Let me add my 2 cents here.
The "delay" solution seems like the right one, with one caveat: two
otherwise identical individuals will no longer have the same hash
value or compare as "=". If you're trying to remove duplicate
individuals or some such, this may be a problem; otherwise, probably
not.
user> (= (delay 2) (delay 2))
false
If this is the case, you probably want to store the delayed fitness
value as metadata on your individual:
(with-meta individual {:fitness (delay ...)})
(force (:fitness ^individual))
If you don't like delay/force, an alternative is to just wrap the
computation of the fitness in a single-element lazy-seq:
(lazy-seq (list (fitness ...))).
The first time you access this lazy-seq, the value will be computed
and then cached for future calls. An advantage of this approach is
that you don't have to put the lazy-seq in metadata to save equality
semantics, because calling (= ...) or (hash ...) on your individual
will force the seq. But, for efficiency, you'd probably still want to
store it in metadata ...
Cheers,
Jason
On Mar 4, 12:09 pm, bOR_ <[email protected]> wrote:
> If you use hash-maps, or a struct-map as the basis of your individual,
> you can just make a key 'fitness', and store the once-calculated
> fitness in there.
>
> I'm not sure how, but it might be possible that at the creation of
> your animals, you assign the key 'fitness' a basic function that upon
> being accessed replaces itself with its outcome. I would be interested
> in seeing that coded. It would be a very neat way to do what you want.
>
> proof of concept, which is not going to win a beaty contest.
>
> user=> (def myhost (hash-map :c 15 :b 3 :a (fn [hostname] (assoc
> hostname :a (* (hostname :b) (hostname :c))))))
> #'user/myhost
> user=> myhost
> {:a #<user$fn__35 user$fn_...@d85cc>, :b 3, :c 15}
> user=> (def myhost ((myhost :a) myhost))
> #'user/myhost
> user=> myhost
> {:a 45, :b 3, :c 15}
> user=>
>
> On Mar 4, 6:27 pm, Meikel Brandmeyer <[email protected]> wrote:
>
> > Hi,
>
> > Am 04.03.2009 um 18:21 schrieb Dan:
>
> > > How can I have my fitness computation be triggered on first access
> > > and reused afterward?
>
> > Maybe you can use a Delay.
>
> > {:fitness (delay (compute-my-fitness))}
>
> > And when you access the value use force.
>
> > (-> my-thing :fitness force)
>
> > The first time it is accessed, the value is calculated.
> > Afterwards the cached value is returned. If you don't
> > use the value of interim stages, it is not calculated.
>
> > Sincerely
> > Meikel
>
> > <commercial>
> > If you use maps as above I also have a lazy-map
> > package, which evaluates the value of key only
> > in case it is really accessed.
>
> >http://bitbucket.org/kotarak/lazymap
> > </commercial>
>
> > smime.p7s
> > 5KViewDownload
>
>
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