Re: check undefined variable scheme

[Urs Liska]

Am 24.05.2018 um 10:46 schrieb David Kastrup:
> Urs Liska <address@hidden> writes:
>
> > Am 24.05.2018 um 09:56 schrieb Gianmaria Lari:
> > > The following function increase a counter by 1 and return it as string
> > >
> > >      #(define count 0)
> > >      #(define (nextcount) (begin
> > >                            (set! count (+ 1 count))
> > >                            (number->string count)
> > >                            )
> > >         )
> > >
> > > Is my code ok, or I should write it in a different way?
> > Two observations:
> > - You don't need that (begin ...) wrapper because the procedure
> > definition already behaves as such
> > - You shouldn't use reserved words for variable names, so I'd use
> > 'counter' instead.
> > > Is it possible to define "count" inside the function just in case it
> > > is undefined?
> > Yes, see:
> >
> > \version "2.19.80"
> >
> > #(define (nextcount)
> >     (if (not (defined? 'counter)) (ly:parser-define! 'counter 0))
> >     (set! counter (+ 1 counter))
> >     (number->string counter)
> >     )
> You actually don't need to define it externally:
>
>      #(define nextcount
>        (let ((counter 0))
>         (lambda ()
>          (set! counter (1+ counter))
>          (number->string counter))))
>
>      $(nextcount)
>
>      $(nextcount)
>
> will work just fine.

Ah yes, that's more idiomatic and (thus) elegant.
Gianmaria, if you want to understand what's happening there read 
https://www.gnu.org/software/guile/docs/docs-1.8/guile-ref/Closure.html#Closure