Hi David, You can return nothing with (values). The function "values" returns any number of values... (values 1 2 3) returns 3 values, (values 1) returns 1 value, and (values) returns 0 values.
Jay On Fri, Dec 11, 2015 at 1:16 AM, David K. Storrs <[email protected]> wrote: > Question: Is there a way to make a function return nothing -- not '() or > #void, but actually nothing? > > > I'm playing around with HTML parsing as a learning exercise. The current > exercise is writing a look-down function which can be handed an HTML value > produced by the html library and a function, run the function across the > collect the results, and return them. I would use this for things like "find > every link in the following document" or "capitalize every use of the word > 'kumquat'" or etc. > > I would like to have a way to say "this element isn't interesting, just > ignore it" so that I don't get spurious return values in the results list. I > can't see a way to do that...I can exclude the values afterwards, but that's > not really what I was looking for, and opens the door to false negatives. > > Here's some simplified test code (removing all the HTML stuff for clarity) to > show the point: > > > ---------------- > (define (foo-1 x) (if (eq? x "foo") #t '())) > (define (foo-2 x) (if (eq? x "foo") #t (values))) > > (map foo-1 '(a b "foo" "bar")) > (filter (lambda (x) (not (null? x))) (map foo-1 '(a b "foo" "bar"))) > (map foo-2 '(a b "foo" "bar")) > > [dstorrs@localhost:~/personal/study/scheme/spider:<master*>]$ racket test.rkt > racket test.rkt > '(() () #t ()) > '(#t) > result arity mismatch; > expected number of values not received > expected: 1 > received: 0 > values...: > context...: > /Users/dstorrs/personal/study/scheme/spider/test.rkt: [running body] > ---------------- > > Instead of a 'result arity' crash, I would have liked to get '(#t) back, same > a if I'd generated the list and then filtered it. > > Is there a way to do this? > > > For the record, here's the actual look-down function: > > (require (prefix-in h: html) > (prefix-in x: xml)) > > (define (look-down el action) > (match el > [(struct h:html-full (attributes content)) > (apply append (map action content))] > > [(struct h:html-element (attributes)) > (action el)] > > [ _ '()])) > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- Jay McCarthy Associate Professor PLT @ CS @ UMass Lowell http://jeapostrophe.github.io "Wherefore, be not weary in well-doing, for ye are laying the foundation of a great work. And out of small things proceedeth that which is great." - D&C 64:33 -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
