Hi,
2013/4/5 Simon Katz <nomisk...@gmail.com>:
> 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)}}
I think you want fnil:
;; Clojure 1.5.1
=> (update-in {} [:foo :bar] conj :b)
{:foo {:bar (:b)}}
=> (update-in {} [:foo :bar] (fnil conj []) :b)
{:foo {:bar [:b]}}
=> (require 'clojure.repl)
nil
=> (clojure.repl/doc fnil)
-------------------------
clojure.core/fnil
([f x] [f x y] [f x y z])
Takes a function f, and returns a function that calls f, replacing
a nil first argument to f with the supplied value x. Higher arity
versions can replace arguments in the second and third
positions (y, z). Note that the function f can take any number of
arguments, not just the one(s) being nil-patched.
nil
>
> 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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