Re: scheme set list function

[Aaron Hill]

On 2019-04-08 4:35 am, David Kastrup wrote:
> Aaron Hill <address@hidden> writes:
>
> > On 2019-04-08 2:48 am, Thomas Morley wrote:
> > > foo =
> > > #(let ((x (cons 1 0)))
> > >   (define-scheme-function (arg)(symbol?)
> > >     (case arg
> > >       ((indent) (set! x (cons (car x) (1+ (cdr x)))))
> > >       ((increase) (set! x (cons (1+ (car x)) 0)))
> > >       ((reset) (set! x (cons 1 0))))
> > >     (if (zero? (cdr x))
> > >         (format #f "~a" (car x))
> > >         (format #f "~a.~a" (car x) (cdr x)))))
> > >
> > > [ . . . ]
> > >
> > > I'm still not happy with those set-whatever!-thingies. I was beaten
> > > too often. Maybe someone comes up with a better approach.
> >
> > Using set! is perfectly fine as long as you encapsulate things well.
> > Your use of let to define a local variable minimizes the chance that
> > folks would be able to interfere with or even care about such
> > modification.  However, your usage means there is still a "global" x
> > that is shared amongst all of the usage of foo.
>
> There is no global variable x.  There is a binding, but the scope of 
> the
> binding ends with the let.  This binding is anonymous afterwards and 
> has
> no name.  You can call functions manipulating the global variable x 
> from
> inside of foo and their action is not related to having used the 
> binding
> x when defining foo.

I didn't say global, I said "global".  The quotes are there 
intentionally as I know it is not an actual top-level binding, but it is 
effectively no different in practice.

When you use foo there is only one x that matters, and it is shared 
amongst all of the uses of the procedure.

This is not inherently a bad thing, if it is understood that such a 
construct represents a singleton.  And for something like this section 
numbering thing, one could argue a singleton is apropos.

My variation showing an explicit constructor step encapsulates somewhat 
better but is ultimately unnecessary if one were only ever going to 
instanciate it once.


-- Aaron Hill