[Haskell-beginners] Compiling shared (dll) library

[Daniel Fischer]

On Monday 05 December 2011, 13:44:25, Alexander.Vladislav.Popov wrote:
> > Why the separate compilation? Can't you compile them in one go?
> 
>  Sorry for a stupid question, how is it?

Not stupid at all.

I'm not an expert in matters FFI, so I don't know if it works in all 
situations, but for simple cases at least, you can compile in one go 
listing .hs and .c files on the command line.

For a foreign import:

// hsFibo.hs:
{-# LANGUAGE ForeignFunctionInterface #-}
module Main (main) where

import System.Environment (getArgs)

foreign import ccall unsafe "fibo"
    c_fibo :: Int -> Int

main :: IO ()
main = do
    args <- getArgs
    let n = case args of
              (a:_) -> read a
              _ -> 14
    print (c_fibo n)

// Fibonacci.c
int fibo(int n){
    int a = (n&1) ? 0 : 1, b = (n&1) ? 1 : 0;
    while(n > 1){
        a += b;
        b += a;
        n -= 2;
    }
    return b;
}

$ ghc -O2 hsFibo.hs Fibonacci.c 
[1 of 1] Compiling Main             ( hsFibo.hs, hsFibo.o )
Linking hsFibo ...
$ ./hsFibo 21
10946

For a foreign export it's a bit more complicated, as you would have to 
generate the .h file(s) first by other means (if all your exported 
functions are compatible with the implicit types for C functions [pre C99, 
iirc], you can get away without the header, but you'll get warnings about 
implicit declarations). But with appropriate headers, you can compile in 
one go,

$ ghc -O2 -shared -dynamic -fPIC Export.hs useExport.c -o theexport.so

(if you use a C main to create an executable, also pass -no-hs-main).

> 
> And one more question, please: how I can export genexPure :: [String] ->
> [String] to achieve all preferences of lazy list? Not all array at once,
> but to get someting like an iterator, what I can call to get new genex.

Pass. I have no idea how to do that.