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

Gleb Peregud <>
Mon Nov 19 14:18:11 CET 2012


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
>>>>>
>>>>
>>>
>>



More information about the erlang-questions mailing list