[cells-gtk-devel] Re: [cells-devel] Use of :initform c? and c-in

[Peter Denno]

On Monday 03 October 2005 22:16, Kenny Tilton wrote:

> >>From the hyperspec:
> >
> >"The :initform slot option is used to provide a default initial value form
> > to be used in the initialization of the slot. This form is evaluated
> > every time it is used to initialize the slot." -- see, it says "every
> > time it is used to initialize the slot" not "every time the slot is
> > accessed."
>
> Ah, but this describes exactly what is going on. You are confusing the
> stream of values coming from a formula with non-cell values normally
> used to initialize a slot. The one-time intialization is still present,
> it is just that the thing being bound to the slot is a formula or
> c-input (which really does no more than support an optimization).
>
> by the way, you never explained to me how to resolve conflicts in the
> class precedence list where one class specifies an initform and another
> specifies an :initcell.

I think the whole question is confused by not recognizing that the set of 
notions (initargs, :default-initargs, :initform) address a different concern 
than does :formula. Those three are about providing an initial 
value, :formula  (or any declaration that "this slot is a cell-slot") is 
about providing reader and writer functions. Once you assert that access to 
the slot's value is provided through a cell's accessor, you have effectively 
disconnected from the documented meaning of initargs, :default-initargs 
and :initform. You can, as far as I am concerned, disallow or ignore the use 
of any of these on a cells slot. Perhaps you would be better off using a 
cells-specific constructor rather than make-instance, since cells doesn't use 
the usual semantics of the initargs. -- But, hey, if you are going to say 
"its sooo... easy" sure stick with it. I don't care. I'm trying to help, not 
trying to be critical.

>
> Also, what do we do about:
>
>        (make-instance 'republican :indictments (c-formula ()
> (years-in-office self)))
>
> That would override an initform or :default-initarg. How are cells
> supplied at make-instance time in your scheme?

(mk-republican 'indictments (c-formula () (months-in-office self)))

indictments names a slot, it might not be an initarg.

> >I'd call the role :formula or even better :derived-by, not :initformula,
> > but what's in a name? (provided that name doesn't already associate with
> > a meaning, as does :initform).
> >
> >>i think the problem of the initform not actually becoming the slot value
> >>is a problem only for those who have never seen Cells before and have
> >>not had the briefest introduction. So I say leave it as it is an explain
> >>it. :)
> >
> >Sure, it can be learned. Like GTK itself, I have just enough knowledge of
> >Cells to be dangerous. My concern is getting cells-gtk users up to speed
> > as quickly as possible. Definitions of the concepts used, and predictable
> > semantics of the language are the what makes our work effective (and why
> > we like lisp -- compare with Perl or Python, where the semantics change
> > often, and are ill-defined).
> >
> >But of course there are only so many hours in the day, and so much to do.
> > So nothing is ever going to be perfect.
>
> Could someone somewhere explain to me what is so hard to understand about:
>
>    (make-instance 'rect :area (c? (* (length self) (width self))))

Looks fine to me.
>
> Seriously. I realize it is astonishing that it works, but what else?
> Let's add:
>
>    (def-c-output area ((self room) new-value old-value)
>          (when (> new-value (area (carpet self)))
>               <trigger electronic ordering of new carpet>)))

Also looks fine.

>
> There are a lot of examples in the test suite with some pretty simple
> relationships. Am I the only one who prefers examples to turgid,
> abstract documentation trying to sound like a PhD thesis by using big
> words and never saying what they really mean?

Well, I hope I am not being turgid or using big words. After seeing a couple 
of examples, I like definitions. So far I have this:

A C-INPUT CELL is a cell whose value may be set through explicit procedural 
code, using setf on the slot. Setf-ing the cell typically causes the values 
of other cells to be recomputed. 

A C-FORMULA CELL is a cell whose value is obtained through evaluation of a 
formula.

Note that the usual semantics of :initform do not apply when :initform is 
given by c-formula. Instead of just setting the value at initialization, the 
c-formula (an arbitrary lisp form) specifies the dynamic relationship between 
the slot's value and other aspects of the program state. 

I don't have a definition of "cell" but I think it is might be:

 "A CELL is a slot in a CLOS object whose value is managed by readers and 
writers that are part of a cells constraint network."

It would be useful to have definition of the 'fm' methods.

As far as I can tell, (fm-find-one top name) just does this:

(depth-first-search top #'(lambda (x) (eql (md-name x) name)) #'kids)))

So how about 

fm-other - 
fm-other? -
fm-traverse - 
fm-other-v -
fm! -
fm^ -



> I can see questions about obscure syntax or edge cases, but not
> something basic like the use of :initform to supply the cell that will
> be mediating a slot.

It's OK with me if this isn't discussed further. Again, I am just trying to 
get some documentation for cells-gtk.



-- 
- Best regards,
   Peter