Oh, that's interesting. > (void? (values)) Type Checker: expected single value, got multiple (or zero) values in: (values)
On Fri, Dec 11, 2015 at 8:15 PM, Jay McCarthy <jay.mccar...@gmail.com> wrote: > 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 <david.sto...@gmail.com> > 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 racket-users+unsubscr...@googlegroups.com. > > 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 racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- 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 racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
