Exponentiation operator precedence

Brendan Eich brendan.eich at gmail.com
Thu Sep 24 18:17:52 UTC 2015


Right. It's confusing because (as opposed to Math), ** is not a
superscripting operator whose typography suggests higher precedence than -,
and this matters for the sign of the result when exponentiating.

Claude, the Math folks won't often need to negate the result, but when they
do, they'll have to parenthesize. That's the price of the typographic shift
and the precedence inversion that it suggests to many people.

/be

On Thu, Sep 24, 2015 at 11:16 AM, Mark S. Miller <erights at google.com> wrote:

>
>
> On Thu, Sep 24, 2015 at 11:08 AM, Claude Pache <claude.pache at gmail.com>
> wrote:
>
>>
>> Le 24 sept. 2015 à 16:11, Brendan Eich <brendan at mozilla.org> a écrit :
>>
>> And indeed apart from dot (a special form whose right operand must be a
>> lexical identifier-name) and square brackets (which isn't an infix operator
>> per se), unary operators bind tighter than binary in JS as in C and other
>> C-derived languages.
>>
>>
>> I just wonder why it is important that unary binds tighter? For instance,
>> before I carefully studied the issue of this thread, I have never expected
>> that unary minus binds tighter than binary
>>
>
>
> Before Jason pointed out the discrepancy:
>   * all of us on the committee who were engaged with the proposal
>   * including myself,
>   * all those who reviewed the proposal,
>   * and all those who implemented the proposal
> had the opposite naive expectation. That's the point. In the absence of
> learning about this case specifically, many people will be unpleasantly
> surprised by #2, and many by #3. Therefore #4 wins. (Actually, it just won
> ;).)
>
>
>
>
>
>> multiplication operator in expressions like `-2*x` (although it does not
>> matter in that case).
>>
>>
>> without having to parenthesize unduly, but one cannot write
>>
>> let z = -x ** y;
>>
>> The user is forced by an early error to write either (-x)**y or -(x**y).
>>
>>
>> In traditional math notation, when you mean `(-x)**n`, you write (-x)ⁿ
>> with mandatory parentheses, so I don’t expect that many people will be
>> tempted to miswrite it `-x ** n`.
>>
>> Making the parentheses mandatory here will be somewhat annoying in
>> perfectly reasonable expressions, where you usually don’t use parentheses
>> in real math notation., like:
>> ```
>> let s2 =  - x**2 - y**2 - z**2 +  t**2
>> ```
>>
>> —Claude
>>
>>
>>
>
>
> --
>     Cheers,
>     --MarkM
>
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