See points 2, 3 and 8 at http://clojure.org/refs
It is just a way to obtain the "change lock" of the ref in the dosync
transaction, without rewriting it. The alternative would be to
explicitly modify it to the same value as it was before, which is
potentially wasteful.
One possible case where this could be useful would be when you have some
synchronized state that should not be touched while another is.
Fictional elevator example:
(def elevator-doors (ref :closed))
(def elevator-floor (ref 1))
;; Elevator doors should NOT be opened while the elevator changes floor
(dosync (alter elevator-floor inc)
(ensure elevator-doors))
/Linus
On 2014-06-19 17:35, Patrick Kristiansen wrote:
> I believe it is to avoid write skew. Check this Wikipedia page:
>
> http://en.m.wikipedia.org/wiki/Snapshot_isolation
>
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