（def m（update in in ks f＆args））是一个好习惯吗？

Question

I'm new to the world of clojure and I have a doubt. I have a nested map such as

(def accounts (hash-map :XYZ (hash-map :balance (hash-map 171000 0 :171018 500 :171025 200)
                                   :statement (hash-map :171018 [{:desc "purchase" :amount 200}
                                                                 {:desc "deposit" :amount 700}]
                                                        :171025 [{:desc "purchase" :amount 300}]))

And I want to update the statements, so I wrote a simple function:

(defn add-statement
[account date desc amount]
(def accounts (update-in accounts [account :statement date] conj {:desc desc :amount amount}))

But I have a feeling that I'm doing that the wrong way...

Answer 1

You would need to change accounts to be mutable if you want to update them. The usual way to do this is by making accounts an atom . Then your function might look like this:

(defn add-statement! [account date desc amount]
  (swap! accounts update-in [account :statement date]
         (fn [line-items] 
           (conj (or line-items []) {:desc desc :amount amount}))))

This will add a statement line-item at a new or an existing date. The update-in is the same as yours, except that a line-item for a new date will be put into a vector rather than a list. ( conj keeps the type, but it has to know the type: (conj nil :a) gives (:a) ).

To turn accounts into an atom:

(def accounts (atom (hash-map ...)))

I notice your balances are not correct anyway. But if you are updating them be sure to do so in the same swap! function.

To answer your question, "Is (def m (update-in m ...)) a good practice?". Definitely not inside a defn . If you are thinking to put a def inside a defn use a let instead. Outside of a defn it would be fine to have a def that updates another def that has a different name, so (def m2 (update-in m1 ...)) .