operator overloading proposal

Jordan Harband ljharb at gmail.com
Wed May 11 06:45:40 UTC 2016


Why would you ever want to violate the algebraic properties of operators,
such that `a += b` wasn't exactly equivalent to `a = a + b`, `a *= b` not
equivalent to `a = a * b`, etc? I'm quite confident that any proposal that
allowed for that would get tons of pushback.

On Tue, May 10, 2016 at 11:26 PM, Isiah Meadows <isiahmeadows at gmail.com>
wrote:

> 1. Yes, they would be inherited, but not on the prototype itself (it would
> technically be parasitic). It would be modeled with internal slots, so that
> the properties are themselves immutable and transparent, so the only way to
> inherit would be via the class syntax or `Reflect.construct`. Engines could
> model this similarly to prototypes internally, while still appearing to
> conform to spec, since there's no other way to access the function without
> explicit reference via a decorator. And if it's not decorated, you can
> transparently fast path the calls automatically and optimize the function
> at compile time for exactly the number of arguments (any different is a
> syntax error, like with getters and setters).
>
> 2. I'm intentionally trying to avoid any semantics that would rely on
> adding more values to the global scope. First, it's harder to optimize a
> `hasOwnProperty` check. Second, when you allow properties to be dynamically
> added, you make it impossible to lower `foo + bar` to a single instruction
> if they're both numbers, because someone can change the Number prototype to
> have one of the operators on it, and now, the assumption, previously
> prevalent, is now invalid. Third, we shouldn't need to add 15+ new symbols
> to accommodate a simple operation.
>
> 3. If it's pure syntax, you won't have the edge cases of `x += y` having
> to desugar to `x = x[Symbol.assignPlus](y)` and so on. You just look for an
> `[[OpAssignPlus]]` on `x`, and if it exists, call it as `x.[[OpAssignPlus]](y)`.
> Else, you check for `[[OpPlus]]`, and set `x` to `x.[[OpPlus]](y)`. If
> neither exists, you fall back to the old algorithm. This can be easily
> optimized by the fact engines only need to check this if the value is an
> object. Numbers and strings don't have this slot.
>
> Note: If the right side has an operator defined, but the left side
> doesn't, and if the operator checked for isn't an assignment one, the right
> side's operator is checked and called. Or basically, beyond assignment, the
> mere existence of a slot takes precedence over no slot, to make
> transitivity easier with primitives. To clarify, in the below case:
>
> ```js
> class C {
>     constructor(x) { this.x = x }
>     operator +(x) {
>         if (x instanceof C) {
>             return this + x.x * 2
>         }
>         return this.x + x
>     }
> }
>
> assert(new C(1) + 1 === 1 +1)
> assert(1 + new C(1) === 1 + 1)
> assert(new C(1) + new C(2) === 1 + 2*2)
> assert(new C(2) + new C(1) === 2 + 1*2)
> ```
>
> On Wed, May 11, 2016, 01:27 Kevin Barabash <kevinb at khanacademy.org> wrote:
>
>> > I would prefer syntax + internal slots, since you'll know at creation
>> time whether the object has overloaded
>> > operators. It's much simpler for the engine to figure out, and it's
>> more performant because you only need to
>> > check one thing instead of worrying about inheritance, own properties,
>> etc.
>>
>> Will operators defined on a class work with instances of a subclass?
>>
>> > Could += be a special case? i.e.,
>>
>> For sure.  We could define `Symbol.assignPlus`, `Symbol.assignTimes`,
>> etc. with `u += v;` desugaring to `u = u[Symbol.assignPlus](v)`.  The
>> reason why we can't do something do `u[Symbol.assignPlus](v)` is that
>> there's no way to define a method on Number, String, etc. that would
>> reassign their value.
>>
>> > it appears to me that overloading an operator multiple times (e. g.
>> unary/binary plus operator) might become
>> > painful, assuming that the semantics follow the same variadic approach
>> that regular functions do.
