<div dir="ltr">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 <div class="gmail_quote">On Wed, Oct 8, 2008 at 1:11 PM, michael rice <<a href="mailto:nowgate@yahoo.com" target="_blank">nowgate@yahoo.com</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <table border="0" cellpadding="0" cellspacing="0"><tbody><tr><td style="font-family: inherit; font-style: inherit; font-variant: inherit; font-weight: inherit; font-size: inherit; line-height: inherit; font-size-adjust: inherit; font-stretch: inherit;" valign="top"> 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 </td></tr></tbody></table> _________________________________________________  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>