Sunil S Nandihalli <sunil.nandiha...@gmail.com> writes:
> Hello everybody,
> the following code is not producing what was expected of it in clojure
> 1.2 .. has this been fixed the clojure 1.3?
>
> (let [x (transient {})]
> (dotimes [i 10]
> (assoc! x i (* i i)))
> (persistent! x))
This is not a bug, it's a misuse of transients. A transient map is not
an imperative mutable map. They're just a performance optimisation for
certain use cases of an immutable map.
To quote the clojure.org/transients documentation:
> 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. In this way, they support the same code structure as the
> functional persistent code they replace.
There's a simple rule for using transients correctly. First write the
code with a normal immutable data structure:
(let [x {}]
(reduce (fn [m i] (assoc m i (* i i))) x (range 10)))
Then stick "!" on the end of assoc/conj/disj and wrap in the
transient/persistent! calls. Don't make any other changes to the
code structure.
(let [x (transient {})]
(persistent! (reduce (fn [m i] (assoc! m i (* i i))) x (range 10))))
Hope that helps,
Alex
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