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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(2 intermediate revisions by 2 users not shown) | |||
Line 29: | Line 29: | ||
#ifdef __GLASGOW_HASKELL__ |
#ifdef __GLASGOW_HASKELL__ |
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#include "Safe_stub.h" |
#include "Safe_stub.h" |
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− | extern void __stginit_Safe(void); |
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#endif |
#endif |
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#include <stdio.h> |
#include <stdio.h> |
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Line 37: | Line 36: | ||
int i; |
int i; |
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hs_init(&argc, &argv); |
hs_init(&argc, &argv); |
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− | #ifdef __GLASGOW_HASKELL__ |
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− | hs_add_root(__stginit_Safe); |
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− | #endif |
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i = fibonacci_hs(42); |
i = fibonacci_hs(42); |
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Line 56: | Line 52: | ||
the Haskell objects on the command line: |
the Haskell objects on the command line: |
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+ | If GHC version < 7.2 |
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$ ghc -optc-O test.c Safe.o Safe_stub.o -o test |
$ ghc -optc-O test.c Safe.o Safe_stub.o -o test |
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+ | else |
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− | |||
− | Note, this will not work for ghc >= 7.2 (thanks to Daniel Fischer for pointing this out), use the following command instead: |
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− | |||
$ ghc --make -no-hs-main -optc-O test.c Safe -o test |
$ ghc --make -no-hs-main -optc-O test.c Safe -o test |
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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.