[plt-scheme] SRFI 41

Agreed.  I purposely returned (stream #f) because the original poster
expected #f.

By the way, it is easier to write (stream 'a 'b) than (stream-cons 'a
(stream-cons 'b stream-null)).

On Wed, Oct 8, 2008 at 2:40 PM, Jos Koot <jos.koot at telefonica.net> wrote:

> SRFI 41 in PLT is almost an identical copy of the reference implementation.
> I did not yet know it was adopted in PLT, but I am glad to see it there. It
> does not include procedure stream-member. This procedure is given as an
> example in the document. Clearly the example is incorrect.
>
> I suggest:
>
> #lang scheme
> (require "streams.ss")
>
> (define (stream-member eql? obj strm)
>  (let/ec ec
>   (stream-let loop ((strm strm))
>     (cond
>       ((stream-null? strm) strm)
>       ((eql? obj (stream-car strm)) strm)
>       (else (loop (stream-cdr strm)))))))
>
> (define s1 (stream-cons 'a (stream-cons 'b stream-null)))
>
> (stream->list (stream-member eq? 'a s1)) ; --> (a b)
> (stream->list (stream-member eq? 'b s1)) ; --> (b)
> (stream->list (stream-member eq? 'c s1)) ; --> ( ) ; signals that the
> element is not found.
>
> The solution given by Phil goes wrong on
> (stream-member eqv? #f (make-stream #f)) and
> (stream-member eqv? #f stream-null)
> because it returns (stream #f) in both cases. It does not show whether the
> returned value, i.e (stream #f), is the last cdr of the stream or a signal
> that the element has not been found. Since the result cannot be empty if the
> element is found it is better to return null-stream in this case.
>
> Jos
> ----- Original Message ----- From: "Jens Axel Soegaard" <
> jensaxel at soegaard.net>
> To: <nowgate at yahoo.com>
> Cc: <plt-scheme at list.cs.brown.edu>
> Sent: Wednesday, October 08, 2008 8:42 PM
> Subject: Re: [plt-scheme] SRFI 41
>
>
>  michael rice skrev:
>>
>>> 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
>>>
>>>
>>>
>> The documentation for srfi 41 says:
>>
>> (stream-let tag ((var expr) ...) body)
>>
>> ... stream-let evaluates the expressions in its body in an environment
>> containing
>> the newly-bound variables, returning the value of the last expression
>> evaluated,
>> which must yield a stream.
>>
>> Since #f is not a stream, the behaviour is undefined.
>>
>> Note that evaluating (stream-car (stream-member equal? 'c s1)) gives #f.
>>
>> But... The example rises a few questions:
>>
>>  Is PLT using the reference implementation of srfi 41?
>>
>>  If not, it would be a worth chaning the behaviour of stream-let to
>>  match the reference implementation.
>>
>>  Was it intentional that this behaviour is undefined?
>>
>>  If it is unintentional, maybe an error message would be better
>>  than matching the behaviour of the reference implementation?
>>
>>
>>
>> --
>> Jens Axel Søgaard
>>
>> _________________________________________________
>>  For list-related administrative tasks:
>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>
>>
>
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