Thanks.  The source of the problem was that I missed the note in the documentation for scheme/kw saying that "The <a href="file:///C:/Program%20Files/PLT-FULL-3.99.0.26/doc/reference/lambda.html#%28form._%28%28lib._scheme/base..ss%29._lambda%29%29" class="schemesyntaxlink">lambda</a> and procedure-application forms of scheme/base support keyword arguments, and it is not compatible with the mzlib/kw library." and blithely went on thinking they were the same.  I'll pay more attention to the sidenotes in the future.  Thanks again. <div class="gmail_quote">On Fri, Jun 13, 2008 at 2:31 PM, Robby Findler <<a href="mailto:robby@cs.uchicago.edu">robby@cs.uchicago.edu</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">That function accepts a mandatory keyword parameter using the keyword whose name just happens to be #:optional which, I'm guessing, is not what you intended. Anyways, here is its contract:  (-> any/c #:optional any/c false/c) If you wanted a function that takes either one or two arguments (as the contract suggests), I think you meant to write it like this:  (define (foo a [b #f]) #f) (They keyword syntax changed a little bit from 372's (lib "kw.ss") to 4.0. Maybe that's the source of the confusion?) Robby <div><div></div><div class="Wj3C7c"> On Fri, Jun 13, 2008 at 1:28 PM, David Einstein <<a href="mailto:deinst@gmail.com">deinst@gmail.com</a>> wrote: > How do I write a contract for the following > > (define (foo a #:optional b) #f) > > I would have thought that > > (provide/contract >   [foo (->* (any/c) (any/c) false?)] > ) > > would work, but obviously I'm missing something. > > </div></div>> _________________________________________________ >  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>