[erlang-questions] Reading, Learning, Confused
Sat Jul 19 17:15:27 CEST 2008
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?
Not really, but it's a bit cryptic. The text only talks about evaluation
of guard tests, i.e., the complete expressions separated by commas
and/or semicolons. The commas/semicolons are always short-circuited. But
they cannot be nested arbitrarily (you can have semicolon-separated
groups of comma-separated expressions, but not the other way around and
not grouped using parentheses). To write more complex expressions and
still have short-circuiting, you need andalso/orelse.
The strictness of the and/or operators is historical; I think the
reasoning was that evaluating both sides gave a more predictable
behaviour, with regard to possible exceptions and evaluation time. This
was probably a mistake, but it was too late to fix, so andalso/orelse
The short answer to "when would anyone want to use plain and/or?" is
"rarely". A slightly longer answer would be "when readability and
horizontal space is more important than shaving off a few cycles".
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