I've written a macro define-structure-conversion to flesh out what I see as the way to go.  Eventually, I think this would be best as a couple of key arguments in the defcstruct, similar to FSBV's current :constructor and :deconstructor arguments.

I am stuck however on recursive conversion of slots.  What I have works for the complex example, but if I built on that with a  "real-and-complex" structure and try to convert to foreign, I get an error because the complex slot is already converted by the time it gets in the setf.  Even with an existing pointer, as would be obtained from foreign-slot-pointer, the only function I have at my disposal for translating the slot is translate-to-foreign which creates a new foreign struct, it does not write to an existing struct.  It seems I need something like a setf function.

See the define-structure-conversion definition and test case examples.  Any thoughts on how to proceed?

Liam