在Clojure中同时更新两个值

Question

I am wondering how to update two values at the same time. For instance, I want to increase month and decrease age at the same time. My code is

user=>(def users {:month 1 :age 26})
user=>(update-in users [:month :age] inc dec)

I know this syntax is not correct, but how can I fix this? And I need to update at the same time. Since if I update month first and then update age, then the first map will lost when I update the second map. Or is there other way to figure out this problem?

Answer 1

update does not modify a value, it just returns a new value, so it's just a function. If you need to update 2 fields of a map, the straightforward way to do that is just call update twice, first on the original map and then on the result of the first update:

(defn update-month-and-age [user]
  (update (update user :month inc) :age dec))

Which looks nicer using -> :

(defn update-month-and-age [user]
  (-> user
      (update :month inc)
      (update :age dec)))

Answer 2

in this simple case (update functions without additional params) you could also do it like this:

user> (def users {:month 1 :age 26 :records [1 2 3]})
#'user/users

user> (reduce-kv update users {:month inc :age dec :records reverse})
{:month 2, :age 25, :records (3 2 1)}

with additional params it would be a little bit more verbose:

user> (reduce-kv (partial apply update)
                 users
                 {:month [+ 2] :age [dec] :records [conj 101]})
{:month 3, :age 25, :records [1 2 3 101]}

well, it is still worse then simple usage of threading macro.

Answer 3

I'm tempted to solve this problem generally, by writing a function that takes a map: keyword -> function, and turns out a function that applies all the functions to the relevant record/map entries.

Such a function is

(defn field-updater [fn-map]
  (fn [record]
    (reduce
     (fn [m [k f]] (assoc m k (f (m k))))
     record
     fn-map)))

And we use it to generate your required field updater:

(def update-in-users (field-updater {:month inc :age dec}))

... which we can then apply

(update-in-users users)
;{:month 2, :age 25}