Re: Define variable only if it doesn't exist / is unbound

[Richard Shann]

On Thu, 2019-03-28 at 18:22 -0700, Aaron Hill wrote:
> On 2019-03-28 3:14 pm, Thomas Morley wrote:
> > Am Do., 28. März 2019 um 22:22 Uhr schrieb Aaron Hill
> > <address@hidden>:
> > > 
> > > On 2019-03-28 2:18 pm, Valentin Villenave wrote:
> > > > On 3/28/19, Pedro Pessoa <address@hidden> wrote:
> > > > > I imagined something like:
> > > > > 
> > > > > %%% pseudo
> > > > > #(define var
> > > > >     (not (is-bound? var) 0))
> > > > > %%%
> > > > 
> > > > I’d probably probably do something like
> > > > 
> > > > (null? (ly:parser-lookup 'var))
> > > > 
> > > > But I’m sure there are better ways :-)
> > > 
> > > ;;;;
> > > (if (not (defined? 'foo)) (define foo 47))
> > > ;;;;
> > > 
> > > 
> > > -- Aaron Hill
> > 
> > It's worth mentioning, the above will throw an error in guilev2:
> > 
> > error: GUILE signaled an error for the expression beginning here
> > #
> >  (if (not (defined? 'foo)) (define foo 47))
> > definition in expression context, where definitions are not allowed

This sounds like a problem I had when Denemo started running under
Guile 2  - I asked on the guile mailing list and got fixed up with
this:

;;; for guile 2.0 compatibility define the define-once procedure to
work in guile 1.8
(cond-expand
   (guile-2) ; nothing
   (else ; guile < 2.0
    (define-macro (define-once sym exp)
      `(define ,sym
         (if (module-locally-bound? (current-module) ',sym)
             ,sym
             ,exp)))))
             
Apologies in advance if this is un-related, I've not been following
this thread :(

Richard