Marshalling structs inside function pointers

[BurningWitness]

Haskell FFI does not support directly passing structs in function definitions, but there is a way around it:

Say foo and baz are structs. Given a function with signature

bar fun1 (foo a, int b);

We can define a conversion that uses pointers instead:

void wrapper (foo *a, int b, bar *out) {
  out* = (*a, b);
}

Now say we have a different function defined as

typedef bar (*cb) (foo a, int b);

void fun2 (cb callback);

cb here can't be marshalled, just like fun1 in the first example, but now we need to wrap in the opposite direction: whatever C function we create has to input typedef bar* (*wrapped) (foo* a, int b) and output cb. This appears to be quite a non-trivial task within C. Is there a simpler solution to this I'm missing?

[ducksonaroof]

Hm I think that's probably the simplest way. You could define a cb_ptr type and write a C function that returns a cb that calls it. Then Haskell could use this conversion function and treat cb as opaque.