Exponentiation operator precedence

Claude Pache claude.pache at gmail.com
Thu Sep 24 21:05:41 UTC 2015


> Le 24 sept. 2015 à 22:20, Brendan Eich <brendan.eich at gmail.com> a écrit :
> 
> Sez you! :-P
> 
> Seriously, the problem you are dismissing implicitly (bad form :-/) is the one we discussed yesterday, which I've stated explicitly twice now: the typography and plain sense of JS-in-itself suggests unary minus binds tighter than any binary connective (including **).

It’s more a question of convention than of typography, as in: `x + y * z` or: `x || y && z`.

There are two conflicting conventions here, not two conflicting typographies. For the set of operators defined in C, the convention of making every unary operator having a higher precedence, does not conflict with the conventions used in math. (It is not clear for me why that convention is important, just that it is a handy uniform rule that works well for some limited set of operators.)

Note that if you want to disallow `-x**y`, you might want to disallow `x**y**z` (which is imho more confusing), as there is no other right-associative binary operator (except assignment).

—Claude

> 
> Saying "Math!" doesn't overcome this, any more than shouting "typography!" or "JS-in-itself!"
> 
> You don't get to pick a winner just because you root for one team. If you think the operator is worth adding and you concede that the other team's position is tenable because enough people are confused on the committee to predict a wider problem, then you have to consider whether the hardship of mandatory parens in the rare case of negated exponentiation expression is onerous.
> 
> It's not a source of runtime bugs, where test coverage is never good enough. It's a SyntaxError, so (a) Math uber alles people who (b) run into the rare hard case will learn to parenthesize.
> 
> I suspect that many Math-should-prevail types (I am among them in the abstract, prior to engaging with the human factors at play in JS) will want to parenthesize anyway, given the typographic appearance of precedence inversion. I will certainly do it if there's any unary prefixing an exponentiation expression in my code.
> 
> I did find a formula search engine. See http://www.searchonmath.com/result?equation=-+x%5E%7By%7D&page=1&tm=1 <http://www.searchonmath.com/result?equation=-+x%5E%7By%7D&page=1&tm=1> for a taste of the hard cases (and false positives, which do not count).
> 
> Methinks you protest too much. Where is the common case made hard by proposal (4)?
> 
> /be
> 
> 
> On Thu, Sep 24, 2015 at 12:29 PM, Domenic Denicola <d at domenic.me <mailto:d at domenic.me>> wrote:
> I object to #4. Disallowing perfectly reasonable math expressions (Claude's is a good example) makes this operator too surprising to include in the language.
> 
>  
> 
> From: Brendan Eich <brendan.eich at gmail.com <mailto:brendan.eich at gmail.com>>
> Sent: Sep 24, 2015 13:18
> To: Mark S. Miller; Claude Pache
> Cc: es-discuss
> Subject: Re: Exponentiation operator precedence
> 
> 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 <mailto:erights at google.com>> wrote:
> 
> 
> On Thu, Sep 24, 2015 at 11:08 AM, Claude Pache <claude.pache at gmail.com <mailto:claude.pache at gmail.com>> wrote:
> 
>> Le 24 sept. 2015 à 16:11, Brendan Eich <brendan at mozilla.org <mailto: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
> 
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20150924/4a43c5b2/attachment.html>


More information about the es-discuss mailing list