>>
>> Another pain point is handling cases where you want one class to
>> interoperate with another.  In one of the example above methods are defined
>> that allow `Point`s and `Number`s to be added to each other.  In order to
>> maintain the commutativity of `+` we need to define `operator+` /
>> `[Symbol.add]` methods on both `Point` and `Number`.  One potential
>> solution to this problem is create `Symbol.plusRight`, `Symbol.timesRight`
>> for all of the commutative/symmetric operators.
>>
>> I feel like this ends up making things more complex because there are
>> more methods to implement and the methods have to be more complex b/c they
>> have to do type checking when overloaded.
>>
>> Maybe `operator+` could work like the `@operator` decorator by calling
>> `Function.defineOperator` behind the scenes.  In this situation, instead of
>> methods being added to classes, the `Function` object has well-defined
>> methods that look up the correct function to call based on the argument
>> types.  `u + v` desugars to `Function[Symbol.plus](u, v)`.  This is
>> definitely slower than internal slots, but if we're doing runtime type
>> checking in the method we may as well have it be automatic.  My hope is to
>> eventually use static typing (flow b/c I'm using babel) to remove the
>> lookup cost.
>>
>>
>> On Tue, May 10, 2016 at 7:07 PM, Isiah Meadows <isiahmeadows at gmail.com>
>> wrote:
>>
>>> You're correct in that the operator doesn't do any type checking (it
>>> dispatches from its first argument, but that's just traditional OO).
>>>
>>> On Tue, May 10, 2016, 20:28 kdex <kdex at kdex.de> wrote:
>>>
>>>> @Isiah: Comparing your syntax proposal to `Function.defineOperator`, it
>>>> appears to me that
>>>> overloading an operator multiple times (e. g. unary/binary plus
>>>> operator) might become painful,
>>>> assuming that the semantics follow the same variadic approach that
>>>> regular functions do.
>>>>
>>>> That is, of course, unless you intend to handle all operator overloads
>>>> in a single `operator +(...args) {}`
>>>> definition. But then again, something like `Function.defineOperator`
>>>> seems cleaner and suggests implicit
>>>> (optional?) type checks with its second argument.
>>>>
>>>> On Dienstag, 10. Mai 2016 15:25:32 CEST Isiah Meadows wrote:
>>>> > Here's my thought, if we go with syntax.
>>>> >
>>>> > ```js
>>>> > class Point {
>>>> >     // constructor, etc.
>>>> >
>>>> >     operator +(other) {
>>>> >         assert(other instanceof Point)
>>>> >         return new Point(
>>>> >             this.x + other.x,
>>>> >             this.y + other.y)
>>>> >     }
>>>> >
>>>> >     operator +=(other) {
>>>> >         assert(other instanceof Point)
>>>> >         this.x += other.x
>>>> >         this.y += other.y
>>>> >     }
>>>> > }
>>>> > ```
>>>> >
>>>> > On Tue, May 10, 2016, 11:16 Brian Barnes <ggadwa at charter.net> wrote:
>>>> >
>>>> > > A note on this from somebody who's entire existence seems dedicated
>>>> to
>>>> > > stopping as much stuff as possible from getting GC'd, the example
>>>> below:
>>>> > >
>>>> > >  >const u = new Point(5, 10);
>>>> > >  >const v = new Point(1, -2);
>>>> > >  >
>>>> > >  >const w = u + v;  // desugars to u[Symbol.add](v)
>>>> > >  >console.log(w);   // { x: 6, y: 8 };
>>>> > >
>>>> > > Could += be a special case?  i.e.,
>>>> > >
>>>> > > u+=v;
>>>> > >
>>>> > > would call:
>>>> > >
>>>> > > Class Point { ... other stuff ...
>>>> > > [whatever the syntax is](pt)
>>>> > > {
>>>> > > this.x+=pt.x;
>>>> > > this.y+=pt.y;
>>>> > > }
>>>> > > }
>>>> > >
>>>> > > instead of desugaring to:
>>>> > >
>>>> > > u=u+v;          // which would cause the creation of an object and
>>>> > >                 // leave the other to be collected
>>>> > >
>>>> > > For all I know, += might be doing such anyway in some engines, but
>>>> for
>>>> > > my stuff which is a lot of 3D math that could be a performance
>>>> killer.
