[racket] matching behavior in dispatch-rules

I think I've buried the problem inside an inaccurate analogy. 


I'm wondering if there's a generalized idiom for matching an item in a dispatch pattern WITHOUT passing it through as one of the arguments.


For instance. Suppose I have a magic.html page that I generate with a route, so I set up a dispatch pattern like this:


;;;;;;;;;;;;;;;;;;;;;;
#lang racket
(require web-server/dispatch
web-server/http/request-structs
net/url)


(define (url->request u)
(make-request #"GET" (string->url u) empty
(delay empty) #f "1.2.3.4" 80 "4.3.2.1"))


(define-values (test-dispatch _)
(dispatch-rules
[((string-arg) ... "magic.html") (λ(req . args) (print args))]))


(test-dispatch (url->request "http://host.com/some/path/to/magic.html"))

;;;;;;;;;;;;;;;;;;;;;


This dispatch pattern will match the given URL and return as the arguments:


'(("some" "path" "to"))


So far, so good. But suppose I want to put the name of the magic page in a variable, magic-page-name, and match on that instead:


(define magic-page-name "magic.html")


So how do I adjust the dispatch pattern? If I do this:


((string-arg) ... magic-page-name)



It's a syntax error, of course. So instead I change the pattern to:


((string-arg) ... (? (λ(x)(equal? x magic-page-name))))



This seems to work, except that for arguments, I now get:


'(("some" "path" "to") "magic.html")



What I would prefer is to match on magic-page-name without passing through the result as an argument (i.e., get the same behavior as when I was using the string literal)













> On Jan 21, 2014, at 9:10 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> 
> 
> Hi Matthew,
> 
> 
> In your 'match' examples, you are printing out the result of "matched"
> which is bound by (and matched ...) on the outside of the match
> pattern, so it is equivalent to
> 
> 
> (define matched (list ....))
> (match matched
> [...inside... ....])
> 
> 
> This idiom doesn't really have anything to do with what dispatch-rules
> does, because dispatch-rules doesn't have "and". Instead,
> dispatch-rules behaves more like a normal "match" where only the names
> are bound/passed-to-the-function.
> 
> 
> Jay
> 
> 
> 
> 
> 
> On Sat, Jan 18, 2014 at 9:59 PM, Matthew Butterick <mb at mbtype.com> wrote:
> 
> 
>> If you run this code:
>> 
>> 
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>> #lang racket
>> (require web-server/dispatch
>> web-server/http/request-structs
>> net/url)
>> 
>> 
>> (define (url->request u)
>> (make-request #"GET" (string->url u) empty
>> (delay empty) #f "1.2.3.4" 80 "4.3.2.1"))
>> 
>> 
>> (define-values (test-dispatch sum-url)
>> (dispatch-rules
>> [((? string?) "foo" (? string?)) test-route]
>> [((? string?) (? (λ(x) (equal? x "bar"))) (? string?)) test-route]))
>> 
>> 
>> (define (test-route req . args)
>> (displayln (format "args = ~v" args)))
>> 
>> 
>> (test-dispatch (url->request "http://url.com/first-string/foo/second-string.html"))
>> (test-dispatch (url->request "http://url.com/first-string/bar/second-string.html"))
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>> 
>> 
>> You'll get this:
>> 
>> 
>> args = '("first-string" "second-string.html")
>> args = '("first-string" "bar" "second-string.html")
>> 
>> 
>> 
>> 
>> Whereas if you use these patterns with regular match, like so:
>> 
>> 
>> (match (list "first-string" "foo" "second-string.html")
>> [(and matched (list (? string?) "foo" (? string?))) matched])
>> 
>> 
>> (match (list "first-string" "bar" "second-string.html")
>> [(and matched (list (? string?) (? (λ(x) (equal? x "bar"))) (? string?))) matched])
>> 
>> 
>> You'll get:
>> 
>> 
>> '("first-string" "foo" "second-string.html")
>> '("first-string" "bar" "second-string.html")
>> 
>> 
>> 
>> 
>> Thus my question: why doesn't the literal "foo" in the first pattern under dispatch-rules become one of the matched items, as it does with regular match?
>> 
>> 
>> 
>> 
>> ____________________
>> Racket Users list:
>> http://lists.racket-lang.org/users
>> 
>> 
>> 
> 
> 
> 
> 
> 
> 
> 
> 
> --
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://faculty.cs.byu.edu/~jay
> 
> 
> "The glory of God is Intelligence" - D&C 93
> 
> 
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