[racket-dev] [racket] Question about fields in Racket OO

This seems like a trivial point because the class system doesn't have to
track these things and they are in fact part of the closures of the methods,
so I don't see in what sense they are "fields". Perhaps I am blinded by my
reading of the implementation. I certainly agree they are essentially
fields, but I can't but think of them as closed-over variables.

Jay

On Thu, Dec 16, 2010 at 7:00 AM, Carl Eastlund <cce at ccs.neu.edu> wrote:

> To quote the class* documentation:
> (
> http://docs.racket-lang.org/reference/createclass.html#%28part._clfields%29
> )
>
> "Each field, init-field, and non-method define-values clause in a
> class declares one or more new fields for the class. Fields declared
> with field or init-field are public."
>
> So only the public ones are accessible via get-field.
>
> Carl Eastlund
>
> On Thu, Dec 16, 2010 at 8:56 AM, Jay McCarthy <jay.mccarthy at gmail.com>
> wrote:
> > Does 'define' really mean 'make a field'? I thought fields had to be
> > specially designated so that get-field would know about them...
> > Yes, this program errors:
> > #lang racket
> > (define c%
> >   (class* object% ()
> >     (field [x 1])
> >     (define y 2)
> >     (super-new)))
> > (define o (new c%))
> > (field-names o)
> > (get-field x o)
> > (get-field y o)
> > --
> > I agree that 'define' is like making a field, but fields are something
> > special too.
> > Jay
> > On Thu, Dec 16, 2010 at 6:51 AM, Robby Findler <
> robby at eecs.northwestern.edu>
> > wrote:
> >>
> >> On Thu, Dec 16, 2010 at 12:22 AM, Mark Engelberg
> >> <mark.engelberg at gmail.com> wrote:
> >> > OK, it works when the set! occurs after the super-new.  I didn't think
> >> > set!
> >> > would work at all in a class definition (as opposed to within a
> method);
> >> > I
> >> > was thinking of the whole system of defining classes as more of a
> >> > declarative DSL that only allowed certain constructs.
> >>
> >> You've probably already figured this out, but the body of a class is a
> >> series of definitions and expressions like at the top-level but
> >> 'define' taking on the meaning of 'make a field', and a bunch of new
> >> definitions appearing. The new stuff says what the methods are, but
> >> everything else is just executed in sequence as if it were in the body
> >> of the initializer (if this were in Java, say).
> >>
> >> hth,
> >> Robby
>



-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/dev/archive/attachments/20101216/7c3a0593/attachment.html>