> I managed to confuse at least two people! I've stated it in the grammar, but
> forgot to give an example:
>
> ===begin===
> Multiple precedence relationships can be stated for a single precedence
> group. Example:
> ```swift
> precedencegroup A { }
> precedencegroup C { }
> precedencegroup B { precedence(> A) precedence(< C) }
> ```
> By transitivity, precedence of C becomes greater than precedence of A.
> ===end===
>
> As you can see, your suggested syntax would not look good, because there can
> be any number of precedence declarations.
Ah, I see.
> 2. Limit precedence relationships.
>
> Do we really need a full-blown Directed Acyclic Graph?
> Could `above` and `between` be enough?
>
> Example:
>
> precedencegroup B : between(A, C)
>
> This is one of dark places of the proposal, obviously underdiscussed.
> Are there practical situations other than `above` and `between`?
> Do we really need unlimited relationships per one precedencegroup?
We probably do if you're serious about having operators whose precedence
relative to each other is undefined. Moreover, you actually have to be prepared
for *more than* two relationships, or two relationships which are both on the
same "side", so "between" doesn't cut the mustard.
I can see three ways to fit multiple relationships on one line:
1. precedence Multiplicative > Additive < BitwiseShift left
2. precedence Multiplicative > Additive, < BitwiseShift left
3. precedence Multiplicative > Additive, Multiplicative < BitwiseShift left
Another option would be to have `precedence` lines declare-or-redeclare *all*
of the precedence levels in them, not just the one on the left of the operator.
Then you would write something like:
4. precedence @associativity(left) Multiplicative > Additive
precedence Multiplicative < BitwiseShift
It would be an error to have two `precedence` lines which marked the same
precedence level with a different `@associativity`. Of course, we could instead
put associativity on its own line, perhaps allowing multiple declarations for
compactness:
5. precedence Multiplicative > Additive
precedence Multiplicative < BitwiseShift
associativity left Cast, Comparative, Multiplicative, Additive
I think 5 is my preference, but if we want a single-line syntax, I'd probably
favor 2.
--
Brent Royal-Gordon
Architechies
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