[swift-evolution] Standard operator precedence

[Jordan Rose]

A boolean is not a single bit of data in Swift, in the same way that "someIntegerValue & 1" cannot be used as a condition. There is a bijection between boolean values and integers with one bit set or cleared, but they are not the same type.

(You can't use '&' on a Bool either, only '&&'. That does mean Swift has no non-short-circuiting boolean AND operation, but so far we haven't needed one.)

Jordan


> On Feb 15, 2016, at 11:17, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Yes, but not enough to claim limited resources needed for pressing matters. It's not a feature anyone would clamor for, more a product of the foolish consistency hobgoblin. Rationale: it is possible in JavaScript (for instance) to evaluate true * true, but not in Swift. [I'm aware that implicit casting is taking place in JavaScript, and that the return type isn't a Boolean.] It comes into play in exactly one scenario I can think of:
> 
> Student: "Why is & a multiplicative operation?"
> Teacher: "Well, because for two bits A and B, A & B == A * B."
> Student: "That's interesting, let me try it with the only type I know that represents a single bit of data."
> [At this point the student should, IMO, be able to actually evaluate A * B where A and B are booleans.]
> 
> Analogous rationale for defining &+ and &- on booleans.
> 
> On Mon, Feb 15, 2016 at 12:20 PM Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>> On Feb 14, 2016, at 10:25 AM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> Some further study has been helpful. Am I close to the mark in answering my own question?
>> 
>> - Further reading shows that Swift's rationalized operator precedence levels broadly align with those of Erlang and Go. (However, those languages also seem to lack documentation on how they arrived at this set of precedence levels.)
>> 
>> - It is quite evident why << is considered exponentiative.
>> 
>> - Dennis Ritchie has explained <http://www.lysator.liu.se/c/dmr-on-or.html <http://www.lysator.liu.se/c/dmr-on-or.html>> why & has lower precedence than == in C, and why in hindsight that is better off changed. This change has been implemented in Swift/Go/Erlang and also in other languages like Python.
>> 
>> - & is considered multiplicative because for two bits A and B, A & B == A * B.
>> 
>> - ^ and | should have equal precedence to - and +, respectively, by analogous reasoning.
>> 
>> - It also happens to be rational for & to have higher precedence than | by analogy with && and ||.
> 
> Yep, this is all right.
> 
>> It's a bit of a bummer, if this line of reasoning was indeed used in arriving at these operator precedence levels, that Swift will not allow evaluation of true * true (etc.) without casting. I would expect true & true == true * true.
> 
> 
> I’m not sure what you mean here.  You want multiplication defined on booleans?
> 
> -Chris
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