Re: ASSIGN_EVENT_ONCE in Scheme

[Jean Abou Samra]

Le 20/02/2022 à 19:46, Dan Eble a écrit :
> ASSIGN_EVENT_ONCE(cur, new) does this:
>
>    - if cur is nullptr: assign cur = new, return true
>    - if *cur and *new are equal: quietly return false
>    - if *cur and *new are unequal: warn and return false
>
> Would a Scheme analog of ASSIGN_EVENT_ONCE be used like this,
>
>      (let ((my-foo-event #f))
>        ;; . . .
>        (listeners
>          ((foo-event this-engraver event)
>           (if (ly:set-event-once! my-foo-event event)
>             ;; my-foo-event event has been set: handle it
>
> or am I thinking too much like a C++ programmer?



You would have to define it as a macro, a Scheme
procedure (= function) can't set lvalues.

#(define-macro (assign-event-once lvalue rvalue)
  (let ((var (make-symbol "event")))
    `(let ((,var ,rvalue))
       (cond
         ((not ,lvalue)
          (set! ,lvalue ,var))
         ((equal? ,lvalue ,var))
         (else
          (ly:event-warning ,var "conflict bla bla"))))))

However, my personal take is that ASSIGN_EVENT_ONCE
is an attractive nuisance in a number of cases (its
use prevents concurrent events that could very well
be handled), so I would rather take the direction
of reducing its use in C++. That said, if you have
a good reason for it in Scheme, go for it.

HTH

Jean