omg thank you so much!!<div>that's just what I want it!</div><div> <div class="gmail_quote">2010/11/13 Thomas Chust <<a href="mailto:chust@web.de">chust@web.de</a>> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> 2010/11/13 김태윤 <<a href="mailto:kty1104@gmail.com">kty1104@gmail.com</a>>: > [...] <div class="im">> I was told by mailing list that read the racket guide unit section > but sorry I still don't understand. </div>> [...] Hello, you have to use units here because modules do not allow mutually recursive dependencies. An example for your situation may look like this: -----BEGIN "ab-sig.rkt"----- #lang racket (define-signature a^ (a)) (define-signature b^ (b)) (provide (all-defined-out)) -----END "ab-sig.rkt"----- -----BEGIN "a.rkt"----- #lang racket (require "ab-sig.rkt") (define-unit a@ (import b^) (export a^) (define a 1) (printf "a@ sees b = ~s~%" b)) (provide (all-defined-out)) -----END "a.rkt"----- -----BEGIN "b.rkt"----- #lang racket (require "ab-sig.rkt") (define-unit b@ (import a^) (export b^) (define b 2) (printf "b@ sees a = ~s~%" a)) (provide (all-defined-out)) -----END "b.rkt"----- -----BEGIN "ab.rkt"----- #lang racket (require "ab-sig.rkt" "a.rkt" "b.rkt") (define-values/invoke-unit/infer (export a^ b^) (link a@ b@)) (printf "ab.rkt sees a = ~s~%" a) (printf "ab.rkt sees b = ~s~%" b) -----END "ab.rkt"----- As you can see, the module dependencies are cycle free but the unit dependency between a@ and b@ is mutual. Note that, even though a@ and b@ can see each other's variables exported through the signatures a^ and b^ respectively, the units still have to be initialized in some order and hence one of the printf calls in a@ or b@ will show an undefined value when "ab.rkt" is run. I hope this helps :-) Ciao, Thomas -- When C++ is your hammer, every problem looks like your thumb. </blockquote></div> </div>