Re: "Current" solution for generalized set!

[Mikael Djurfeldt <mdj@nada.kth.se>]

Maciej Stachowiak <mstachow@MIT.EDU> writes:

> mdj@nada.kth.se writes:

[...]

> > 4. Assume that G is an instance of <getter-with-setter>.
> >    Then G has the following properties:
> >      a) G has no slots
> >      b) There exists no mutator which can alter G.
> >      c) G =/= (getter G)
> >      d) (G A1 ...) <=> ((getter G) A1 ...)
> >      e) (set! (G A1 ...) V) <=> ((setter G) V A1 ...)

[...]

> > 6. (define-metod (add-method! ((g <getter-with-setter>) (m <method>)))
> >      (add-method! (getter g) m))
>                      ^^^^^^
> 
> Why is this necessary? WOuldn't applying a getter-with-setter apply
> the getter anyway and therefore have the correct semantics?

It's because the default `add-method!' needs a <generic> as its first
argument.  While `g' above is a <getter-with-setter>, (getter g) is a
<generic> (hopefully!).

[...]

> That's the solution I was going to suggest, allong with the following
> convenience macro (which would be useful more generally):
> 
> (defmacro define-setter (name setter)
>   (let ((old-name (gensym)))
>     `(let ((,old-name ,name))
>        (set! ,name (make-getter-with-setter 
> 		    (if (getter-with-setter? ,name) 
> 			(getter ,name) 
> 			,name) 
> 		    ,setter)))))

Good idea.


[...]

>  A smarter
> `define-setter' could work more like define [allowing, e.g.
> 
> (define-setter (getter-name arg1 arg2 arg3 value)
>   (some code))
> 
> ] but this may be confusing.

Why?

> > 8. He can add new methods to the setter with
> > 
> >    (add-method! (setter G) M)
> > 
> 
> I am pretty sure this would work without the need for point 6, and
> while making use of my above suggestion.

I don't understand.  Could you explain it a bit further?

> Noting my above suggestion, it would even be possibly to have
> `define-setter-method' automatically create a setter generic for the
> generic by doing a trick like `define-setter' if it is not already a
> `setter-with-getter'.

Yes.

> So you'd copy the innards of the closure when you call
> `make-getter-with-setter'? For correct semantics they would have to
> share code and environments....

Why?  The semantics will also be correct if the <getter-with-setter>
closure calls the getter.

(As an optimization, though, the code and environment pointers will be
copied from the getter if the getter is a closure.)

> I still think it should be called a <procedure-with-setter> (`getter'
> could be interpreted as possibly referring to a macro) but I don't
> care too much.

I think I like the name <procedure-with-setter> as well, but you still
haven't given me any arguments!  :)

/mdj