On Wed, Apr 26, 2017 at 12:21 AM, Jon Zeppieri <[email protected]> wrote:
> 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. > Yeah, that works well. Thanks! Any thoughts on how to do it for non-nullary functions? > > > > 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? > > > > -- > > You received this message because you are subscribed to the Google Groups > > "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to [email protected]. > > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
