... or the no-sentinel find-based approach:
(if-let [[_ v] (find a-map :b)]
v
(my-foo))
Cheers,
Dave
On Mon, Oct 28, 2013 at 9:08 AM, Alex Baranosky <
alexander.barano...@gmail.com> wrote:
> 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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