<div dir="ltr">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)). <div class="gmail_quote">On Wed, Oct 8, 2008 at 2:40 PM, Jos Koot <<a href="mailto:jos.koot@telefonica.net">jos.koot@telefonica.net</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> 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<div class="Ih2E3d">   (stream-let loop ((strm strm))     (cond </div>       ((stream-null? strm) strm)<div class="Ih2E3d">       ((eql? obj (stream-car strm)) strm) </div>       (else (loop (stream-cdr strm)))))))<div class="Ih2E3d"> (define s1 (stream-cons 'a (stream-cons 'b stream-null))) </div> (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" <<a href="mailto:jensaxel@soegaard.net" target="_blank">jensaxel@soegaard.net</a>><div class="Ih2E3d"> To: <<a href="mailto:nowgate@yahoo.com" target="_blank">nowgate@yahoo.com</a>> Cc: <<a href="mailto:plt-scheme@list.cs.brown.edu" target="_blank">plt-scheme@list.cs.brown.edu</a>> </div> Sent: Wednesday, October 08, 2008 8:42 PM<div class="Ih2E3d"> Subject: Re: [plt-scheme] SRFI 41 </div><div><div></div><div class="Wj3C7c"><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> michael rice skrev: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> 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 </blockquote> 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:  <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote> </div></div></blockquote></div> </div>