Who said swap *was* CAS, rather than was implemented *in terms of* CAS? In
any event, your claim that "no comparison is done unless it's done in the
supplied function" is just plain wrong. Comparison *is* done, outside that
function, to make sure the atom wasn't changed by another thread while the
function was executing.
Or just look at the source for clojure.lang.Atom.swap():
public Object swap(IFn f) {
for(; ;)
{
Object v = deref();
Object newv = f.invoke(v);
validate(newv);
if(state.compareAndSet(v, newv))
{
notifyWatches(v, newv);
return newv;
}
}
}
The compareAndSet method of a java.util.concurrent.AtomicReference is
invoked, and other than the watcher and validator stuff you can see that
it's semantically the same as the Clojure loop I posted previously.
On Sat, Jun 29, 2013 at 8:52 PM, Ben Wolfson <[email protected]> wrote:
> On Sat, Jun 29, 2013 at 5:27 PM, Cedric Greevey <[email protected]>wrote:
>
>>
>> On Sat, Jun 29, 2013 at 8:21 PM, Ben Wolfson <[email protected]> wrote:
>>
>>> On Sat, Jun 29, 2013 at 5:07 PM, Brandon Bloom <
>>> [email protected]> wrote:
>>>
>>>> > Can anyone explain the relationship between swap! and reset! ?
>>>>
>>>> swap! is for "CAS"
>>>>
>>>> See: http://en.wikipedia.org/wiki/Compare-and-swap
>>>>
>>>>
>>> How so? No comparison is done (unless it's done in the supplied
>>> function, which is entirely up to the user of swap!). There's a separate
>>> compare-and-set! function for atoms.
>>>
>>
>> Er, swap! is basically
>>
>> (loop []
>> (let [x @a y (f x)]
>> (if (compare-and-set! a x y)
>> y
>> (recur))))
>>
>> though it actually calls a .swap method of the Atom object that has
>> equivalent semantics.
>>
>>
> The fact that swap! can be implemented with compare-and-set! doesn't make
> it enlightening (or even correct) to say that swap! is CAS, especially
> since compare-and-swap as explicated by the linked wikipedia page is
> actually compare-and-set! in Clojure terms (there's no transformation by a
> function and it's not unconditional).
>
> --
> Ben Wolfson
> "Human kind has used its intelligence to vary the flavour of drinks, which
> may be sweet, aromatic, fermented or spirit-based. ... Family and social
> life also offer numerous other occasions to consume drinks for pleasure."
> [Larousse, "Drink" entry]
>
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