[racket] How can I use the results of a pattern match to produce a value in a match-expander?

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.lambda at gmail.com> wrote:

> I just posted a question on Stack Overflow asking about using racket/match. 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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>  http://lists.racket-lang.org/users

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