wow.. thank you for your very kind explain<div>it is very helpful and I notice how do they works</div><div>thanks a lot! <div class="gmail_quote">2010/10/29 synx <<a href="mailto:plt@synx.us.to">plt@synx.us.to</a>> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Racket is different from most languages in that it has a flexible syntax. That means you can define new syntactical constructs, even entirely new languages that barely resemble the original (see Honu) (module weird racket/base (define-syntax-rule (weird (a) (b) (c)) (list a b c)) (weird ("this") ("is") ("weird")) ...will simplify down to (list "this" "is" "weird") and not be incorrect even though the strings are right next to the parentheses. (module weird2 racket (define-syntax (weird2 form) (syntax-case form () ((_ (((a)))) #'23))) (weird2 (((1)))) ...will simplify down to 23, ignoring all the parentheses in there and even the 1! You can think of the (procedure argument argument argument...) thing as a default syntax for the racket language and the racket/base language. *Most* of the time you can only put a procedure on the left of such a parenthetical statement, since in *most* syntaxes that instructs to apply the right arguments to the leftmost procedure. Except for a *very* few examples (like you noticed with instantiate), and except for any weird stuff you provide yourself, that is what you can expect from racket. Another more commonly used exception is the let syntax. (module let-example racket/base (let ((a 1) (b 2)) (list a b))) ... produces (list 1 2). If you could never put anything on the left side of a parenthetical statement except a procedure, then the above syntax would produce an error, since neither a nor b are procedures. ((a 1) (b 2)) by itself produces an error, since by default it assumes a and b are procedures. But within (let ...) it does not produce an error, because the let syntax is different from the normal syntax. ...I always use "new" not "instantiate," myself. It's a little jarring to read the syntaxof instantiate for me. (define frame (new frame% (label "Example")) ...is much preferable for me. It's really a matter of opinion at that point though. "label" isn't a procedure either, since new itself has its own syntax, but it just looks nicer to me. <div class="im"> > and I don't understand how such a code like > ((func arg) arg2) works too. </div>In racket, functions (or "procedures") are 1st class. That means they can be stored in variables, applied as arguments, and returned from other procedures. When you see ((func arg) arg2) what you can deduce is either it produces an error, or else (func arg) must return a procedure, to which arg2 is then applied. (module bb racket/base (define plus +) (define (doubleplus n) +) (list (plus 2 4) ((doubleplus 'ignored) 2 4) (+ 2 4))) ...produces (list 3 3 3). Procedures that return procedures are more useful in the sense of /closures/. You can make a procedure that already contains information previously known, then apply more arguments later to get the end result: (module name racket/base (define (make-show-name last) (define (show-name first) (list first last)) show-name) (define show-full-name (make-show-name "Smith")) (list (show-full-name "Bob") (show-full-name "Alice")) ((make-show-name "Last") "First")) You can see the above lets you first specify a last name, then get the full name both first and last by repeatedly specifying first names. With the ((a "b") "c") syntax, it's just skipping the repeating part, and going straight to one single first/last name pair. -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.14 (GNU/Linux) Comment: Using GnuPG with Mozilla - <a href="http://enigmail.mozdev.org/" target="_blank">http://enigmail.mozdev.org/</a> iQEcBAEBAgAGBQJMyjuKAAoJECC/cKf8E7UIkQIH/3/7GqjppV+dKl65EZGDs11q F5vvLSSUyPMo5CeedXTA7ObVGM5vqoAo7pK7nmcEmOE5S8QapReYnjGQwWnEueW4 l6WDapQg8zkaaAZjIyVxOCgwZaoEAlPgwm2B86q8ivJ447xfghyrFv5ifsVXp9Hp 1Jqu9m6chfXaMtbv4Vp/frWDLwmIhrDjx7aQoGhiVGUhYYvq9CNkzTrzVhM/I1ib +OsvKH3Pg63FxHrQhII29DdYFMfFmTk3uFMjSw1ZI/Cc38F/BAbqogILxKVr9f4f Qz7UoR5FWe36vZUdy43VDNxiHlsY0B1clCGSn2duP335KLrwIpuS/GjjQWC4CrE= =9Ial -----END PGP SIGNATURE----- </blockquote></div> </div>