(get a-map :b my-foo) will result in the function object itself being
returned if :b is not found. If you want it to be called only in the event
of not found, you need either
(if (contains? a-map :b) (a-map :b) (my-foo)) -- which may perform the
lookup twice -- or
(if-let [r (a-map :b)] r (my-foo)) -- which does not, but treats a {:b nil}
entry as the same as absence of :b -- or
(let [sentinel (Object.)
r (get a-map :b sentinel)]
(if (identical? r sentinel)
(my-foo)
r)) -- which is longer and more complex, but calls my-foo (and returns
what it returns) if and only if :b is genuinely absent from the map (like
the first solution above) and performs the lookup only once (like the
second solution above). If this has to be done in a tight loop, the extra
efficiency over the second solution may be worth it, but you'll want to
avoid repeatedly creating and discarding the sentinel object as well,
resulting in something like
(def {^:private} sentinel (Object.))
...
(defn ...
...
(loop ...
...
(let [r (get a-map :b sentinel)]
(if (identical? r sentinel)
(my-foo)
r)) ... ) ... )
On Sun, Oct 27, 2013 at 1:00 PM, Ryan <arekand...@gmail.com> wrote:
> Silly me, thank you for your replies guys!
>
> One more question though, what if my-foo had parameters?
>
> Ryan
>
>
> On Sunday, October 27, 2013 6:55:34 PM UTC+2, Luc wrote:
>
>> You are getting my-foo evaluated, remove the parens around it.
>>
>> Luc P.
>>
>>
>> > Hello,
>> >
>> > I am trying to understanding why is this happening:
>> >
>> > > (defn my-foo [] (println "Why do I get printed?"))
>> > > #'sandbox4724/my-foo
>> > > > (get {:b 1} :b (my-foo))
>> > > Why do I get printed?
>> > > 1
>> > > >
>> >
>> >
>> > Shouldn't (my-foo) only be called in case the key isn't found? Why am I
>> > seeing the above behavior instead?
>> >
>> > Thank you for your time,
>> >
>> > Ryan
>> >
>> > --
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