[erlang-questions] Reading, Learning, Confused

Toby Thain <>
Sun Jul 20 19:53:34 CEST 2008


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.

(Compare, e.g. C's | and ||, where | may be used deliberately for  
side-effects on the RHS.)

--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.
>>
>
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