[erlang-questions] Futures/promises and ability to "call" abstract modules

Gleb Peregud <>
Mon Nov 19 15:05:30 CET 2012


Then I wouldn't be able to set future's value after it being defined.
Like in this example:

7> F = future:new().
{future,<0.47.0>,#Ref<0.0.0.27235>,undefined}
8> spawn(fun() -> timer:sleep(10000), F:set(42) end).
<0.49.0>
9> F:get().
42

Without this ability such futures are useless with request-reply pattern:
1) Client sends request
2) Server creates a future and sends it back to client
3) Client does it's work with the value of the future (without
bothering the fact that value may become available in uncertain
future)
4) Server finishes computations and sets future's value


On Mon, Nov 19, 2012 at 3:01 PM, Robert Virding
<> wrote:
> Wouldn't it be easier if future:new just returned a fun? Then you could do F() without any changes?
>
> Robert
>
> ----- Original Message -----
>> From: "Gleb Peregud" <>
>> To: "Vlad Dumitrescu" <>
>> Cc: "Erlang" <>
>> Sent: Monday, 19 November, 2012 2:18:11 PM
>> Subject: Re: [erlang-questions] Futures/promises and ability to "call" abstract modules
>>
>> With the these changes [1] the following code works as intended:
>>
>> 7> F = future:new(fun() -> timer:sleep(5000), 42 end).
>> {future,<0.44.0>,#Ref<0.0.0.71>,undefined}
>> 8> F().
>> 42
>>
>> Aside from a problem where it improperly reports arity in case of
>> undefined function:
>>
>> 9> F(1,2,3).
>> ** exception error: undefined function future:call/3
>>
>> 1: https://github.com/gleber/otp/compare/call-abstract-module
>>
>> On Mon, Nov 19, 2012 at 11:48 AM, Gleb Peregud <>
>> wrote:
>> > Sverker Eriksson wrote the following in [1]:
>> >
>> >> But even if the resource terms look alike, they are unique and
>> >> there is
>> >> no bug leaking NIF resources (that I know of). A resource is
>> >> released
>> >> (and destructor called) when the last reference is garbage
>> >> collected.
>> >> The shell can fool you however, as it keeps a command history that
>> >> can
>> >> retain terms even though you think the variables are forgotten.
>> >> Test NIF
>> >> resource cleanup by running a test module and call
>> >> erlang:garbage_collect to force destructors to be called.
>> >
>> > This seems to mean that they are "shared" and garbage collected
>> > just once.
>> >
>> > 1:
>> > http://erlang.org/pipermail/erlang-questions/2011-January/055524.html
>> >
>> > On Mon, Nov 19, 2012 at 11:46 AM, Vlad Dumitrescu
>> > <> wrote:
>> >> I have no idea, that's why I asked :-)
>> >> /Vlad
>> >>
>> >>
>> >> On Mon, Nov 19, 2012 at 11:44 AM, Gleb Peregud
>> >> <> wrote:
>> >>>
>> >>> I assumed that NIF-generated resources are shared between
>> >>> processes (the
>> >>> same way as large binaries are), and I haven't done any tests on
>> >>> this. Are
>> >>> you sure it is garbate collected multiple times (once per
>> >>> referencing
>> >>> process)?
>> >>>
>> >>>
>> >>>
>> >>> On Mon, Nov 19, 2012 at 11:41 AM, Vlad Dumitrescu
>> >>> <>
>> >>> wrote:
>> >>>>
>> >>>> Hi Gleb,
>> >>>>
>> >>>> just a quick observation about garbage collecting futures: would
>> >>>> the
>> >>>> NIF-generated resource keep track of usage across processes? I
>> >>>> fI send a
>> >>>> future as a message, it may be referenced by multiple processes
>> >>>> which have
>> >>>> their own heap and garbage collection...
>> >>>>
>> >>>> regards,
>> >>>> Vlad
>> >>>>
>> >>>>
>> >>>> On Mon, Nov 19, 2012 at 11:32 AM, Gleb Peregud
>> >>>> <>
