Haskell Language Foreign Function Interface Calling C from Haskell
Example
For performance reasons, or due to the existence of mature C libraries, you may want to call C code from a Haskell program. Here is a simple example of how you can pass data to a C library and get an answer back.
foo.c:
#include <inttypes.h>
int32_t foo(int32_t a) {
return a+1;
}
Foo.hs:
import Data.Int
main :: IO ()
main = print =<< hFoo 41
foreign import ccall unsafe "foo" hFoo :: Int32 -> IO Int32
The unsafe keyword generates a more efficient call than 'safe', but requires that the C code never makes a callback to the Haskell system. Since foo is completely in C and will never call Haskell, we can use unsafe.
We also need to instruct cabal to compile and link in C source.
foo.cabal:
name: foo
version: 0.0.0.1
build-type: Simple
extra-source-files: *.c
cabal-version: >= 1.10
executable foo
default-language: Haskell2010
main-is: Foo.hs
C-sources: foo.c
build-depends: base
Then you can run:
> cabal configure
> cabal build foo
> ./dist/build/foo/foo
42
