Thanks Matthias, that makes sense.
On Tue, Feb 12, 2019 at 8:50 PM Matthias Felleisen
<matth...@felleisen.org> wrote:
>
>
>
> > On Feb 12, 2019, at 5:28 PM, David Storrs <david.sto...@gmail.com> wrote:
> >
> > 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)
>
>
>
> The line above exports two pieces: the constructor and compile-time
> information about the structure, including the functions as you saw. If you
> want to protect your invariants, you replace this provide with something like
>
> (provide (contract-out (person (-> string natural-number/c person?))))
>
> See end for a simpler way to test such things.
>
>
>
> > ;; 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
> >
> > -
>
>
>
>
> #lang racket
>
> (module test1 racket
> (provide (contract-out (person (-> string natural-number/c person?))))
> (struct person (name age)))
>
> (module test2 racket
> (require (submod ".." test1)
> (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
> )
>
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