I might be wrong, but I don’t think you can do that with the existing cond.
What you can do though is make your own version of cond that does that. I actually did something like that here for a for/cond-clause form: https://github.com/AlexKnauth/my-cond If you wanted you could define a version of cond that had something like a “cond expander”, and then define $: as one. Or I could add cond expanders to https://github.com/AlexKnauth/my-cond. On Nov 1, 2014, at 4:15 PM, Andrew Kent <andmk...@indiana.edu> wrote: > Good afternoon! > > Macro newbie here, > > I wanted to define a macro $: so that I could write this: > > (cond > [$: p (Point x y) => (+ x y)]) > > and it would expand into this: > > (cond > [(if (Point? p) p #f) > => (match-lambda [(Point x y) (+ x y)])] > [else #f]) > > I gave it a try: > > (define-syntax ($: stx) > (syntax-case stx (=>) > [(_ arg (struct-name field ...) => body) > (with-syntax ([pred? (format-id stx "~a?" #'struct-name)]) > #'[(if (pred? arg) > arg > #f) > => > (match-lambda ([(struct-name field ...) body]))])])) > > > But this macro expands (or attempts to) after cond has expanded... which is > not what I wanted. > > Is there a quick fix for this? Or can someone point me towards a particular > spot in the documentation where I might educate myself? > > I know about match (we're great friends)---I was just toying around with this > to see if I could get cond to look pretty w/ structs as well. > > Best, > Andrew > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
