Or a shorter variant of the sentinel approach:
(let [r (get a-map :b ::unfound)]
(if (= r ::unfound)
(my-foo)
r))
On Sun, Oct 27, 2013 at 2:57 PM, Cedric Greevey <cgree...@gmail.com> wrote:
> (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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