Why inserting 1000 000 values in a transient map in Clojure yields a map with 8 items in it?

Question

If I try to do 1000 000 assoc! on a transient vector, I'll get a vector of 1000 000 elements

(count
  (let [m (transient [])]
    (dotimes [i 1000000]
      (assoc! m i i)) (persistent! m)))
; => 1000000

on the other hand, if I do the same with a map, it will only have 8 items in it

(count
  (let [m (transient {})]
    (dotimes [i 1000000]
      (assoc! m i i)) (persistent! m)))
; => 8

Is there a reason why this is happening?

Answer 1

The transient datatypes' operations don't guarantee that they will return the same reference as the one passed in. Sometimes the implementation might decide to return a new (but still transient) map after an assoc! rather than using the one you passed in.

The ClojureDocs page on assoc! has a nice example that explains this behavior:

;; 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.

I'd like to repeat that last part because it's very important: the original collection you passed in should be treated as having an undefined value. Only the return value is predictable.

Here's a modified version of your code that works as expected:

(count
  (let [m (transient {})]
    (persistent!
      (reduce (fn [acc i] (assoc! acc i i))
              m (range 1000000)))))

---

As a side note, the reason you always get 8 is because Clojure likes to use a clojure.lang.PersistentArrayMap (a map backed by an array) for maps with 8 or fewer elements. Once you get past 8, it switches to clojure.lang.PersistentHashMap.

user=> (type '{1 a 2 a 3 a 4 a 5 a 6 a 7 a 8 a})
clojure.lang.PersistentArrayMap
user=> (type '{1 a 2 a 3 a 4 a 5 a 6 a 7 a 8 a 9 a})
clojure.lang.PersistentHashMap

Once you get past 8 entries, your transient map switches the backing data structure from an array of pairs (PersistentArrayMap) to a hashtable (PersistentHashMap), at which point assoc! returns a new reference instead of just updating the old one.