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

[Aaron Hill]

On 2019-03-29 3:37 am, Thomas Morley wrote:
> Am Fr., 29. März 2019 um 10:00 Uhr schrieb Richard Shann
> <address@hidden>:
> > 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
>
> Hi Richard,
>
> I tried to understand what you've got from the guile-list.
>
> Thus I opened a guilev2 prompt and simply typed cond-expand:
>
> scheme@(guile-user)> (version)
> $1 = "2.2.4.9-71f536"
> scheme@(guile-user)> cond-expand
> While compiling expression:
> Syntax error:
> unknown location: source expression failed to match any pattern in
> form cond-expand
> scheme@(guile-user)>
>
> Well, ok, the guile-manual says:
> 7.5.2 SRFI-0 - cond-expand
> [...]
>
> So I tried:
>
> scheme@(guile-user)> (use-modules (srfi srfi-0))
> While compiling expression:
> no code for module (srfi srfi-0)
>
> What am I missing?

Looks like srfi-0 is built into guile 2, so there is no module to use 
anymore.

But cond-expand does work if you do something with it:

scheme@(guile-user)> (cond-expand (guile-2 'hi))
$2 = hi

-- Aaron Hill