>>>> > > It would be nice to be able to just add points and such, as long as
>>>> the
>>>> > > overhead is negligible.
>>>> > >
>>>> > > [>] Brian
>>>> > >
>>>> > > On 5/10/2016 10:52 AM, Isiah Meadows wrote:
>>>> > > > I would prefer syntax + internal slots, since you'll know at
>>>> creation
>>>> > > > time whether the object has overloaded operators. It's much
>>>> simpler for
>>>> > > > the engine to figure out, and it's more performant because you
>>>> only need
>>>> > > > to check one thing instead of worrying about inheritance, own
>>>> > > > properties, etc.
>>>> > > >
>>>> > > > Also, it would be IMHO easier to read than a symbol (the computed
>>>> > > > property syntax is ugly IMO). Using a different concept than
>>>> symbols
>>>> > > > would also fit better with value types whenever any of those
>>>> proposals
>>>> > > > make it into the language (either the struct or special syntax).
>>>> > > >
>>>> > > >
>>>> > > > On Tue, May 10, 2016, 04:03 G. Kay Lee
>>>> > > > <balancetraveller+es-discuss at gmail.com
>>>> > > > <mailto:balancetraveller%2Bes-discuss at gmail.com>> wrote:
>>>> > > >
>>>> > > >     Yes, I think exposing operators through well-known symbols is
>>>> an
>>>> > > >     interesting idea worthy of more exploration because it's
>>>> precisely
>>>> > > >     the purpose of well-known symbols to expose and allow
>>>> manipulation
>>>> > > >     to previously inaccessible internal language behaviors.
>>>> > > >
>>>> > > >     On Tue, May 10, 2016 at 1:59 PM, Kevin Barabash
>>>> > > >     <kevinb at khanacademy.org <mailto:kevinb at khanacademy.org>>
>>>> wrote:
>>>> > > >
>>>> > > >         > And remember that decorators are essentially just a
>>>> syntax to
>>>> > > >         apply functions to objects/classes at design time, so what
>>>> > > >         you're proposing is essentially some new global function,
>>>> which
>>>> > > >         is going against the current trend and effort to better
>>>> > > >         modularize/namespace all these utility functions/methods.
>>>> > > >
>>>> > > >         That's a really good point.
>>>> > > >
>>>> > > >         > It has been mentioned and discussed in numerous places
>>>> over the
>>>> > > >         years, you can find more info on this with some casual
>>>> googling.
>>>> > > >         For example:https://news.ycombinator.com/item?id=2983420
>>>> > > >
>>>> > > >         Thanks for the link.  I played around with sweet.js a bit
>>>> over
>>>> > > >         the weekend.  Using macros should work if we went with
>>>> Python
>>>> > > >         style operator overloading.  Instead of defining methods
>>>> like
>>>> > > >         _ADD_, _SUB_ etc. we could create some well-known
>>>> symbols, maybe
>>>> > > >         Symbol.plus, Symbol.times, etc.
>>>> > > >
>>>> > > >         ```
>>>> > > >         class Point {
>>>> > > >           constructor(x, y) {
>>>> > > >             Object.assign(this, {x, y});
>>>> > > >           }
>>>> > > >
>>>> > > >           [Symbol.add](other) {
>>>> > > >             return new Point(this.x + other.x, this.y + other.y);
>>>> > > >           }
>>>> > > >         }
>>>> > > >
>>>> > > >         const u = new Point(5, 10);
>>>> > > >         const v = new Point(1, -2);
>>>> > > >
>>>> > > >         const w = u + v;  // desugars to u[Symbol.add](v)
>>>> > > >         console.log(w);   // { x: 6, y: 8 };
>>>> > > >         ```
>>>> > > >
>>>> > > >         This would require default implementations to be defined
>>>> on
>>>> > > >         Object.prototype for Symbol.plus, Symbol.times, etc.
