[racket-users] Re: Best way to write run-once-and-cache functions?

[Jack Firth]

A `define/delay` macro for this might be a good addition to `racket/promise`:

(define-simple-macro (define/delay id:id expr:expr)
  (begin (define p (delay expr)) (define (id) (force p

(define/delay conf
  (with-input-from-file "db.conf" read-json))

(conf) ;; forces promise

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[racket-users] Re: Best way to write run-once-and-cache functions?

[Alex Harsanyi]

On Wednesday, April 26, 2017 at 12:12:26 PM UTC+8, David K. Storrs wrote:

> 
> I could do it with a parameter but that's only sweeping the above ugliness 
> under the rug:
> 
> (define conf (make-parameter #f))
> 
> (define (read-conf)
>    (or (conf)
>  (begin
>    (conf (with-input-from-file "db.conf" (thunk (read-json
>    (conf

This is probably the wrong way, as the parameter will only be set in the 
current thread.  The code will read the contents of "db.conf" in every thread 
that `read-conf` is called.

Best Regards,
Alex.

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