[erlang-questions] Process Dictionary limitations??

Michael Truog <>
Thu Oct 11 01:21:29 CEST 2012


On 10/10/2012 03:55 PM, Charles Hixson wrote:
> I'm choosing a language to implement a ... well, neural network is wrong, and so is cellular automaton, but it gives the idea.  Anyway, I'm going to need, in each cell, a few stateful items, e.g. activation level.
>
> When I look at what Erlang can do, I see that the Process Dictionary looks as if it would serve my needs, but then I am immediately warned not to use it, that it will cause bugs.  These stateful terms will not be exported from the cell within which they are resident.  Is this still likely to cause problems?  Is there some better approach to maintaining state?  (I can't just generate a new process, because other cells will need to know how to access this one, or to test that it has been rolled out.)
>
This explains some basics about the process dictionary: http://www.erlang.org/course/advanced.html#dict
Quoted below:

    * Destroys referential transparency
    * Makes debugging difficult
    * Survives Catch/Throw

So, it is much better to use variables, so side-effects are more explicit (i.e., function variables).  This is the equivalent to the State variable of a gen_server behaviour (http://www.erlang.org/doc/man/gen_server.html).  Depending on the expected state-handling, you might want a gen_server, a gen_event, or a gen_fsm for each cell.  Otherwise, if you want to avoid OTP behaviour usage, you could just do plain Erlang code, but your code might then be more error-prone (especially since you are asking this question).

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