calling FunPtr's?

[Alastair Reid]

The simplest would be to generate some C code at runtime, pass it to gcc and 
dynamically load the resulting .o file. 

Since the Storable class is for marshalling Haskell values into and out of 
contiguous memory, the C code you need would read values out of contiguous 
memory, pass them as arguments to your C function, then copy the result back 
into a block of contiguous memory.

Details:

1) It's going to be challenging to use the resulting Haskell functions because 
Haskell's typechecker won't know their type at runtime.  You'll probably need 
to use something like the Dynamic library's runtime typechecking.


2) If you don't want to invoke gcc, you could look at code from StgHugs (badly 
bitrotted):

http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/rts/Attic/ForeignCall.c

Versions around 1.20 seem to be about the best.

This provided a function with type something like the following:

     HsThunk mkFun(char* argtype, char* restype, HsFunPtr fun)

The two string arguments contained encodings of the argument and result types.  
For example, "II" translated to the C type "int, int".


Finally, the Storable class doesn't seem quite right:

1) You can't use type classes with types that aren't known
   until runtime.
   (Well, you can but you have to jump through hoops to do it.)

   Easier to simply have a lookup table from type names to 
   marshalling help code.

2) You almost certainly want to map a C type to a Haskell type.
   Storable is much more setup for mapping a Haskell type
   onto a list of C types.  Fighting or ignoring that detail 
   will require some more hoop jumping.


Hope this helps,

--
Alastair Reid         www.haskell-consulting.com


On Friday 17 October 2003 11:09 am, John Meacham wrote:
> I need to call some dynamically linked routines recieved via
> Posix.DynamicLinker but am unsure how to do this from haskell. I do not
> know the types of the functions I wish to call at compile time so can't
> create appropriate foreign import statements. What would be nice is
> something like
>
> callFunPtr  :: (Storable a, Storable b) => FunPtr z -> a -> IO b
>
> with a being a tuple if I want to push multiple arguments onto the
> stack perhaps.
>
> this function should be straightforward to implement, as it just needs
> to place the arguments on the stack in the appropriate fashion and pull
> the return value of out the proper register or memory location, but
> would obviously be very architecture specific so should be provided by
> the compiler. does something like this exist? is there another way to do
> this?
>         John