[racket] Persistent struct API

From: Roman Klochkov (kalimehtar at mail.ru)
Date: Tue Aug 19 10:45:38 EDT 2014

 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 at cs.indiana.edu>:
>On Tue, Aug 19, 2014 at 8:32 AM, Roman Klochkov < kalimehtar at 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.

Roman Klochkov
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