[英]How to update records in a ref map in Clojure?
Given the following scenario: 鉴于以下情况:
(defrecord Person [firstname lastname])
(def some-map (ref {}))
(dosync
(alter some-map conj {1 (Person. "john" "doe")})
(alter some-map conj {2 (Person. "jane" "jameson")}))
To change the firstname of "joe" to "nick", I do the following: 要将“joe”的名字改为“nick”,我会执行以下操作:
(dosync
(alter some-map (fn [m]
(assoc m 1
(assoc (m 1) :firstname "nick")))))
What is the idiomatic way of doing this in Clojure? 在Clojure中这样做的惯用方法是什么?
无需使用update-in,对于这种情况,assoc-in正是您想要的。
(dosync (alter some-map assoc-in [1 :firstname] "nick"))
Edit: For your example assoc-in
is better, since you ignore the previous value. 编辑:对于您的示例,
assoc-in
更好,因为您忽略了以前的值。 Keeping this answer for cases where you actually need the previous value: 在实际需要以前值的情况下保留此答案:
The update-in
is there to update nested structures: update-in
用于更新嵌套结构:
(alter some-map update-in [1 :firstname] (constantly "nick"))
The last argument is a function on the value to be "replaced" (like assoc
, it does not replace but returns a new structure.) In this case the old value is ignored, hence the constantly
function that always returns "nick". 最后一个参数是要被“替换”的值的函数(如
assoc
,它不替换但返回一个新结构。)在这种情况下,旧值被忽略,因此常常返回“nick”的constantly
函数。
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