[erlang-questions] Reading, Learning, Confused

Toby Thain <>
Tue Jul 22 15:44:16 CEST 2008


On 20-Jul-08, at 3:52 PM, Alpár Jüttner wrote:

> On Sun, 2008-07-20 at 14:53 -0300, Toby Thain wrote:
>> On 19-Jul-08, at 11:31 AM, Alpár Jüttner wrote:
>>
>>> Btw. the Erlang Reference Manual says that
>>>
>>>         As of Erlang 5.5/OTP R11B, short-circuit boolean
>>> expressions are
>>>         allowed in guards. In guards, however, evaluation is always
>>>         short-circuited since guard tests are known to be free of  
>>> side
>>>         effects.
>>>         (Section 6.14, Short-Circuit Boolean Expressions)
>>>
>>> Something is wrong here, isn;t it?
>>
>>
>> I also did a double take on this text, but my reading of "always
>> short-circuited" is "it is always safe to short circuit [since...]",
>> so a (normally) non-short-circuit operator can always be short-
>> circuited.
>
> Do you mean that 'or' can always be replaced by 'orelse'? We've learnt
> that it is not true.
>
>> (Compare, e.g. C's | and ||, where | may be used deliberately for
>> side-effects on the RHS.)
>
> It is a different story. In C, '|' is the bitwise or operation,

Yes I am aware. However, you may choose to use it in a logical  
context where you explicitly don't want short circuit OR (that was  
the point of my example).

--Toby

> it
> corresponds to 'bor' in Erlang. ('||' in C is the same as 'orelse' and
> there is no C equivalent for the 'or' operator of Erlang.)
>
> Regards,
> Alpar
>
>>
>> --Toby
>>
>>>
>>> Regards,
>>> Alpar
>>>
>>> On Sat, 2008-07-19 at 06:50 -0700, Lev Walkin wrote:
>>>> Sean Allen wrote:
>>>>> by a small bit of example code in Programming Erlang related to
>>>>> guards
>>>>> and short circuit booleans:
>>>>>
>>>>>   f(X) when (X == 0) or (1/X > 2) ->
>>>>>      ...
>>>>>
>>>>> g(X) when (X == 0) orelse ( 1/X > 2) ->
>>>>>     ...
>>>>>
>>>>> The guard in f(X) fails when X is zero but succeeds in g(X)
>>>>>
>>>>> Can someone explain why?
>>>>
>>>>
>>>> Sean,
>>>>
>>>> The thing is, "or" does not short-circuit evaluation when left side
>>>> succeeds, whereas "orelse" does. Same short-circuit logic is
>>>> behind the differences between "and" and "andalso".
>>>>
>>>> Actually, the very book you read explains these differences and  
>>>> warns
>>>> about caveats a couple pages later (or earlier). Don't stop  
>>>> reading.
>>>>
>>>
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
>>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>
>




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