[swift-evolution] [Discussion] Breaking precedence

Xiaodi Wu xiaodi.wu at gmail.com
Tue Aug 2 12:42:37 CDT 2016

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> 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> 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> 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> a écrit :
>>> On Tue, Aug 2, 2016 at 10:41 AM, Félix Cloutier <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> a
>>>> écrit :
>>>> 2016-08-02 7:18 GMT+03:00 Félix Cloutier <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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