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.
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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