>>>> > > >
>>>> > > >
>>>> > > >         On Sun, May 8, 2016 at 10:38 PM, G. Kay Lee
>>>> > > >         <balancetraveller+es-discuss at gmail.com
>>>> > > >         <mailto:balancetraveller+es-discuss at gmail.com>> wrote:
>>>> > > >
>>>> > > >             > Why not? The standard defines well-known symbols.
>>>> Maybe
>>>> > > `@operator` could be a well known decorator (assuming decorators get
>>>> > > approved).
>>>> > > >
>>>> > > >             Well... you make something into the standard with
>>>> proposals,
>>>> > > >             not why-nots, so in order to make that happen you
>>>> need to
>>>> > > >             draft another proposal for well-known decorators. And
>>>> > > >             remember that decorators are essentially just a
>>>> syntax to
>>>> > > >             apply functions to objects/classes at design time, so
>>>> what
>>>> > > >             you're proposing is essentially some new global
>>>> function,
>>>> > > >             which is going against the current trend and effort to
>>>> > > >             better modularize/namespace all these utility
>>>> > > >             functions/methods. And maybe a new mechanism could be
>>>> > > >             drafted for these new well-known decorators, so that
>>>> we can
>>>> > > >             hide these new functions somewhere... but by now I
>>>> hope it's
>>>> > > >             becoming clear that it's introducing way too much new
>>>> > > >             surface area for the language in exchange for one
>>>> small
>>>> > > feature.
>>>> > > >
>>>> > > >             > I haven't seen any proposals for macros, could you
>>>> post a
>>>> > > link?
>>>> > > >
>>>> > > >             It has been mentioned and discussed in numerous
>>>> places over
>>>> > > >             the years, you can find more info on this with some
>>>> casual
>>>> > > >             googling. For example:
>>>> > > >             https://news.ycombinator.com/item?id=2983420
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >             On Sun, May 8, 2016 at 2:51 PM, Kevin Barabash
>>>> > > >             <kevinb at khanacademy.org <mailto:
>>>> kevinb at khanacademy.org>>
>>>> > > wrote:
>>>> > > >
>>>> > > >                 I should update the demo code to show the
>>>> `@operator`
>>>> > > >                 decorator in addition to
>>>> `Function.defineOperator`.
>>>> > > >
>>>> > > >                 Initially I started out with just the `@operator`
>>>> > > >                 decorator, but that meant that each class would
>>>> have to
>>>> > > >                 have knowledge of each of the classes it might
>>>> want to
>>>> > > >                 interact with before hand.  Having a separate
>>>> > > >                 `defineOperator` function avoids this situation.
>>>> > > >
>>>> > > >                 It means that prototype style classes must be
>>>> converted
>>>> > > >                 to the new class syntax before operator
>>>> overloading
>>>> > > >                 could be used.  Lastly, there may be some cases
>>>> where it
>>>> > > >                 makes sense to overload operators with existing
>>>> 3rd
>>>> > > >                 party code or built-in classes, e.g. adding set
>>>> > > >                 operations to Set using operator overloading.
>>>> > > >
>>>> > > >                 > It's also apparent that the `@operator
>>>> decorator` part
>>>> > > >                 of the proposal is an effort trying to address
>>>> this
>>>> > > >                 issue, but it really is not the responsibility of
>>>> the
>>>> > > >                 standard to try to define such a thing.
>>>> > > >
>>>> > > >                 Why not?  The standard defines well-known symbols.
>>>> > > >                 Maybe `@operator` could be a well known decorator
>>>> > > >                 (assuming decorators get approved).
