[Haskell-cafe] Calling a C function that returns a struct by value

Matthew Steele mdsteele at alum.mit.edu
Wed May 25 22:19:50 CEST 2011

 From Haskell, I want to call a C function that returns a struct by  
value (rather than, say, returning a pointer).  For example:

   typedef struct { double x; double y; } point_t;
   point_t polar(double theta);

I can create a Haskell type Point and make it an instance of Storable  
easily enough:

   data Point = Point CDouble CDouble
   instance Storable Point where -- insert obvious code here

And now I want to do something like this:

   foreign import ccall unsafe polar :: CDouble -> IO Point

...but that doesn't appear to be legal (at least in GHC 6.12).  Is  
there any way to import this C function into Haskell _without_ having  
any additional wrapper C code?  I mean, I suppose I could write  
something like:

   // C:
   void polar_wrapper(double theta, point_t* output) {
     *output = polar(theta);
   -- Haskell:
   foreign import ccall unsafe polar_wrapper :: CDouble -> Ptr Point - 
 > IO ()
   polar :: CDouble -> IO Point
   polar theta = do -- insert obvious code here

...but that's a lot of extra boilerplate if I have many such functions  
to wrap, and it seems like GHC should be able to do that sort of thing  
for me.  P-:


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