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

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 <alexander at 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(.+)$ <smb://s(.+)//s(.+)$>" (list _ a b c))
>                    (foo a b c)]
>                   [_ #f]))
>               (? foo? result))])))
> (define (foo-from-str str)
>   (match str
>     [(foo-string value) value]
>     [_ #f]))
> 
> (check-match (foo-from-str "1 2 3") (foo "1" "2" "3"))
> 
> 
> On Jan 16, 2015, at 4:24 PM, Alexis King <lexi.lambda at gmail.com <mailto:lexi.lambda at gmail.com>> wrote:
> 
>> I just posted a question on Stack Overflow asking about using racket/match <http://stackoverflow.com/q/27992566/465378>. Since I’m not sure how active the racket tag is on SO, I figured I’d also post it here. For those who don’t want to look at the SO question, here’s the question’s contents.
>> 
>> In a racket/match pattern, I want to match some values, then create a struct that wraps them. For example, take the following (contrived) code:
>> 
>> (struct foo (a b c))
>> 
>> (define (foo-from-string str)
>>   (match str
>>     [(pregexp #px"^(.+)\\s(.+)\\s(.+)$ <smb://s(.+)//s(.+)$>" a b c)
>>      (foo a b c)]
>>     [_ #f]))
>> I frequently find myself matching this list of three elements in other patterns, then creating a struct from it. Therefore, I wanted to simplify this by writing a custom match expander. Ideally, it would work like this:
>> 
>> (struct foo (a b c))
>> 
>> (define (foo-from-str str)
>>   (match str
>>     [(foo-string value) value]
>>     [_ #f]))
>> That is, it should automatically match a string that satisfies the regex, then store the values into a foo struct on success and bind it to value. I tried writing something like the following to implement this:
>> 
>> (define-match-expander foo-string
>>   (λ (stx)
>>     (syntax-case stx ()
>>       [(_ result)
>>        #'(and (pregexp #px"^(.+)\\s(.+)\\s(.+)$ <smb://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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> 

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