[erlang-questions] style question - best way to test multiple non-guard conditions sequentially
Thu Jun 20 18:27:25 CEST 2013
On 06/20/2013 11:42 AM, Anthony Ramine wrote:
> Because changing it at this point would be weird. And because guards have special semantics and can't fail.
> Supporting expressions now would mean weird semantics like in comprehensions, where you can't distinguish a guard from an expression and know that they are compiled differently, or breaking existing code such as "if length(X) =:= 3 -> ... ; ... end": if X is not a list and that expression is evaluated with guard semantics, the first clause is skipped, whereas if it is evaluated with expression semantics, the code crashes.
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.
If I had read some code that just said:
if length(X) =:= 3 -> ...
I would have inferred (incorrectly) that it would be an error for X to
not be a list.
> If I understood things correctly, Erlang is supposed to have a cond Clauses end expression —try to evaluate "cond" in Erlang, it's a syntax error because cond is a reserved keyword—, which is the same as if but with tests compiled with expression semantics. Maybe it should be implemented for real.
Perhaps that is a remnant of "the long-ago-proposed 'cond' form" that
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