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

[Aaron Hill]

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
>
> Probably better get used to below:
>
> #(if (not (defined? 'foo)) (ly:parser-define! 'foo 47))

Wouldn't...

(define foo (if (defined? 'foo) foo 123))

...work?  Since foo is not defined (or redefined) until the S-expression 
is fully evaluated, it should be able to resolve the existing value 
should it exist.  This also keeps the define at the top-level.

-- Aaron Hill