[erlang-questions] Reading, Learning, Confused
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
> (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-
(Compare, e.g. C's | and ||, where | may be used deliberately for
side-effects on the RHS.)
> 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
>>> 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?
>> 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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