I think it is because concurrent stuff can happen between let and
alter in the first example. Is it true that in that case, one would
have to use "ensure" to make the operation correct?
On Nov 29, 5:53 pm, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> 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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