[swift-evolution] [Proposal] Custom operators

Fri Apr 8 16:17:14 CDT 2016

Based on commit ‘01317e1a’:

I think that it makes sense for membership to live outside of a precedence group. An example is if I wanted to add a new assignment operator ( maybe ??= 'assign if nil’), I would want it to have the same precedence as existing assignment operators, including precedence with other custom operators.

Secondly, I think the relationships between precedence groups should be defined outside of an existing precedence group. If I have two libraries that declare operators in their own custom groups, I may need to define a relationship between those groups.

This leads the group declaration itself being just of a name and associativity.

If group precedence is declared externally, there isn’t a need to support ‘>’. Since we are declaring a relationship and not evaluating a relationship, there are side-effects that developers will need to understand if they are trying to comprehend precedence groups in aggregate. Having the groups appear consistently in the same order when defining precedence may help with this.

I still assume these relationships are meant to be constrained to a DAG, although there might be cases where cycles (or even having multiple graphs) would still be unambiguous. I can’t wrap my head around implementation to the point of understanding evaluation if not a DAG yet, nor practical reasons to have cycles in relations.

Two groups may be unable to be declared to be equivalent. First, they need to be of the same associativity. Second are also possibilities of graph cycles once the relationships of both groups are overlaid. This is actually the trouble I alluded to in my first email in the thread.

Finally, an infix operator is part of one and only one group. It might make sense to have a default group (with no associativity) for operators to fall into if they do not declare a precedence group.

Oh wait, Yet another quasi-syntax based on the above:

precedencegroup Additive, associativity: left
precedencerelation Additive < Multiplicative
precedencerelation Range < Additive

infix operator +, group: Additive

-DW

> On Apr 8, 2016, at 1:28 PM, Антон Жилин via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The question 4 (`prefix operator ! { }` or `prefix operator !`) seems dead simple, because if we need to declare precedence group in body of infix operators, then other operators should have it for consistency.
> 
> It's not.
> I suggest an alternative syntax for that:
> infix operator <> : Comparative
> 
> Colon immediately after the operator may not look the best, but it's the only disadvantage I can find. It looks like inheritance and has similar meaning.
> So, in this scheme, all operators will have no body.
> 
> I also described two methods to declare that operator belongs to a precedence group: in `members` and in operator declaration.
> I suggest that this new syntax looks brief/natural enough to remove `members` option entirely. "There should be one - and preferably only one - obvious way to do it."
> 
> - Anton
> 
> 2016-04-08 19:59 GMT+03:00 Антон Жилин <antonyzhilin at gmail.com <mailto:antonyzhilin at gmail.com>>:
> Thank you for your reply, Chris!
> I was thinking about purging directives from the proposal, and that was what I needed to do it.
> So, the proposal is now completely overhauled:
> https://github.com/Anton3/swift-evolution/blob/operator-precedence/proposals/NNNN-operator-precedence.md <https://github.com/Anton3/swift-evolution/blob/operator-precedence/proposals/NNNN-operator-precedence.md>
> 
> Yes, Maximilian and I have considered operator/precedence groups and they have now moved from alternatives to main part of the proposal.
> 
> Questions:
> 1. Is it OK that associativity is moved to precedence groups and that every operator must belong to a precedence group?
> 2. Dictionary-like or "functional keywords"? That is, `associativity: left` or `associativity(left)`? So far, only second form has been used somewhere inside declarations.
> 3. First-lower or first-upper? `additive` or `Additive`?
> 4. Empty body or no body? `prefix operator ! { }` or `prefix operator !`?
> 
> Just in case, some questions/concerns copied from previous discussion:
> 
> 1. All precedence groups have a "parent".
> It means, all operators will want to have precedence higher than Comparative or Ternary, or, at least, Assignment.
> 
> 2. Moreover, I could not find any case where I had to write anything other than precedence(>, ...)
> Of cause, I cheated, because I can control all these declarations.
> Mere people will have to use `<` to say that Additive, for example, should have less priority than their custom operator.
> 
> But... can you build a custom operator where `<` will actually be needed? I have even stronger doubts on `=`.
> Maybe we can even contract this feature to `parent(Comparative)` or something without losing any expressivity?
> 
> 3. Can we allow operators to have less priority than `=`?
> If yes, can you give an example of such operator?
> 
> - Anton
> 
> 2016-04-08 8:59 GMT+03:00 Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>>:
> 
>> On Apr 7, 2016, at 1:39 PM, Антон Жилин via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> First of all, sorry for the delay. I still hope to finish the discussion and push the proposal to review for Swift 3.0.
>> Link for newcomers:
>> https://github.com/Anton3/swift-evolution/blob/operator-precedence/proposals/NNNN-operator-precedence.md <https://github.com/Anton3/swift-evolution/blob/operator-precedence/proposals/NNNN-operator-precedence.md>
>> 
>> Sadly, I've moved into the territory opposite to what I had in mind in the beginning: absense of conflict resolution.
>> I wanted lightweight directives, but am moving to closed precedence groups.
>> 
>> It's just IMHO, and I think I just need input on this from more people. I still have not heard anything from Core team.
> 
> Hi Антон,
> 
> I’m sorry for the delay, I have been out of town recently.  I haven’t read the upstream thread so I hope this isn’t too duplicative.  Here is my 2c:
> 
> - I completely agree that numeric precedences are lame, it was always the “plan” that they’d be removed someday, but that obviously still hasn’t happened :-)
> - I definitely agree that a partial ordering between precedences is all that we need/want, and that unspecified relations should be an error.
> 
> That said, I feel like #operator is a major syntactic regression, both in consistency and predictability.  We use # for two things: directives (like #if) and for expressions (#file).  The #operator is a declaration of an operator, not an expression or a directive.  For declarations, we consistently use a keyword, which allows contextual modifiers before them, along with a body (which is sometimes optional for certain kinds of decls).  I feel like you’re trying to syntactically reduce the weight of something that doesn’t occur very often, which is no real win in expressiveness, and harms consistency.
> 
> Likewise #precedence is a relationship between two operators.  I’d suggest putting them into the body of the operator declaration.
> 
> OTOH, the stuff inside the current operator declaration is a random series of tokens with no apparent structure.  I think it would be reasonable to end up with something like:
> 
> infix operator <> {
>   associativity: left
>   precedenceLessThan: *
>   precedenceEqualTo: -
>  }
> 
> Or whatever.  The rationale here is that “infix” is primal on the operator decl (and thus is outside the braces) but the rest of the stuff can be omitted, so it goes inside.
> 
> Just in terms of the writing of the proposal, in the "Change precedence mechanism” keep in mind that swift code generally doesn’t care about the order of declarations (it doesn’t parse top down in the file like C does) so the example is a bit misleading.
> 
> Question for you: have you considered introducing named precedence groups, and having the relationships be between those groups?  For example, I could see something like:
> 
> 	operator group additive {}
> 	operator group multiplicative { greaterThan: additive }
> 	operator group exponential { greaterThan: additive }
> 
> Then:
> 
> infix operator + {
>   associativity: left
>   precedence: additive
>  }
> infix operator - {
>   associativity: left
>   precedence: additive
>  }
> 
> etc.
> 
> -Chris
> 
> 
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