`clojure.core/assoc!`¶

Type: function Added: Clojure 1.1 Examples: 2 Runnable: 2

([coll key val] [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.

Examples¶

by yasuto on 2011-06-10

;; 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.  See one example for conj! that contains a detailed
;; example of a wrong result that can occur if you do not use its return value.

;; 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.

by dalzony on 2016-12-29

clojure.core/assoc!¶

Examples¶

See also¶

`clojure.core/assoc!`¶