I also answered on SO, but you just want to do more in the `app` pattern:
#lang racket
(struct foo (a b c) #:transparent)
(define-match-expander foo-string
(λ (stx)
(syntax-case stx ()
[(_ result)
#'(app (λ (v) (apply (λ (_ a b c) (foo a b c))
(regexp-match #px"^(.+)\\s(.+)\\s(.+)$" v)))
result)])))
(define (foo-from-str str)
(match str
[(foo-string value) value]
[_ #f]))
(foo-from-str "1 2 3")
Sam
On Fri Jan 16 2015 at 4:42:56 PM Alexis King <lexi.lam...@gmail.com> wrote:
> Yes, chandler already suggested something like that on #racket. I didn’t
> think of that, but it should work. It’d be nice if there were a cleaner way
> to do it, but if you want to post that as an answer on SO for the imaginary
> internet points, I’d accept it.
>
> On Jan 16, 2015, at 13:38, Alexander D. Knauth <alexan...@knauth.org>
> wrote:
>
> Would this work for what you want?
> #lang racket
> (require rackunit)
> (struct foo (a b c))
> (define-match-expander foo-string
> (λ (stx)
> (syntax-case stx ()
> [(_ result)
> #'(app (λ (v)
> (match v
> [(pregexp #px"^(.+)\\s(.+)\\s(.+)$" (list _ a b c))
> (foo a b c)]
> [_ #f]))
> (? foo? result))])))
> (define (foo-from-str str)
> (match str
> [(foo-string value) value]
> [_ #f]))
>
> (check-match (foo-from-str "1 2 3") (foo "1" "2" "3"))
>
>
> On Jan 16, 2015, at 4:24 PM, Alexis King <lexi.lam...@gmail.com> wrote:
>
> I just posted a question on Stack Overflow asking about using racket/match
> <http://stackoverflow.com/q/27992566/465378>. Since I’m not sure how
> active the racket tag is on SO, I figured I’d also post it here. For those
> who don’t want to look at the SO question, here’s the question’s contents.
>
> In a racket/match pattern, I want to match some values, then create a
> struct that wraps them. For example, take the following (contrived) code:
>
> (struct foo (a b c))
> (define (foo-from-string str)
> (match str
> [(pregexp #px"^(.+)\\s(.+)\\s(.+)$" a b c)
> (foo a b c)]
> [_ #f]))
>
> I frequently find myself matching this list of three elements in other
> patterns, then creating a struct from it. Therefore, I wanted to simplify
> this by writing a custom match expander. Ideally, it would work like this:
>
> (struct foo (a b c))
> (define (foo-from-str str)
> (match str
> [(foo-string value) value]
> [_ #f]))
>
> That is, it should automatically match a string that satisfies the regex,
> then store the values into a foo struct on success and bind it to value.
> I tried writing something like the following to implement this:
>
> (define-match-expander foo-string
> (λ (stx)
> (syntax-case stx ()
> [(_ result)
> #'(and (pregexp #px"^(.+)\\s(.+)\\s(.+)$" a b c)
> (app (λ (v) (foo a b c)) result))])))
>
> Unfortunately, this fails because a, b, and c, are unbound when the
> function passed to the app pattern gets called. Is there any way to
> implement such a match expander so that it can perform some arbitrary
> procedure on the matched values?
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