<html> <head> <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type"> </head> <body bgcolor="#FFFFFF" text="#000000"> I think you are running into the issue described here: <a class="moz-txt-link-freetext" href="http://docs.racket-lang.org/guide/module-set.html">http://docs.racket-lang.org/guide/module-set.html</a> "Along the same lines, when a module contains no <a href="http://docs.racket-lang.org/reference/set_.html#%28form._%28%28quote._%7E23%7E25kernel%29._set%21%29%29" class="RktStxLink" pltdoc="x">set!</a> of a particular identifier that is defined within the module, then the identifier is considered a constant that cannot be changed—not even by re-declaring the module." Try in your definitions window: #lang racket (define counter 0) (set! counter 0) Thanks, Dave On 10/31/2011 09:58 AM, <a class="moz-txt-link-abbreviated" href="mailto:jkaczorek@aol.pl">jkaczorek@aol.pl</a> wrote: <blockquote cite="mid:8CE660583067BA6-E18-78A4E@Webmail-d112.sysops.aol.com" type="cite">Hi, I’m using DrRacket. In an edit window I have written: #lang racket (define counter 0) I’m pressing “Run” button and next, in a command line, after running a command: (set! counter (add1 counter)) I receive a message “set!: cannot modify a constant: counter” The problem can be resolved by changing the language (Language/Choose language/R5RS and unchecking “Disallow redefinition of initial binding”) but I suppose that in this way I can lose an access to the racket libraries. Of course, I realize, that redefinition of the variables is not good (functional) programming style but sometimes it’s necessary. Is there a possibility of such modifications without changing language? Jerzy <fieldset class="mimeAttachmentHeader"></fieldset> <pre wrap="">_________________________________________________ For list-related administrative tasks: <a class="moz-txt-link-freetext" href="http://lists.racket-lang.org/listinfo/users">http://lists.racket-lang.org/listinfo/users</a></pre> </blockquote> </body> </html>