>> >>>> wrote:
>> >>>>>
>> >>>>> Hello
>> >>>>>
>> >>>>> Last evening I was trying to implement futures/promise
>> >>>>> mechanism in
>> >>>>> Erlang (mostly for fun, since I am still unsure if it is
>> >>>>> useful). I got
>> >>>>> inspired with the presentation [1], which mentioned using
>> >>>>> futures as a
>> >>>>> foundation of building services, where things like timeouts,
>> >>>>> tracing,
>> >>>>> authentication, etc. is built by composing futures (see slide
>> >>>>> 41).
>> >>>>>
>> >>>>> Do you think that such composition of futures could be useful
>> >>>>> as a tool
>> >>>>> to improve code reuse of communication patterns in Erlang (as
>> >>>>> described in
>> >>>>> the presentation)?
>> >>>>>
>> >>>>> I've implemented futures using processes and message passing
>> >>>>> and
>> >>>>> stumbled upon two issues:
>> >>>>> 1) garbage collection of futures
>> >>>>> 2) slightly too much code when using them
>> >>>>>
>> >>>>> Example of the first problem is here:
>> >>>>>
>> >>>>> 1> F = future:new(fun() -> timer:sleep(10000), 10 end).
>> >>>>> {future,<0.36.0>,#Ref<0.0.0.1736>,undefined}
>> >>>>> 2> F:get(). %% it hangs for 10 seconds
>> >>>>> 10
>> >>>>>
>> >>>>> Since future F is represented as a process <0.36.0> it will
>> >>>>> stay running
>> >>>>> forever till it's timed out (which is not a good solution,
>> >>>>> since someone may
>> >>>>> still have a reference to this future) or F:done() manually
>> >>>>> called.
>> >>>>>
>> >>>>> My idea is to insert into 'future' tuple a NIF-generated
>> >>>>> resource, which
>> >>>>> will have a destructor attached (called upon garbage collection
>> >>>>> of the
>> >>>>> resource) which will call F:done(). Will it work?
>> >>>>>
>> >>>>> The second issue is illustrated here:
>> >>>>>
>> >>>>> 7> F = future:new().
>> >>>>> {future,<0.47.0>,#Ref<0.0.0.27235>,undefined}
>> >>>>> 8> spawn(fun() -> timer:sleep(10000), F:set(42) end).
>> >>>>> <0.49.0>
>> >>>>> 9> F:get().
>> >>>>> 42
>> >>>>>
>> >>>>> In ideal world it should be enough to just write "F" (without
>> >>>>> :get()) to
>> >>>>> fetch future's value, but it seems too far fetched for Erlang.
>> >>>>> Slightly
>> >>>>> better solution would be to allow calling future with "F()".
>> >>>>>
>> >>>>> This can be done by extending concept of "abstract modules"
>> >>>>> with
>> >>>>> "default call". Currently abstract modules allow the following:
>> >>>>>
>> >>>>> {future, Pid, Ref, undefined}:get() which is translated to
>> >>>>> future:get({future, Pid, Ref, undefined})
>> >>>>>
>> >>>>> With a simple change in beam_emu.c in call_fun function (which
>> >>>>> would
>> >>>>> replace obsolete fun tuples) we can allow for the following:
>> >>>>>
>> >>>>> {future, Pid, Ref, undefined}() which COULD be translated to
>> >>>>> future:call({future, Pid, Ref, undefined})
>> >>>>>
>> >>>>> hence allowing to use just "F()" to read a value of the future.
>> >>>>> This
>> >>>>> will also extend "metaprogramming" capabilities of Erlang for
>> >>>>> some other
>> >>>>> quirky use, which may or may not be a Good Thing(tm).
>> >>>>>
>> >>>>> Thoughts?
>> >>>>>
>> >>>>> Cheers,
>> >>>>> Gleb Peregud
>> >>>>>
>> >>>>> 1: http://monkey.org/~marius/talks/twittersystems/
>> >>>>>
>> >>>>> _______________________________________________
>> >>>>> erlang-questions mailing list
>> >>>>> 
>> >>>>> http://erlang.org/mailman/listinfo/erlang-questions
>> >>>>>
>> >>>>
>> >>>
>> >>
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