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 <sa...@cs.indiana.edu>: >On Tue, Aug 19, 2014 at 8:32 AM, Roman Klochkov < kalimeh...@mail.ru > 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
____________________ Racket Users list: http://lists.racket-lang.org/users
