id,summary,reporter,owner,description,type,status,priority,milestone,component,resolution,keywords,cc,planetversion,pltversion
18,stream-split-at,dherman,dherman,"Steve Huwig wrote:

> Hi,
> 
> `stream-split-at' in stream.plt 1 1 only returns one value, and
> attempting to use that value results in errors. I looked at the source
> and I think there's a simple oversight:

{{{
  (define (stream-split-at i stream)
    (stream-delay
;    ^^^^^^^^^^^^ 
     (cond
       [(zero? i)
        (values stream-null stream)]
       [(stream-null? stream)
        (values stream-null stream-null)]
       [else
        (let-values ([(prefix suffix) (stream-split-at (sub1 i)
(stream-cdr stream))])
          (values (stream-cons (stream-car stream) prefix) suffix))])))
}}}

> I don't think stream-delay is correct here! At least, when I changed the
> code to:

{{{
  (define (stream-split-at i stream)
     (cond
       [(zero? i)
        (values stream-null stream)]
       [(stream-null? stream)
        (values stream-null stream-null)]
       [else
        (let-values ([(prefix suffix) (stream-split-at (sub1 i)
(stream-cdr stream))])
          (values (stream-cons (stream-car stream) prefix) suffix))]))
}}}

> the routine behaved as I expected.
> 
> Is this a bug or am I missing something basic? Do you have a good
> example of using stream-split-at? With the original code, I tried:
> 
>> > (stream-split-at 3 (list->stream (list 1 2 3 4 5 6)))
> #<struct:stream>
>> > (stream-car (stream-split-at 3 (list->stream (list 1 2 3 4 5 6))))
> . context (lexical binding) expected 2 values, received 1 value:
> #<struct:stream>
>> > 
> 
> which left me cold, but with the new code, I was able to do:
> 
>> > (stream-split-at 3 (list->stream (list 1 2 3 4 5)))
> #<struct:stream>
> #<struct:stream>
>> > (let-values ([(front back) (stream-split-at 3 (list->stream (list 1 2
> 3 4 5)))])
>     (values (stream->list front) (stream->list back)))
> (1 2 3)
> (4 5)
> 
> which is about what I expected to need to do.
> 
> If this is a bug, then I think stream-partition and stream-span suffer
> the same bug. If it's not a bug, then apparently the bug is in my brain
> for not figuring out the right API.
> 
> Thanks,
> Steve Huwig
> 
> P.S. stream-split-at reverses the order of split-at arguments from
> SRFI-1, not sure if that's deliberate.
> 
",defect,new,major,,dherman/stream.plt,,,,,