[swift-users] Associativity of && and || operator

Jordan Rose jordan_rose at apple.com
Mon Feb 20 12:16:08 CST 2017

Remember too that in Swift the operator is decoupled from its implementation. Plenty of people are using && for things like constraint or predicate DSLs, where it doesn't short-circuit. Of course, a non-associative version of && would be quite surprising, so even a custom implementation probably shouldn't rely on the left-associativity.

There's one other reason to prefer this way, though, and that's complexity in the AST. Recall that the RHS of the standard && (and ||, and ??) is an auto-closure—Swift's way to avoid the eager evaluation of the argument. Right-associativity would result in heavily-nested autoclosures, and even though they'll all be flattened as part of compilation, the AST representation still has to model them that way. So purely from a compiler-implementation perspective left-associativity has a small edge.

I think you're right that we should have picked right-associativity anyway, but at this point I'm not sure it's worth changing it.


> On Feb 17, 2017, at 01:03, Saagar Jha via swift-users <swift-users at swift.org> wrote:
> Left associativity is most likely just a holdover from the C family–not conforming with it would break expectations for programmers coming from these languages. And as you mentioned, the compiler will short-circuit the condition and stop evaluating as soon as it encounters a false condition, so there’s no measurable benefit.
> Saagar Jha
>> On Feb 17, 2017, at 12:54 AM, rintaro ishizaki via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> Hello all,
>> Why the associativity of Logical{Conjunction,Disjunction}Precedence is "left"?
>> If you write: A && B && C, it's grouped as (A && B) && C.
>> This means that the && function is always called twice: (&&)((&&)(A, B), C).
>> I feel "right" associativity is more natural:  (&&)(A, (&&)(B, C)),
>> because the && function is called only once if A is false.
>> I know that redundant && calls are optimized away in most cases.
>> I also know C and C++ standard says: "The && operator groups left-to-right", and most programming languages follow that.
>> But why not "right" associativity?
>> What is the difference between logical operators and ?? operator that has "right" associativity?
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