Thanks - this solved the problem!  Still not quite familiar with parameters I guess ;)  And yes I simply copy/paste redefined code into REPL without clearing the old state - I will remember to do that going forward. Thanks, yinso <div>On 4/16/07, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.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;"> At Mon, 16 Apr 2007 16:30:36 -0700, "Yin-So Chen" wrote: >   (define dispatcher (make-parameter (make-hash-table 'equal))) ; run-time > dispatcher... `make-parameter' returns a value that acts like a procedure. >   (define-syntax (register stx) >     (syntax-case stx () >       ((_ key value) >        #'(hash-table-put! dispatcher key value)))) Use `(dispatcher)' instead of `dispatcher' to get the current dispatcher. >   (define-syntax (dispatch stx) >     (syntax-case stx () >       ((_ key args ...) >        #'((hash-table-get dispatcher key) args ...)))) Same here. > (require bar) ; error = define-values: cannot change identifier that is > instantiated as a module constant: dispatcher I get a different result: hash-table-put!: expects type <mutable hash-table> as 1st argument, given: #<primitive:parameter-procedure>; other arguments were: test #<procedure:test> For your error: I think you must be using raw mzscheme and trying to re-define the `dispatcher' module (as opposed to, say, using DrScheme with its "Run" button that clears out old state before running program). Re-defining a module conflicts with certain optimizations that you'd normally like the compiler to perform. To disable the optimizations and allow module re-definition, use `(compile-enforce-module-constants #f)'. Matthew </blockquote></div> -- <a href="http://www.yinsochen.com">http://www.yinsochen.com</a> ...continuous learning...