What timing, I was in the middle of writing the same question to the list when I got this. I can only add that the problem is also present in 5.3.0.20, and that, if I take (for instance, the stream-map) definition in racket/stream and execute it again, stream-map begins working as expected again. Maybe a problem with generics? <div class="gmail_quote">On Wed, Aug 22, 2012 at 1:49 PM, Stephen Chang <<a href="mailto:stchang@ccs.neu.edu" target="_blank">stchang@ccs.neu.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> I've been playing with Racket streams and I am confused about what exactly counts as a stream? For example, are lists streams? I think the answer is yes but then are the stream- functions supposed to work on regular lists? Some do but some don't. Welcome to DrRacket, version 5.3.0.16 [3m]. Language: racket [custom]. > (stream-map (ë (x) x) null) . . plt/collects/racket/stream.rkt:113:0: stream-accessor: contract violation expected: stream? given: '() > (stream? null) #t > (stream-map (ë (x) x) (list 1 2)) . . plt/collects/racket/stream.rkt:113:0: stream-accessor: contract violation expected: stream? given: '(1 2) > (stream? (list 1 2)) #t > (stream-first (list 1 2)) 1 _________________________ Racket Developers list: <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a> </blockquote></div>