Exponentiation operator precedence

Domenic Denicola d at domenic.me
Thu Sep 24 19:29:53 UTC 2015


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>
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

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