> On May 19, 2016, at 3:07 PM, Антон Жилин via swift-evolution
> <[email protected]> wrote:
>
> Thanks Brent,
>
> 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.
>
> But I agree that bulkiness of my syntax is a problem.
> I can think of two solutions:
>
> 1. Global-scope precedence relationships. Example:
>
> precedencegroup B : associativity(left)
> precedencerelation B > A
> precedencerelation B < C
> infix operator <$> : B
>
> It's already included as an alternate solution.
Your argument against this in the alternatives is compelling. We should avoid
this one.
>
> 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)
Brainstorming some other options here based on Brent’s syntax
A list:
precedence NewGroup > ExistingGroup, < OtherExistingGroup left
Require braces for multiple precedence levels:
precedence NewGroup {
left
> ExistingGroup
< OtherExistingGroup
}
I think my preference is this “lightweight” braced option when multiple
relations are necessary, with Brent’s syntax as a shorthand for the common case.
>
> 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 precedence group?
I don’t have a concrete example, but those would not allow you to place a group
above two unrelated groups. I can’t think of a good reason for this
restriction unless there are implementation considerations that make it
necessary.
>
> - Anton
>
> Brent Royal-Gordon wrote:
> I like this proposal, except for the `precedencegroup` syntax, which I think
> is a bit overwrought. Rather than this proposal's:
> precedencegroup Multiplicative {
> associativity(left)
> 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`.
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