<HTML><BODY>Serialize is good for one-shot import/export. By persistence I mean possibility to get objects from persistent storage by reference on-demand. For example I have facebook-like site: (persistent-struct user (login password photo ...)) (persistent-struct message (user text seen-by)) If i read concrete message for its text, I don't want to pull all user information for its author and list of users, that have seen the message. Then I need to make load only references to these users. But when I write (user-login (message-user the-message)), the library must make in field user real struct from reference (and memoize it). I tried to use impersonator, but it doesn't allow to have own fields in impersonator (and prop:impersonator-of doesn't replace accessors). generic doesn't add accessors to fields. It only gives common function for different structs. Tue, 19 Aug 2014 10:52:23 -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_14084599650000000400_BODY">First, for persistence, can you use either prefab structs or the serialize library? If you can't, what I'd do is implement a generic called `persist` using `racket/generic`, and then have all the structs you wanted to be persistent implement that generic. Sam On Tue, Aug 19, 2014 at 10:45 AM, Roman Klochkov <<a href="/compose?To=kalimehtar@mail.ru">kalimehtar@mail.ru</a>> wrote: > 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 > <<a href="/compose?To=samth@cs.indiana.edu">samth@cs.indiana.edu</a>>: > > 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 > > > > -- > Roman Klochkov </div> <base target="_self" href="https://e.mail.ru/"> </div> </div> </div> </blockquote> -- Roman Klochkov </BODY></HTML>