[erlang-questions] Re: Concept of Side-effect

[Jayson Vantuyl]

A small addendum.

Note that calling unsafe functions can create side-effects.  Things  
like gen_server:call, for example, send messages.  So you need to be  
careful with library functions.

Thinking a bit, other times that you can modify state:

* Changing the process dictionary (most people don't really do this)
* ETS tables (messages in disguise)
* DETS tables (messages in disguise)
* Network Connections (messages in disguise)
* digraphs (messages in disguise)

On Sep 17, 2009, at 6:35 PM, Jayson Vantuyl wrote:

> I'll do better.  Here's an example that actually is necessary.
>
> Assume you have two processes.  One updates a record in Mnesia, the  
> other receives an acknowledgement that it did it.
>
> Mnesia can replay a transaction in times of extreme contention.   
> Here's a buggy version of a function that charges some account.   
> Look at the fun() inside.  It has the side-effect that it sends a  
> message to another process.
>
>> charge_account(Requestor,UserId,Amount) ->
>>  {atomic,ok} = mnesia:transaction(
>>    fun() ->
>>      [ Current ] = mnesia:read(account,UserId),
>>      OldAmount = Current#account.amount,
>>      New = Current#account{ amount = OldAmount - Amount },
>>      ok = mnesia:write(New),
>>      Requestor ! {updated,New},
>>      ok
>>    end
>>  ).
>
> This function will potentially send the message to Requestor  
> multiple times if the transaction has to be retried.
>
> If you want to do this correctly, you can simply modify the function  
> like this:
>
>> charge_account(Requestor,UserId,Amount) ->
>>  {atomic,Result} = mnesia:transaction(
>>    fun() ->
>>      [ Current ] = mnesia:read(account,UserId),
>>      OldAmount = Current#account.amount,
>>      New = Current#account{ amount = OldAmount - Amount },
>>      ok = mnesia:write(New),
>>      New
>>    end,
>>    Requestor ! {updated,Result},
>>  ).
>
> The inner fun() has no side-effects and can now be safely retried by  
> Mnesia.
>
> This is only one instance where side-effects matter.  The important  
> thing to remember is that state data in Erlang takes the form of  
> recursion, processes and messages.  If you don't create recurse with  
> a modified value, create a process, or send a message, then no state  
> is changed.  Even when you recurse, your state only affects a single  
> process.  This property makes it very easy to create very stable  
> programs, because you can contain the places that state can get  
> corrupted or lost.
>
> Hopefully this helps.
>
> On Sep 17, 2009, at 6:17 PM, Kaiduan Xie wrote:
>
>> Thanks all for the reply. Can someone provide an example in code? For
>> example, a side-effect free function, and its counterpart function
>> with side-effect.
>>
>> Thanks,
>>
>> kaiduan
>>
>> On Thu, Sep 17, 2009 at 11:07 AM, Gene Tani <gene.tani@REDACTED>  
>> wrote:
>>>
>>>
>>> On Sep 16, 3:26 pm, Kaiduan Xie <kaidu...@REDACTED> wrote:
>>>> Hi,
>>>>
>>>> Can someone explain what does side-effect means in Erlang? What is
>>>> side-effect free function, and why we need to write side-effect  
>>>> free
>>>> function? A concrete example is preferred.
>>>>
>>>> Thanks,
>>>>
>>>> kaiduan
>>>>
>>>> ________________________________________________________________
>>>> erlang-questions mailing list. Seehttp://www.erlang.org/faq.html
>>>> erlang-questions (at) erlang.org
>>>
>>> http://blog.tornkvist.org/blog.yaws?id=1239107937892262
>>>
>>> http://www.cs.chalmers.se/Cs/Grundutb/Kurser/ppxt/HT2007/general/languages/armstrong-erlang_history.pdf
>>> (PDF page 14)
>>>
>>> and a FP vs. OO debate:
>>>
>>> http://news.ycombinator.com/item?id=493963
>>>
>>> ________________________________________________________________
>>> erlang-questions mailing list. See http://www.erlang.org/faq.html
>>> erlang-questions (at) erlang.org
>>>
>>>
>>
>> ________________________________________________________________
>> erlang-questions mailing list. See http://www.erlang.org/faq.html
>> erlang-questions (at) erlang.org
>>
>
>
>
> -- 
> Jayson Vantuyl
> kagato@REDACTED
>
>
>



-- 
Jayson Vantuyl
kagato@REDACTED