2010/11/29 Stuart Halloway <stuart.hallo...@gmail.com>
> I must respectfully disagree with James's first point here. The first
> pattern (read-ponder-update) is not concurrency-friendly. It isn't about
> atom vs. ref, the important distinction is whether all the work can be done
> in a function that gets sent *to* the ref. The latter formulation also can
> be extracted into a pure function, which is easier to test.
>
Yep, and in this case it *seems* that using commute could be useful as well
(and thus one more point for the latter form).
Though, I fail to see from a point of view of "correctness", why the former
form is not concurrency-friendly ?
>
> The bang suffix "!" is generally reserved for functions that are not safe
> in a transaction. If you are using atoms, you should have the bang, if using
> refs, you shouldn't.
>
> Stu
>
> > I'd say the former was more idiomatic Clojure. There's no need to make
> > refs compatible with atoms (otherwise why have two different
> > concurrency primitives in the first place).
> >
> > Additionally, earmuffs (like *this*) should only be used on vars that
> > you expect to override in a binding, like *out* or *err*. They aren't
> > for all global vars. You should rename "*counts*" to just "counts".
> >
> > - James
> >
> > On 28 November 2010 22:36, Takahiro Hozumi <fat...@googlemail.com>
> wrote:
> >> Hi,
> >> This is trivial question.
> >> Which style do you like?
> >>
> >> (def *counts* (ref {}))
> >>
> >> (defn dec-or-dissoc! [key]
> >> (dosync
> >> (let [n (@*counts* key)]
> >> (if (< 1 n)
> >> (alter *counts* assoc key (dec n))
> >> (alter *counts* dissoc key)))))
> >>
> >> (defn dec-or-dissoc! [key]
> >> (dosync
> >> (alter *counts*
> >> (fn [m]
> >> (let [n (m key)]
> >> (if (< 1 n)
> >> (assoc m key (dec n))
> >> (dissoc m key)))))))
> >>
> >> I think former style is normal, but latter is easy to replace ref with
> >> atom.
> >> Thanks.
> >>
> >> --
> >> Takahiro Hozumi
> >>
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