(assoc! coll key val) (assoc! coll key val & kvs)
When applied to a transient map, adds mapping of key(s) to val(s). When applied to a transient vector, sets the val at index. Note - index must be <= (count vector). Returns coll.
;; The key concept to understand here is that transients are
;; not meant to be `bashed in place`; always use the value
;; returned by either assoc! or other functions that operate
;; on transients.
(defn merge2
"An example implementation of `merge` using transients."
[x y]
(persistent! (reduce
(fn [res [k v]] (assoc! res k v))
(transient x)
y)))
;; Why always use the return value, and not the original? Because the return
;; value might be a different object than the original. The implementation
;; of Clojure transients in some cases changes the internal representation
;; of a transient collection (e.g. when it reaches a certain size). In such
;; cases, if you continue to try modifying the original object, the results
;; will be incorrect.
;; Think of transients like persistent collections in how you write code to
;; update them, except unlike persistent collections, the original collection
;; you passed in should be treated as having an undefined value. Only the return
;; value is predictable.
(def m (assoc! (transient {}) :x 1 :y 2))
(:x m)
;; 1
(:y m)
;; 2
(count m)
;; 2