<div dir="ltr">Thank you. Reading the match docs, I didn't infer the significance of quasipatterns for XML parsing.</div><div class="gmail_extra"> <div class="gmail_quote">On Tue, Jun 25, 2013 at 6:14 AM, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu" target="_blank">matthias@ccs.neu.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"> On Jun 24, 2013, at 9:52 PM, Matthew Butterick wrote: > I've seen this technique recommended numerous times. Would anyone have a link to an idiomatic example of the technique in use? Thank you. </div>> ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> Your question is somewhat generic. So here is a generic answer: #lang racket (require xml) (module+ test (require rackunit)) ;; ----------------------------------------------------------------------------- ;; recognizing an XML that belongs to a grammar ;; E = <plus>E E</plus> | <number value=String /> (define (ex i) (format "<number value =\"~a\" />" i)) (define (ex2 i j) (string-append "<plus>" (ex i) (ex j) "</plus>")) ;; String -> Boolean ;; does E produce e? (module+ test (check-true (parse (ex2 5 42))) (check-false (parse (ex "hello world"))) (check-false (parse (ex2 5 "hello world")))) (define (parse e:str) (define e:xml (read-xml/element (open-input-string e:str))) (define e:xexpr (xml->xexpr e:xml)) (let parse ((e e:xexpr)) (match e [`(number ((value ,v))) (number? (string->number v))] [`(plus () ,e1 ,e2) (and (parse e1) (parse e2))] [else #f]))) ;; ----------------------------------------------------------------------------- ;; creating a parse tree for a valid XML ;; A = Number | (list '+ Number Number) ;; String -> [maybe/c A] ;; does E produce e? if so, create an a, otherwise #f (module+ test (check-equal? (parse-to (ex2 5 42)) '(+ 5 42)) (check-false (parse-to (ex "hello world"))) (check-false (parse-to (ex2 5 "hello world")))) (define (parse-to e:str) (define e:xml (read-xml/element (open-input-string e:str))) (define e:xexpr (xml->xexpr e:xml)) (let parse ((e e:xexpr)) (match e [`(number ((value ,v))) (define x (string->number v)) (if (number? x) x #f)] [`(plus () ,e1 ,e2) (define p1 (parse e1)) (cond [(boolean? p1) #f] [else (define p2 (parse e2)) (and p2 `(+ ,p1 ,p2))])] [else #f]))) I have not include exception handling for invalid and/or incomplete XML strings. I have also skipped treatment of white space. I tend to create macros with which I can specify X-expression grammars so that I can easily write down parsers and unparsers. That way I don't ever see match in real code: (define (state-writer s) (state->xexpr s)) (define state-parser (xml-parser (state () (b board-parser) (p player-parser) ... #:action (*create-state b p)))) (module+ test (check-equal? (state-parser (state-writer s0)) s0) (check-equal? (state-parser (state-writer s1)) s1)) These are from a recent game. The tests ensure that the two functions are compatible. -- Matthias </blockquote></div> </div>