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)]
[ _ '()]))
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