[racket] Expanding a macro inside a cond

From: Alexander D. Knauth (alexander at knauth.org)
Date: Sat Nov 1 16:35:37 EDT 2014

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:

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 <andmkent at 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
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