[plt-scheme] SRFI 41

The typing you describe is in fact consistent with that of ordinary (R5RS) 
member, which does not end in a question mark.

On Wed, 8 Oct 2008, Phil Bewig wrote:

> The definition of stream-member given in the text of SRFI-41 is incorrect.
> The problem is inconsistent typing: one branch of the cond returns #f, which
> is not a stream, while another branch of the cond returns strm, which is a
> stream.  Here is a definition of stream-member? which returns either #t or
> #f; it uses normal define, not stream-define, and normal let, not
> stream-let:
>
> (define (stream-member? eql? obj strm)
>  (let loop ((strm strm))
>    (cond ((stream-null? strm) #f)
>          ((eql? (stream-car strm) obj) #t)
>          (else (loop (stream-cdr strm))))))
>
> Given that, (stream-member? equal? 'c (stream 'a 'b)) evaluates to #f, as
> you expect.
>
> Another definition of stream-member, the one that was intended by SRFI-41,
> is:
>
> (define-stream (stream-member eql? obj strm)
>  (stream-let loop ((strm strm))
>    (cond ((stream-null? strm) (stream #f))
>          ((eql? obj (stream-car strm)) strm)
>          (else (loop (stream-cdr strm))))))
>
> That definition type-checks because both branches of the cond evaluate to a
> stream.  Given that definition, (stream-car (stream-member equal? 'c (stream
> 'a 'b))) evaluates to #f, and (stream-car (stream-member equal? 'a (stream
> 'a 'b))) evaluates to a.
>
> Note that stream-member is not defined in SRFI-41; it is just an example in
> the text (worse, it is an incorrect example in the text).  If PLT includes
> stream-member in its version of SRFI-41, that is an extension of SRFI-41.
> If PLT wants to extend SRFI-41, it would be better to define the library in
> such a way that it is loaded by (require srfi/41-extended-by-PLT) or some
> such mechanism.  By taking the liberty of extending the base definition of
> SRFI-41, PLT confuses users and makes code written using its extended
> version of SRFI-41 non-portable to other Scheme systems that adhere to
> SRFI-41 as it is written.
>
> I will forward this note to the SRFI-41 mailing list.
>
> Phil
>
> On Wed, Oct 8, 2008 at 1:11 PM, michael rice <nowgate at yahoo.com> wrote:
>
>>  There's a stream-member function given in SRFI 41 that doesn't seem to
>> work as expected.
>>
>> =================
>>
>> (require srfi/41)
>>
>> (define s1 (stream-cons 'a (stream-cons 'b stream-null)))
>>
>> (define-stream (stream-member eql? obj strm)
>>   (stream-let loop ((strm strm))
>>     (cond ((stream-null? strm) #f)
>>           ((eql? obj (stream-car strm)) strm)
>>           (else (loop (stream-cdr strm))))))
>>
>> ===================
>>
>> Welcome to DrScheme, version 4.1 [3m].
>> Language: Swindle; memory limit: 128 megabytes.
>>> (stream-member equal? 'c s1)
>> #<stream>
>>>
>>
>> ===================
>>
>> Shouldn't the answer be #f?
>>
>> Michael
>>
>>
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>>
>