[erlang-questions] update_counter on steroids

[Anthony Ramine]

Indeed.

And now we can have 4-ary BIFs. :p

I plan to implement ets:take_all_objects(Table) which would empty out a table and return all its rows. Would you have a use for such a thing too?

Regards,

-- 
Anthony Ramine

Le 11 mai 2014 à 13:52, Ulf Wiger <ulf@REDACTED> a écrit :

> 
> Hi Anthony,
> 
> That’s certainly a function that many erlangers have had to implement over and over. :)
> 
> And of course, having to write it in Erlang by combining several operation into a non-atomic function introduces some problems of its own.
> 
> BR,
> Ulf W
> 
> On 11 May 2014, at 13:12, Anthony Ramine <n.oxyde@REDACTED> wrote:
> 
>> Hello Ulf,
>> 
>> Not what you asked for, but I implemented ets:update_counter/4 which takes a default value yesterday:
>> 
>> 	ets:new(counters, [set,named_table]),
>> 	1 = ets:update_counter(counters, c1, 1, {c1,1}),
>> 	3 = ets:update_counter(counters, c1, 2, {c1,1}),
>> 	[{c1,3}] = ets:tab2list(counters).
>> 
>> Regards,
>> 
>> 	https://github.com/erlang/otp/pull/362
>> 
>> -- 
>> Anthony Ramine
>> 
>> Le 7 août 2013 à 13:39, Ulf Wiger <ulf@REDACTED> a écrit :
>> 
>>> 
>>> There was some discussion recently on teaching update_counter some new tricks.
>>> 
>>> Here is a suggestion:
>>> 
>>> Right now, the function takes (Tab, Key, UpOp | [UpOp]), where
>>> UpOp ::== {Pos, Incr} | {Pos, Incr, Threshold, WrapTo}
>>> 
>>> I'd like to save the result of an operation in a temporary variable.
>>> 
>>> An atomic reset, for example. (let's say counter c1 has the value 17):
>>> 
>>> ets:update_counter(counters, c1, [{2, 0, '$1'}, {2, {'-', '$1'}}]) ->
>>> [17, 0]
>>> 
>>> (Increment by 0 means we read the existing value, just like today).
>>> 
>>> Why use this instead of simply ets:insert/2? Well, for one thing, we get the old value back, so it's an atomic read-reset.
>>> 
>>> Sum counter:
>>> 
>>> ets:update_counter(counters, c2, [{2,0,'$1'}, {3,0,'$2'}, {4, {'+', '$1', '$2'}}])
>>> 
>>> Wrap with parameterized threshold and reset value:
>>> 
>>> ets:update_counter(counters, c3, [{3,0,'$1'}, {4,0,'$2'}, {2, Incr, '$1', '$2'}])
>>> 
>>> This assumes the following changes:
>>> 
>>> - A number of temp variables, like in match specs.
>>> - A 3-tuple  {Pos, Incr, SaveTo}, where SaveTo is a variable name (e.g. '$1')
>>> - A 5-tuple  {Pos, Incr, Threshold, WrapTo, SaveTo}
>>> - The possibility to use, wherever an integer() is expected
>>> VariableName | {UnaryOp, VariableName} | {BinaryOp, Var1, Var2}
>>> 
>>> We could also use this to e.g. store a value together with an increment operation. A reset with timestamp:
>>> 
>>> TS = timestamp(),  % in milliseconds
>>> ets:update_counter(counters, c4, [{2, 0, '$1'}, {2, {'-', '$1'}, {3, 0, '$2'}, {3, {'-', TS, '$2'}}])
>>> 
>>> …a bit bizarre perhaps, but still.
>>> 
>>> Comments?
>>> 
>>> BR,
>>> Ulf W
>>> 
>>> 
>>> Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
>>> http://feuerlabs.com
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> erlang-questions mailing list
>>> erlang-questions@REDACTED
>>> http://erlang.org/mailman/listinfo/erlang-questions
>> 
> 
> Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
> http://feuerlabs.com
> 
> 
>