aaa Laurent beat me to it! :)
it seems we both understood the same thing so fnil is indeed your friend...
Jim
On 05/04/13 12:29, Jim foo.bar wrote:
if I understood correctly you're looking for 'fnil' :
=>(update-in {} [:foo :bar] (fnil conj []) :a :b :c)
{:foo {:bar [:a :b :c]}}
Jim
On 05/04/13 12:25, 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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