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

[Ovidiu Deac]

Exceptions too?

On Fri, Sep 18, 2009 at 4:47 AM, Jayson Vantuyl <kagato@REDACTED> wrote:
> 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
>
>
>
>
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