On Tue, Feb 12, 2019 at 5:03 PM <jackhfi...@gmail.com> wrote:
>>>
>>> This is nice for defining abstract types, but it can be pretty inconvenient
>>> for defining plain old aggregated data types that just have a bundle of
>>> fields. When defining those types as structs, consider using the
>>> #:transparent option. This means "use no inspector at all" (roughly) and
>>> lets `struct-info` Just Work (TM) without any inspector wrangling. The
>>> downside is that other modules may be able to break your type's invariants
>>> and possibly circumvent your contracts.
>>
>>
>> That's what I expected, but it doesn't seem to work:
>>
>> > (struct person (name age) #:transparent)
>> > (struct-info person)
>> #f
>> #t
>>
>> What am I missing?
>
>
> I was stumped on this for a while, but then realized the problem:
>
> > (struct-info person)
> #f
> #t
> > (struct-info (person "Alyssa P. Hacker" 42))
> #<struct-type:person>
> #f
Ah, I see. Thanks, I wouldn't have guessed that an instance of a
struct satisfied `struct-type?`
On the other hand, it seems relatively easy to break these
protections. Is there a way to prevent that?
; file test1.rkt
#lang racket
(struct person (name age))
(provide person)
;; end of test1.rkt
; file test2.rkt
#lang racket
(require "test1.rkt"
(for-syntax racket
syntax/parse
syntax/parse/experimental/template
syntax/parse/class/struct-id))
(define p (person 'bob 19))
(displayln "required ctor only of a struct defined in another file.
Can create, is opaque:")
p
(displayln "struct-info returns (values #f #t) since the struct isn't
inspectable here:")
(struct-info p)
(define-syntax struct-funcs (syntax-parser [(_ s:struct-id) (template
(list s.constructor-id s.accessor-id ...))]))
(define lst (struct-funcs person))
(displayln "show list of constructor and accessors that we retrieved
via macro:")
lst
(displayln "Fetch name ('bob) by way of accessor returned through macro:")
((second lst) p)
; Uncommenting the following will cause compilation to fail, since
person-name was not exported
;(person-name p)
;; end of test2.rkt
;; command line:
$ racket test2.rkt
required ctor only of a struct defined in another file. Can create, is opaque:
#<person>
struct-info returns (values #f #t) since the struct isn't inspectable here:
#f
#t
show list of constructor and accessors that we retrieved via macro:
'(#<procedure:person> #<procedure:person-name> #<procedure:person-age>)
Fetch name ('bob) by way of accessor returned through macro:
'bob
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