[swift-evolution] [Proposal] Custom operators

Ross O'Brien narrativium+swift at gmail.com
Fri Apr 8 12:31:52 CDT 2016

If I want to define a new operator, it seems like an unnecessary overhead
to have to immediately decide which precedence group it should belong to
before it can be used (assuming it doesn't interact with other operators).
At the moment, new operators are implicitly assigned a 'default' precedence
of 100; can we make it so that new operators are implicitly assigned to a
'default' group with an effective precedence of 100? (I believe this is
currently the precedence of Ternary, but I'm not sure if I'd have Ternary
be the default group).

On Fri, Apr 8, 2016 at 5:59 PM, Антон Жилин <swift-evolution at swift.org>

> 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
> 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>:
>> On Apr 7, 2016, at 1:39 PM, Антон Жилин via swift-evolution <
>> 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
>> 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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