[swift-evolution] [swift-evolution-announce] [Review] SE-0077: Improved operator declarations

Brent Royal-Gordon brent at architechies.com
Wed May 18 04:50:42 CDT 2016

> 	* What is your evaluation of the proposal?

I like this proposal, except for the `precedencegroup` syntax, which I think is a bit overwrought. Rather than this proposal's:

	precedencegroup Multiplicative {
		precedence(> Additive)

I would prefer to see:

	precedence Multiplicative > Additive left

(Or possibly, if the `left` is deemed too inexplicable by itself, `associativity(left)`.) I don't really think the `precedence` keyword or the curly brackets bring much to the declaration, and dropping them allows us to replace the awkward compound `precedencegroup` with the shorter and equally explanatory `precedence`.

> 	* Is the problem being addressed significant enough to warrant a change to Swift?

Yes. The numeric precedence levels are a significant wart, an impedance to both reading the standard library and implementing new custom operators.

> 	* Does this proposal fit well with the feel and direction of Swift?

Yes. Magic numbers are un-Swifty.

> 	* If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?

The only language I know which has equivalently powerful custom operator specification is Perl 6. When you define a new operator in Perl 6, you're asked to declare it `is tighter`, `is looser`, or `is equiv` to another operator. `tighter` and `looser` both introduce a new precedence level between the operator you specified and the next closest existing precedence level.

The Swift solution seems better because it asks you to design your precedence hierarchy first and then assign operators to its levels. It also allows for two operators to have no precedence relative to each other, so they have to be parenthesized.

However, one good thing about the Perl 6 solution is that it lets you introduce new precedence levels between the existing ones. It's not clear to me if this proposal allows that, and if so, how. (I don't believe any of the examples demonstrate that.)

> 	* How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

Quick reading.

Brent Royal-Gordon

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