For the record, you can also use fnil:
=> (defn mystery [m k] (update-in m [k] (fnil inc 0)))
#'user/mystery
=> (mystery {} :a)
{:a 1}
=> (mystery {:a 4} :a)
{:a 5}
2011/6/1 Ken Wesson <kwess...@gmail.com>:
> On Wed, Jun 1, 2011 at 1:05 AM, Andreas Kostler
> <andreas.koestler.le...@gmail.com> wrote:
>> Hi all,
>> I find myself using the following pattern quite often:
>> (assoc m :key (inc (or (:kay m) 0))
>>
>> To increment or somehow transform a value in a map that I'm not sure it
>> exists.
>> Is there an idiomatic way of doing this sort of thing in a short and concise
>> way?
>
> Well, these also work:
>
> (assoc m :key (inc (m :key 0)))
>
> (merge-with + m {:key 1})
>
> (update-in m [:key] #(if % (inc %) 1))
>
> The merge-with looks shortest and cleanest of the four to me, but
> that's just my opinion.
>
> Incidentally, if you forget the brackets around :key in the update-in
> the error message you get is rather less specific than you'd hope:
>
> => (update-in {:a 1} :key #(do (println %) (inc %)))
> #<CompilerException java.lang.UnsupportedOperationException: nth not
> supported on this type: Keyword (NO_SOURCE_FILE:0)>
>
> A :pre clause in update-in checking for coll? on the second argument
> could be added to produce an IAE with a more informative message
> instead of this somewhat vague UOE.
>
> --
> Protege: What is this seething mass of parentheses?!
> Master: Your father's Lisp REPL. This is the language of a true
> hacker. Not as clumsy or random as C++; a language for a more
> civilized age.
>
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