[plt-scheme] Re: Cloning objects?

I don't have any better ideas for you on cloning objects, but I can
answer the macro question:

At Wed, 9 Dec 2009 11:38:48 +0100, Laurent wrote:
> On Tue, Dec 8, 2009 at 17:32, Laurent <laurent.orseau at gmail.com> wrote:
> 
> >
> > And, BTW, despite the use of `define-syntax-rule', when omitting the
> > `source' argument in the `init-field-clone' definition,
> > the form still works but becomes non-hygienic! Now I have to completely
> > rewire my brain again...
> >
> >
> More exactly:
> `source' can be omitted from the arguments:
> 
> (define-syntax-rule (init-field-clone [var val] ...)
> >   (begin
> >     (init [source #f])
> >     (init-field [var (if source (get-field var source) val)] ...)))
> >
> 
> it seems that `source' is inaccessible, but then it cannot be used as a
> field anymore:
> 
> > (define c1%
> >   (class object%
> >     (init-field-clone [source "z"])
> >     (super-new)
> >     ))
> > error: class: duplicate declared external init name in: source
> >
> 
> how can this be if the macro is hygienic?
> Well, of course, if it were completely internalized, I could not instantiate
> c1% with `new'...
> Does that means that it is `init' or `init-field' that are non-hygienic?

Each initialization argument, field, and method has two names: an
internal name and an external name. The internal name is an identifier,
so it is subject to the usual scoping and hygiene rules. An external
name, however, is merely a symbol.

For example, when you write

 (send o m 1 2 3)

then the method name `m' doesn't have to be bound in the environment of
the `send' form. The symbol 'm is used as the external name of the
method.

Note that the error message above is specifically about the external
name.

The `define-local-member-name' form creates a scoped binding to be used
as an external name. That's not quite the same thing as having external
names be identifiers, but it provides similar functionality.