[erlang-questions] style question - best way to test multiple non-guard conditions sequentially
Thu Jun 20 19:33:11 CEST 2013
> OK - that is surprising. That certainly changes my thinking about if
> statements. Is this particular semantic difference between guards and
> general conditions ever expressed in the reference manual? If so, I
> certainly missed it.
On closer inspection:
"If an arithmetic expression, a boolean expression, a short-circuit
expression, or a call to a guard BIF fails (because of invalid
arguments), the entire guard fails. "
Before understanding this semantic peculiarity of guards, the
parenthetical portion of the above statement in the reference manual
didn't make much of an impression on me. Now, I see it is conveying
this semantics. Actually, when I read that sentence alone, it still
doesn't quite do the trick. It's the following sentence:
"If the guard was part of a guard sequence, the next guard in the
sequence (that is, the guard following the next semicolon) will be
that completes the meaning by disambiguating "fails" as "evaluates to
false" instead of "throws an exception". Although, still, I'd probably
make the mistake of typing "length(0)." into the interpreter, seeing the
error, and think that this experience overrules what I might have
otherwise inferred from the manual. To further confuse matters (or,
maybe just me), many guards are is_type tests - which would seem to be
useful, but are not, as protection for other guards in the same guard
if is_list(X), length(X) =:= 3 -> ...
Previous to this discussion, the reference manual statement: "The reason
for restricting the set of valid expressions is that evaluation of a
guard expression must be guaranteed to be free of side effects." was the
one that I based my (mis)understanding of the guard/condition
distinction on. I could understand why that would be an important
restriction for guards on things like function and receive clauses, but
not so much on if statements - or, at least, why there would not be any
Erlang statement that would allow testing a sequence of arbitrary
Doesn't guard semantics violate the Principle of Least Astonishment?
I'd be astonished if I was alone in thinking that...
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