Oooohh, I think I understand now: the way it was is already sufficiently general. The writer of the initial code took a different direction. Instead of defining a union type, he changes the tag of the pointer depending on what kind of type he needs in the union: (cpointer-push-tag! e XAnyEvent-tag) (case (XAnyEvent-type e) ((KeyPress) (cpointer-push-tag! e XKeyPressedEvent-tag)) ((KeyRelease) (cpointer-push-tag! e XKeyReleasedEvent-tag)) .... It's pretty smart. Is it the right way to go in general, and should _unions be avoided? Laurent <div class="gmail_quote">On Fri, Oct 26, 2012 at 6:39 PM, Laurent <<a href="mailto:laurent.orseau@gmail.com" target="_blank">laurent.orseau@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Hi, I am trying to convert an FFI cstruct definition into a union. More exactly, the previous definition was : (define-cstruct _XEvent ((type EventType))) with EventType being defined elsewhere. But this was an incomplete definition, and I now want to translate the following C union: typedef union _XEvent { int type; /* must not be changed; first element */ XAnyEvent xany; XKeyEvent xkey; // ... the rest for later } XEvent; I searched the collects directory, but I see only a single and too simple _union use, so I'm not sure how to use it. I guessed the following: (define _XEvent (_union EventType ; type _XAnyEvent ; xany _XKeyEvent ; xkey ; etc. )) (define (XEvent-type ev) (union-ref ev 0)) (define (XEvent-xany ev) (union-ref ev 1)) (define (XEvent-xkey ev) (union-ref ev 2)) However, now I don't use define-cstruct anymore (it would not work with _union, would it?), so I don't have all the -tag, -pointer, etc. bindings, which I need. What should I do ? Thanks, Laurent </blockquote></div>