I also found it counterintuitive that #lang racket (define x 0) (set! x 1)
works, but moving the set! out of the module and into the REPL does not. But Dave has pointed to the exact line in the Guide that makes this clear: A module-level definition is mutable only if there is a set! for it in the module. Once you know that "reasonable compromise" [as I put it in my emerging lecture materials], everything works as expected. --Dan On Mon, Oct 31, 2011 at 10:20 AM, David Vanderson <david.vander...@gmail.com> wrote: > I think you are running into the issue described here: > http://docs.racket-lang.org/guide/module-set.html > > "Along the same lines, when a module contains no set! 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, jkaczo...@aol.pl wrote: > > 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 > > > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
