TLDR -- Calling D code from Haskell through the FFI works with DMD but not with GDC or LDC2.

The time consuming version:

Since D allows C to directly call it, if the D code is wrapped in extern (C){ ... }, I thought it should be possible to call such D code from Haskell using the FFI.

The FFI in Haskell allows directly calling C code given a header file, the name of a function, and its C-type.  So I made a D file with the functions I want to call, FunctionsInD.d

    extern (C){
        int d_f(int x){
            return x+2;
        }
        int d_g(int x, int y){
            return x+y;
        }
    }

Then made a header file, FunctionsInD.h

    int d_f(int x);
    int d_g(int x, int y);

Then the Haskell wrapper ToD.hs:

    module ToD where

    import Foreign.C

    foreign import ccall "FunctionsInD.h d_f"
        c_f :: CInt -> CInt

    foreign import ccall "FunctionsInD.h d_g"
        c_g :: CInt -> CInt -> CInt

    h_f :: Int -> Int -- the "pure" haskell version
    h_f x = fromIntegral (c_f (fromIntegral x))

    h_g :: Int -> Int -> Int
    h_g x y = fromIntegral (c_g (fromIntegral x) (fromIntegral y))

For reasons I don't completely understand, you also need a fake main function, dummy.d:

    void main(){}

And a driver for test, Main.hs:

    module Main where

    import ToD

    main :: IO ()
    main = do
        let (a,b) = (h_f 3, h_g 3 4)
        sequence_ [putStrLn (show a), putStrLn (show b)]

To put this all together, here is the makefile.

    DC = dmd
    # DC = ldc2
    # DC = ldmd2
    # DC = gdc

    main: FunctionsInD.o FunctionsInD.h dummy.o
        ghc Main.hs --make -o main_ffi_test FunctionsInD.o dummy.o -lphobos2

    clean:
        rm FunctionsInD.o dummy.o main_ffi_test

    FunctionsInD.o FunctionsInD.d:
        $(DC) -c FunctionsInD.d

    dummy.o: dummy.d
        $(DC) -c dummy.d

The question: if I comment out DC=dmd, and uncomment any of the other options, the build fails.  The problem is that ghc complains of multiple definitions of main (not with DMD only GDC and LDC2).  If I remove dummy.o from the ghc line, I get, undefined reference to _DMain.

But wait!  It gets weirder.  I can compile FunctionsInD.d with GDC or LDC2, as long as I compile dummy.d with DMD, everything works fine.

Once I get through this hump, I will try to get on with more interesting interfaces, like strings and structs.

Don't forget to call rt_init: http://dlang.org/phobos/core_runtime.html#.rt_init

On Monday, 4 August 2014 at 21:10:46 UTC, safety0ff wrote:
> Don't forget to call rt_init: http://dlang.org/phobos/core_runtime.html#.rt_init

Where/when should I call this?

On Monday, 4 August 2014 at 21:14:17 UTC, Jon wrote:
> On Monday, 4 August 2014 at 21:10:46 UTC, safety0ff wrote:
>> Don't forget to call rt_init: http://dlang.org/phobos/core_runtime.html#.rt_init
>
> Where/when should I call this?

Before calling any D functions, but usually it's simplest to call it early in main.

It initializes the GC and notifies the D runtime of its existence.
For simple D functions you might get away without calling it.

I get Error: core.runtime.rt_init is private.  And Error: core.runtime.init is not accessible.

On Monday, 4 August 2014 at 21:22:37 UTC, safety0ff wrote:
> On Monday, 4 August 2014 at 21:14:17 UTC, Jon wrote:
>> On Monday, 4 August 2014 at 21:10:46 UTC, safety0ff wrote:
>>> Don't forget to call rt_init: http://dlang.org/phobos/core_runtime.html#.rt_init
>>
>> Where/when should I call this?
>
> Before calling any D functions, but usually it's simplest to call it early in main.
>
> It initializes the GC and notifies the D runtime of its existence.
> For simple D functions you might get away without calling it.

On Monday, 4 August 2014 at 21:35:21 UTC, Jon wrote:
> I get Error: core.runtime.rt_init is private.  And Error: core.runtime.init is not accessible.
>

I would add them to the header and Haskell wrapper (FunctionsInD.h and ToD.hs.)
The signatures are:
int rt_init();
int rt_term();

When it is linked it will find the symbols in druntime.

Yes, thank you.  That is exactly what I did.

On Monday, 4 August 2014 at 21:48:40 UTC, safety0ff wrote:
> On Monday, 4 August 2014 at 21:35:21 UTC, Jon wrote:
>> I get Error: core.runtime.rt_init is private.  And Error: core.runtime.init is not accessible.
>>
>
> I would add them to the header and Haskell wrapper (FunctionsInD.h and ToD.hs.)
> The signatures are:
> int rt_init();
> int rt_term();
>
> When it is linked it will find the symbols in druntime.

As a note, I can interact with strings as expected, but working with structs looks like it will take a little bit of work.

On Monday, 4 August 2014 at 22:17:36 UTC, Jon wrote:
> Yes, thank you.  That is exactly what I did.
>
> On Monday, 4 August 2014 at 21:48:40 UTC, safety0ff wrote:
>> On Monday, 4 August 2014 at 21:35:21 UTC, Jon wrote:
>>> I get Error: core.runtime.rt_init is private.  And Error: core.runtime.init is not accessible.
>>>
>>
>> I would add them to the header and Haskell wrapper (FunctionsInD.h and ToD.hs.)
>> The signatures are:
>> int rt_init();
>> int rt_term();
>>
>> When it is linked it will find the symbols in druntime.

On Monday, 4 August 2014 at 20:48:09 UTC, Jon wrote:

> For reasons I don't completely understand, you also need a fake main function, dummy.d:
>
>     void main(){}
>

Note that this is not necessary if you compile with -lib e.g.:

  dmd -lib -oflibtest.a test.d

and then

  ghc Main.hs --make -omain libtest.a

I don't have gdc or ldc installed but as far as I know ldc has a -lib flag as well.

Oh great thank you.  I think that might solve the majority of the confusion I was having.  One thing I can't figure out though, is getting garbage collection to work as expected.  If I have a function that allocates a pointer to a struct using new, I get an error on linking _dAlloc... not found.  But maybe compiling as a lib will solve this too.

On Tuesday, 5 August 2014 at 21:28:08 UTC, David Soria Parra wrote:
> On Monday, 4 August 2014 at 20:48:09 UTC, Jon wrote:
>
>> For reasons I don't completely understand, you also need a fake main function, dummy.d:
>>
>>    void main(){}
>>
>
> Note that this is not necessary if you compile with -lib e.g.:
>
>   dmd -lib -oflibtest.a test.d
>
> and then
>
>   ghc Main.hs --make -omain libtest.a
>
> I don't have gdc or ldc installed but as far as I know ldc has a -lib flag as well.