>>>> > > >
>>>> > > >                 Slide 15
>>>> > > >                 from
>>>> http://www.slideshare.net/BrendanEich/js-resp shows
>>>> > > >                 syntax for defining operators in value types
>>>> which could
>>>> > > >                 be adapted as follows for regular classes:
>>>> > > >
>>>> > > >                 ```
>>>> > > >                 class Point {
>>>> > > >                    constructor(x, y) {
>>>> > > >                        this.x = +x;
>>>> > > >                        this.y = +y;
>>>> > > >                    }
>>>> > > >                    Point + Number (a, b) {
>>>> > > >                        return new Point(a.x + b, a.y + b);
>>>> > > >                    }
>>>> > > >                    Number + Point (a, b) {
>>>> > > >                        return new Point(a + b.x, a + b.y);
>>>> > > >                    }
>>>> > > >                    Point + Point (a, b) {
>>>> > > >                        return new Point(a.x + b.x, a.y + b.y);
>>>> > > >                    }
>>>> > > >                 }
>>>> > > >                 ```
>>>> > > >
>>>> > > >                 Having to define `+` twice for `Point + Number`
>>>> and
>>>> > > >                 `Number + Point` seems like busy work, but maybe
>>>> it's
>>>> > > >                 better to be explicit.  What are you thoughts
>>>> about this
>>>> > > >                 syntax?
>>>> > > >
>>>> > > >                 > Another thing is that, IMHO, currently there
>>>> are too
>>>> > > >                 much quirks/conventions in the proposal that feel
>>>> > > >                 non-evident and non-flexible which is destined to
>>>> trip
>>>> > > >                 people over from time to time. It would be great
>>>> to make
>>>> > > >                 a proposal that's simple and don't include too
>>>> much
>>>> > > >                 assumptions.
>>>> > > >
>>>> > > >                 Could you elaborator what quirks/conventions
>>>> might trip
>>>> > > >                 people up?
>>>> > > >
>>>> > > >                 > Finally, I'm not sure about the current status
>>>> of
>>>> > > >                 macros, but last I heard of it, they say it's
>>>> going to
>>>> > > >                 make its way into the standard pretty soon (TM),
>>>> and
>>>> > > >                 macros can do much of the things overloading
>>>> could, and
>>>> > > >                 much more.
>>>> > > >
>>>> > > >                 I haven't seen any proposals for macros, could
>>>> you post
>>>> > > >                 a link?
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >                 On Sat, May 7, 2016 at 9:55 PM, G. Kay Lee
>>>> > > >                 <balancetraveller+es-discuss at gmail.com
>>>> > > >                 <mailto:balancetraveller+es-discuss at gmail.com>>
>>>> wrote:
>>>> > > >
>>>> > > >                     I'd say it's way too early to ask for a
>>>> champion on
>>>> > > >                     this because just a quick skimming revealed a
>>>> lot of
>>>> > > >                     places that didn't add up. For example, the
>>>> proposal
>>>> > > >                     suggested that overloading is primarily
>>>> targeted at
>>>> > > >                     making it easier to work with user-defined
>>>> classes,
>>>> > > >                     but curiously a `Function.defineOperator()`
>>>> method
>>>> > > >                     is proposed instead of some syntax that feels
>>>> more
>>>> > > >                     tightly integrated with the class definition
>>>> syntax.
>>>> > > >
>>>> > > >                     ```
>>>> > > >
>>>> > > >                     class Point {
>>>> > > >                         constructor(x, y) {
>>>> > > >                             Object.assign(this, { x, y });
>>>> > > >                         }
>>>> > > >
>>>> > > >                         toString() {
>>>> > > >                             return `(${this.x}, ${this.y})`;
>>>> > > >                         }
>>>> > > >                     }
>>>> > > >
>>>> > > >                     Function.defineOperator('+', [Point, Point],
>>>> (a, b)
>>>> > > => new Point(a.x + b.x, a.y + b.y));
>>>> > > >
>>>> > > >                     ```
>>>> > > >
>>>> > > >                     The demo code made this flaw evident - it
>>>> looks like
>>>> > > >                     a giant step backward to define an instance
>>>> method
>>>> > > >                     like this, don't you agree?
