问题
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?
回答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.
回答2:
The simplest explanation is from the Clojure documentation itself (emphasis mine):
Transients support a parallel set of 'changing' operations, with similar names followed by ! - assoc!, conj! etc. These do the same things as their persistent counterparts except the return values are themselves transient. Note in particular that transients are not designed to be bashed in-place. You must capture and use the return value in the next call.
来源：https://stackoverflow.com/questions/29684803/why-inserting-1000-000-values-in-a-transient-map-in-clojure-yields-a-map-with-8