I don't know that there's a right way, but if your functions are
nullary, then promises are a decent fit:
(define conf
(delay
(with-input-from-file ...)))
Then just (force conf) whenever you want the value.
On Wed, Apr 26, 2017 at 12:12 AM, David Storrs <[email protected]> wrote:
> Reading configuration files is a good example of a run-once function. It's
> effectively self-memoizing -- it should run once, cache its result, and on
> future calls just return the cached value. I could pull in
> https://docs.racket-lang.org/memoize/index.html or
> http://docs.racket-lang.org/mischief@mischief/memoize.html to do that, but
> adding an extra module to the project just for one or two functions feels
> pretty heavy. (Also, memoizing is overkill if the functions are thunks.) Is
> there a simple way to do this in pure Racket?
>
> In Perl I would do something like this (for simplicity I'm ignoring encoding
> and assuming that the config file is JSON):
>
> use JSON;
> sub read_conf {
> state $conf = do {
> local $/;
> open my $fh, "<", "db.conf" or die "failed to open config: $!";
> from_json( <$fh> )
> };
> $conf
> }
>
>
> I could do this in Racket using set!, but that's not very Rackety:
>
> (require json)
> (define conf #f)
> (define (read-conf)
> (or conf
> (begin
> (set! conf (with-input-from-file "db.conf" (thunk (read-json))))
> conf)))
>
>
> 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))))
>
> What is the right way?
>
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