>>>> > > >
>>>> > > >                     It's also apparent that the `@operator
>>>> decorator`
>>>> > > >                     part of the proposal is an effort trying to
>>>> address
>>>> > > >                     this issue, but it really is not the
>>>> responsibility
>>>> > > >                     of the standard to try to define such a thing.
>>>> > > >
>>>> > > >                     What I'd suggest is that perhaps you should
>>>> rethink
>>>> > > >                     your proposed syntax and redesign it to
>>>> become an
>>>> > > >                     extension of the ES6 class definition syntax.
>>>> > > >
>>>> > > >                     Another thing is that, IMHO, currently there
>>>> are too
>>>> > > >                     much quirks/conventions in the proposal that
>>>> feel
>>>> > > >                     non-evident and non-flexible which is
>>>> destined to
>>>> > > >                     trip people over from time to time. It would
>>>> be
>>>> > > >                     great to make a proposal that's simple and
>>>> don't
>>>> > > >                     include too much assumptions.
>>>> > > >
>>>> > > >                     Finally, I'm not sure about the current
>>>> status of
>>>> > > >                     macros, but last I heard of it, they say it's
>>>> going
>>>> > > >                     to make its way into the standard pretty soon
>>>> (TM),
>>>> > > >                     and macros can do much of the things
>>>> overloading
>>>> > > >                     could, and much more.
>>>> > > >
>>>> > > >                     On Sun, May 8, 2016 at 8:51 AM, Kevin Barabash
>>>> > > >                     <kevinb at khanacademy.org
>>>> > > >                     <mailto:kevinb at khanacademy.org>> wrote:
>>>> > > >
>>>> > > >                         I forgot to mention in my last email that
>>>> I'm
>>>> > > >                         looking for a champion for this proposal.
>>>> > > >
>>>> > > >                         On Sat, May 7, 2016 at 5:24 PM, Kevin
>>>> Barabash
>>>> > > >                         <kevinb at khanacademy.org
>>>> > > >                         <mailto:kevinb at khanacademy.org>> wrote:
>>>> > > >
>>>> > > >                             Hi everyone,
>>>> > > >
>>>> > > >                             I've been working on implementing
>>>> operator
>>>> > > >                             overloading and would like to submit a
>>>> > > proposal.
>>>> > > >
>>>> > > >                             I think operator overloading would be
>>>> a
>>>> > > >                             useful addition to the language.  In
>>>> > > >                             particular I think it would be useful
>>>> for
>>>> > > >                             defining operations on common
>>>> mathematical
>>>> > > >                             object types such as complex numbers,
>>>> > > >                             vectors, matrices, and sets.
>>>> > > >
>>>> > > >                             I've create a working prototype that
>>>> > > >                             consists of:
>>>> > > >
>>>> > > >                               * babel plugin that rewrites
>>>> operators as
>>>> > > >                                 function calls
>>>> > > >                               * a polyfill which defines these
>>>> functions
>>>> > > >                                 and which call the correct
>>>> > > >                                 argument-specific function based
>>>> on the
>>>> > > >                                 arguments' prototypes
>>>> > > >                               * Function.defineOperator which can
>>>> be
>>>> > > >                                 used to define which function an
>>>> > > >                                 operator should use for the
>>>> specified
>>>> > > types
>>>> > > >                               * "use overloading" directive which
>>>> allows
>>>> > > >                                 users to opt-in
>>>> > > >
>>>> > > >                             More details can be found
>>>> > > >                             at
>>>> > > https://github.com/kevinbarabash/operator-overloading.
>>>> > > >                             The babel plugin can be found
>>>> > > >                             at
>>>> > > https://github.com/kevinbarabash/babel-plugin-operator-overloading.
>>>> > > >                             I also have a demo project at
>>>> > > >
>>>> > > https://github.com/kevinbarabash/operator-overloading-demo.
>>>> > > >
>>>> > > >                             The design was inspired by some of the
>>>> > > >                             slides from
>>>> > > >
>>>> > > http://www.slideshare.net/BrendanEich/js-resp.
>>>> > > >
>>>> > > >                             – Kevin
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
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