<HTML><BODY>So you mean, that struct should be only struct, not like Common Lisp CLOS or C++ classes, that are "anything with fields". Then I need an advice. I want to make persistent structs. There are several options: 1) Build "`struct?' with #:reader and #:writer options". Then I have to make a struct, redefine accessors and mutators and somehow tell (match ...) that it is not genuine struct (I don't know, how to redefine syntax). 2) Build my own persistent-struct, that is not `struct?'. Then it cannont be used where strut? may be used and I have to write own match-expander. 3) Don't mimic struct. Maybe classes. What would you advice? What is more correct? Tue, 19 Aug 2014 09:35:48 -0400 от Sam Tobin-Hochstadt <samth@cs.indiana.edu>: <blockquote style="border-left:1px solid #0857A6; margin:10px; padding:0 0 0 10px;"> <div id=""> <div class="js-helper js-readmsg-msg"> <style type="text/css"></style> <div> <base target="_self" href="https://e.mail.ru/"> <div id="style_14084553690000001016_BODY">On Tue, Aug 19, 2014 at 8:32 AM, Roman Klochkov <<a href="/compose?To=kalimehtar@mail.ru">kalimehtar@mail.ru</a>> wrote: > #lang racket > (require (for-syntax racket racket/struct-info)) > (struct foo (a b)) > (set! foo-a (let ([old foo-a]) > (λ (x) (displayln x) (old x)))) > (define t (foo 1 2)) > > (displayln "Before change") > (match t [(foo a b) a]) In this code, `match` looks at `foo`, and sees that it's definitely a struct descriptor created by `struct`, and so that it knows that `t` will really be an instance of `foo`. Then it generates code that uses `unsafe-struct-ref` to extract the fields. That's why it never uses the modified `foo-a`. To check this, it uses the `checked-struct-info?` predicate. > > (let-syntax ([foo (extract-struct-info (syntax-local-value #'foo))]) > (displayln "After change") > (match t [(foo a b) a])) In this code, `match` can't be sure that `foo` refers to a real struct, so it just treats the struct info as describing some procedures and a predicate, and it generates the "obvious" code, which calls `foo-a`, which you've modified. I could change `match` so that it also uses `variable-reference-mutated?` for all of the identifiers where it uses checked struct info, and so behaves the same in both cases. However, I'd prefer to reject the first program instead, by hiding `foo-a` behind a macro that rejects assignment. Sam </div> <base target="_self" href="https://e.mail.ru/"> </div> </div> </div> </blockquote> -- Roman Klochkov </BODY></HTML>