[plt-scheme] SRFI 41

Sorry, the let/ec can be omitted. It stems from an earlier trial.
Jos

----- Original Message ----- 
From: "Jos Koot" <jos.koot at telefonica.net>
To: "Jens Axel Soegaard" <jensaxel at soegaard.net>; <nowgate at yahoo.com>; "Phil 
Bewig" <pbewig at gmail.com>
Cc: <plt-scheme at list.cs.brown.edu>
Sent: Wednesday, October 08, 2008 9:40 PM
Subject: Re: [plt-scheme] SRFI 41


> 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
>>
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