You can't define the reader part of a #lang in the same file and then use it, but you can definitely define the bindings, from things like #%module-begin and #%app on up, and use it in the same file. The following works fine for me: #lang racket (module greeting racket (provide #%module-begin) (require (for-syntax syntax/parse)) (define-syntax #%module-begin (syntax-parser [(_ one:str two:str) #'(#%plain-module-begin (printf "Hello, ~a!\n" 'one) (printf "Salutations, ~a!\n" 'two))]))) (module main (submod ".." greeting) "Santa Claus" "Easter Bunny") <div class="gmail_extra"> Carl Eastlund <div class="gmail_quote">On Mon, Dec 3, 2012 at 3:46 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"> When doing quick experiments, I've often wanted the ability to define a new language and then use it in the same file. I thought submodules might allow me to do this, but I've realized that I still can't. However, in trying to do so I ran into some interesting behavior: Here is a file named test.rkt that is saved to disk: #lang racket (define-syntax-rule (app x ...) (begin (displayln "hello") (#%app x ...))) (provide (except-out (all-from-out racket) #%app) (rename-out [app #%app])) If I then add to the same file: (module test "test.rkt" (+ 1 2)) but *don't* save, pressing run in drracket displays: Welcome to DrRacket, version 5.3.1.1 [3m]. Language: racket [custom]. hello 3 > Then if I save and run again, I get an error: Welcome to DrRacket, version 5.3.1.1 [3m]. Language: racket [custom]. . . ../plt/collects/compiler/cm.rkt:350:0: standard-module-name-resolver: cycle in loading at path: /home/stchang/tmp/test.rkt paths: /home/stchang/tmp/test.rkt > Is this the intended behavior? I guess this is what I would expect but it feels funny since an identical buffer produces different output depending on it's saved status. _________________________ Racket Developers list: <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a> </blockquote></div> </div>