There is of course a short and pithy way to do it with anonymous functions,
too: #(conj (or %1 []) %2))
On Fri, Apr 5, 2013 at 8:03 AM, Simon Katz <nomisk...@gmail.com> wrote:
> Thanks Laurent and Jim — yes, fnil is what I was looking for.
>
>
> On Friday, 5 April 2013 12:25:17 UTC+1, Simon Katz wrote:
>>
>> Hi.
>>
>> Is there an idiomatic way to have update-in create a vector when the
>> supplied keys do not already exist? (Or maybe I should use something other
>> than update-in?)
>>
>> For example...
>>
>> This gives me a sequence of things in the reverse of the order I want:
>>
>> (update-in {} [:foo :bar] conj :a)
>> ;; => {:foo {:bar (:a)}}
>>
>> (update-in *1 [:foo :bar] conj :b)
>> ;; => {:foo {:bar (:b :a)}}
>>
>> This does what I want, but at the expense of creating a new vector each
>> time:
>>
>> (update-in {} [:foo :bar] (comp vec conj) :a)
>> ;; => {:foo {:bar [:a]}}
>>
>> (update-in *1 [:foo :bar] (comp vec conj) :b)
>> ;; => {:foo {:bar [:a :b]}}
>>
>> (I know about laziness and structure sharing, and that this expense may
>> not be large.)
>>
>> The following works, but at the expense of a new function that the reader
>> has to understand:
>>
>> (defn conj-vec
>> "Like conj, but returns a vector if coll is empty.
>> Useful in conjunction with update-in."
>> [coll x]
>> (if (empty? coll) [x] (conj coll x)))
>>
>> (update-in {} [:foo :bar] conj-vec :a)
>> ;; => {:foo {:bar [:a]}}
>>
>> (update-in *1 [:foo :bar] conj-vec :b)
>> ;; => {:foo {:bar [:a :b]}}
>>
>> Is there a better way?
>>
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