Re: Newbie question: bind a variable on the fly

[Jonathan Wilson]

Hi Neil,

Neil Jerram wrote:
> Jon Wilson <address@hidden> writes:
> > (define-macro (dyn-set! var val)
> >               `(begin (if (not (defined? (quote ,var)))
> >                         (primitive-eval `(define ,(quote ,var) #f)))
> >                       (set! ,var ,val)))
> >
> > (defined? 'undefined-symbol) ; => #f
> > ;(set! undefined-symbol #t)  Gives an error.
> > (dyn-set! undefined-symbol #t) ; No error.
> > (defined? 'undefined-symbol) ; => #t
>
>
> - isn't ,(quote xxx) equivalent to just xxx?

You'd think so.  But it didn't wind up working out that way.  Try it and
see.  Perhaps this is a bug?  I suspect not, however, what with having
the nested quasiquotes, with one of them inside the primitive-eval.

I'm afraid I don't 100% understand just what I wrote...  With the nested
quasiquotes here, just what gets expanded when?  I guess that

(dyn-set! yyz 5)

becomes (under the macro expansion)

(begin (if (not (defined? 'yyz)))
         (primitive-eval `(define ,(quote yyz) #f)))
       (set! yyz 5)))

And then the other quasiquote gets expanded to
(define yyz #f)

But, I'm not quite sure why this should work this way.  It seems like
the first expansion should be

(begin (if (not (defined? 'yyz)))
         (primitive-eval `(define ,var #f)))
       (set! yyz 5)))

But in this expression, is var still bound to yyz?  It seems like this
expansion shouldn't work, we should get a "symbol var is undefined" sort
of error.  But we don't.

> - as you kind of said, it will go wrong if ,var is defined locally:
>   defined? will be #f for a local variable, so a top-level variable
>   with the same name will be created, but the set! will then set the
>   local variable.

Ah yes, that is an important twist I hadn't thought of.  Is there a
`local-defined?' function anywhere?

Also, I found myself completely unable to bind this macro to the symbol
set!, even by strange things like defining real-set! to be set!, and
then defining the macro, and then (real-set! set! dyn-set!).  Bad things
happened to my poor abused guile.

Thanks for taking a look!

Regards,
Jon