginit_Safe(void);
+
#endif
-
#endif
+
#include <stdio.h>
-
#include <stdio.h>
+
-
 
+
int main(int argc, char *argv[])
-
int main(int argc, char *argv[]) {
+
{
-
int i;
+
int i;
-
hs_init(&argc, &argv);
+
hs_init(&argc, &argv);
-
#ifdef __GLASGOW_HASKELL__
+
#ifdef __GLASGOW_HASKELL__
-
hs_add_root(__stginit_Safe);
+
hs_add_root(__stginit_Safe);
-
#endif
+
#endif
-
 
+
-
i = fibonacci_hs(42);
+
i = fibonacci_hs(42);
-
printf("Fibonacci: %d\n", i);
+
printf("Fibonacci: %d\n", i);
-
 
+
-
hs_exit();
+
hs_exit();
-
return 0;
+
return 0;
-
}
+
}
-
</haskell>
+
Now, first compile the Haskell file:
Now, first compile the Haskell file:
-
$ ghc -c -O Safe.hs
+
$ ghc -c -O Safe.hs
Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, which you import into
Which creates Safe_stub.c, Safe_stub.o, Safe_stub.h, which you import into
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the Haskell objects on the command line:
the Haskell objects on the command line:
-
$ ghc -optc-O test.c Safe.o Safe_stub.o -o test
+
$ 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
+
(Alternatively, fewer files to enumerate: <code>ghc --make -no-hs-main -optc-O test.c Safe -o test</code>)
-
)
+
Then run your C code:
Then run your C code:
-
$ ./test
+
$ ./test
-
Fibonacci: 267914296
+
Fibonacci: 267914296
And that's it.
And that's it.
[[Category:Tutorials]]
[[Category:Tutorials]]

Revision as of 09:48, 5 March 2012

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.

Retrieved from "http://www.haskell.org/haskellwiki/Calling_Haskell_from_C"

Category: Tutorials