# [swift-evolution] [Discussion] Breaking precedence

Daniel Duan daniel at duan.org
Tue Aug 2 13:41:11 CDT 2016

```> On Aug 2, 2016, at 11:09 AM, Nevin Brackett-Rozinsky <nevin.brackettrozinsky at gmail.com> wrote:
>
> @Xiaodi
> Actually, I think just about all the rest of the precedence rules “make sense” intuitively:
>
> `a | b == c % d`
> `a < b ? c : d * e`
> `a ?? b - c`
> `a...b+c`
>
> These all do what they ought to, and of course assignment naturally has low precedence. Really the only confusing ones are operators that “seem like peers” but actually have different precedences. Namely the two groups I mentioned: logical operators and bitwise operators.
>
> @Daniel
> Making it easy to write code that is unclear to other people who read it, is an explicit anti-goal for Swift.
>

Again, one’s “easy” might not apply to others, it’s easy *to him/her*. For people who intuitively pick out these precedences, extra parens is just noise that makes them second guess their understandings.

The goal we should pursue is *clarity*. Today, if one wants to emphasize the precedence, they can add parens anyways:  ` a || (b && c)`. But if we make `a || b && c` have different meaning, that makes the code really really unclear (to the rest of us, who have assumptions about how precedences from our backgrounds) and will be a source for bugs. Assuming neither option is objectively better than the other, you need to convince us why the change is worth this cost.

> Nevin
>
>
> On Tue, Aug 2, 2016 at 1:42 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
> This is an expansive argument you advance. Should users be expected to learn *any* rules of precedence beyond those of basic arithmetic? It would seem that you are arguing no. Yet Swift just went through an arduous redesign to permit--nay, improve--exactly that.
>
> On Tue, Aug 2, 2016 at 12:30 Nevin Brackett-Rozinsky <nevin.brackettrozinsky at gmail.com <mailto:nevin.brackettrozinsky at gmail.com>> wrote:
> Speaking for myself, I will *never* remember which of `&&` and `||` has higher precedence. I think of them as peers, so I always use parentheses around them, and whenever I read code that mingles them without parentheses its meaning is *unclear* to me.
>
> One of Swift’s main goals is clarity at the point of use. After all, code is read far more often than it is written. To me, an expression like `a && b || c && d` is not clear when I read it.
>
> The same goes for bitwise operators: I view them as peers. I do not think of them as “additive” or “multiplicative” (and definitely not “subtractive”), so code that relies on their precedences will always send me scrambling to look up which comes first.
>
> Certainly something like `a + b | c & d - e * f ^ g` is meaningless to me without parentheses.
>
> Nevin
>
>
> On Tue, Aug 2, 2016 at 12:08 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> That's an excellent point, actually. Would there be downsides not yet considered?
>
>
> On Tue, Aug 2, 2016 at 11:03 Félix Cloutier <felixcca at yahoo.ca <mailto:felixcca at yahoo.ca>> wrote:
> These expressions mix two types of logic that have different implications. For instance, `a * 16` and `a << 4` are "mostly equivalent", except that `a * 16` will crash on overflow. In these cases, I find that grouping provides some visual insulation that groups off the somewhat subtle differences.
>
> Félix
>
>> Le 2 août 2016 à 08:49:07, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> a écrit :
>>
>> On Tue, Aug 2, 2016 at 10:41 AM, Félix Cloutier <felixcca at yahoo.ca <mailto:felixcca at yahoo.ca>> wrote:
>> I don't think that "intuitive" or "non-intuitive" is what you'd be looking for. There is nothing intuitive about multiplications having a higher precedence than additions; it's just a matter of conventions. I'm not a maths expert (as Stephen showed, I didn't even give the right explanation to binary operators!), but it seems to me that there could well be a parallel universe in which additions have precedence over multiplications without other serious implications.
>>
>> And as it happens, a majority of people don't know that there is one for binary operators. I believe that the right question should be: do we want to pretend that this convention doesn't exist, to the benefit of people who don't know about it, and the detriment of those who do? Also, do we want to break it for && and || too?
>>
>> I think that the biggest use case for binary operators in other languages are flags, and in Swift we treat these as collections. I'd venture that &, | and ^ would show up about as frequently as UnsafePointers and the like. It seems to me that Swift's approach has been to make things easy by default without locking away the power tools, and my personal expectation is that if you have to write code that has binary operators despite everything else that Swift has for you, you can be bothered to learn a precedence rule.
>>
>> That said, one thing that I could definitely get behind is breaking precedence between binary operators and arithmetic operators. I don't think that it makes sense to write something like "a & b / c". Looking at my code, the only place where I needed to mix binary operators and arithmetic operators were `a & (a - 1)` (results in 0 if a is a power of two), and that one needs parentheses anyway.
>>
>> Although here, your same argument applies. If you need to write `a & b / c`, then you can be bothered either to learn or look up a table, or you can just put in the parenthesis yourself. Likewise, if you're a reader of the code, it's highly likely that this is a complex formula anyway; you can either know the relative precedence or look it up, but that's the *least* of your worries in terms of what it will take to understand that code. I see no reason to force parentheses unless it actually prevents user error.
>>
>>
>>
>> Félix
>>
>>> Le 2 août 2016 à 02:29:41, Anton Zhilin <antonyzhilin at gmail.com <mailto:antonyzhilin at gmail.com>> a écrit :
>>>
>>> 2016-08-02 7:18 GMT+03:00 Félix Cloutier <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>> I disagree. The binary operators have properties that are comparable to arithmetic operators, and their precedence is easy to define as such. & has multiplication-like properties (0*0=0, 0*1=0, 1*0=0, 1*1=1); | has addition-like properties (0+0=0, 0+1=1, 1+0=1, 1+1=2); ^ has subtraction-like properties (0-0=0, 0-1=-1, 1-0=1, 1-1=0), and their precedences are set accordingly (& is multiplicative, | and ^ are additive).
>>>
>>> The same applies to && and ||. Bit shifts are exponentiative.
>>>
>>> I believe that such way of thinking is non-intuitive. In C, bitwise operators are not intervened by any others, except for comparison operators (agreed, it was a mistake). We now have possibilities to do so in Swift, even better. I suggest to branch off right before AdditionPrecedence:
>>>
>>> RangeFormation < Addition < Multiplication
>>> RangeFormation < BitwiseOr < BitwiseAnd < LogicalShift
>>>
>>> Another concern is NilCoalescing, which can be though to be semantically similar to Ternary. And at the same time it looks like || and &&, which would bring it between LogicalConjunction and Comparison.
>>> Also, do Casting and RangeFormation stand where they should?
>>>
>>> Next, Ternary operator is unique. Noone would ever like to put operators in this precedence group, because it would be confusing. Why not simplify our model and say that ?: has lower precedence than all binary operators, including Assignment? Unary > binary > ternary, sounds good?
>
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