[Proposal] New syntax for lazy getters

Andrea Giammarchi andrea.giammarchi at gmail.com
Tue Jun 12 09:52:42 UTC 2018


Actually, never mind. I've just realized this would work as well, and it's
clean enough.

```js
class Test {
  x = 123;
  lazy random = this.x + Math.random();
}

const test = {
  x: 123,
  lazy random: this.x + Math.random()
};
```

My only concern is if that class method is created N times per each
instance (one of the reasons I use lazy getters in classes prototypes
instead of per object) as it is AFAIK for current class fields if you
attach an arrow function, instead of once per class, which seems to be
preferable in a class context.

Otherwise, I'd be happy to see that happen.

Best Regards



On Tue, Jun 12, 2018 at 11:44 AM, Andrea Giammarchi <
andrea.giammarchi at gmail.com> wrote:

> it's not about being difficult, it's about compatibility with classes,
> where getters are OK since ever. However, since classes fields are in Stage
> 3 [1] maybe it's OK to have:
>
> ```js
> class Test {
>   x = 123;
>   lazy random = () => this.x + Math.random();
> }
> ```
>
> However, like you've noticed, while it's easy to reason about the context
> within a class declaration, it's easy to create a footgun outside that.
>
> ```js
> const test = {
>   x: 123,
>   lazy random: () => this.x + Math.random()
> };
> ```
>
> That is a whole new meaning of arrow function I'd rather like not to ever
> encounter.
>
> So what about extra new syntax?
>
> ```js
> class Test {
>   x = 123;
>   lazy random() => this.x + Math.random();
> }
>
> const test = {
>   x: 123,
>   lazy random() => this.x + Math.random()
> };
> ```
>
> But I'm pretty sure at methods shorthand discussions that came up and for
> some reason didn't work.
>
>
>
> [1] https://github.com/tc39/proposal-class-fields#field-declarations
>
> On Tue, Jun 12, 2018 at 11:32 AM, Aadit M Shah <aaditmshah at fastmail.fm>
> wrote:
>
>> Actually, from a parsing perspective I believe it shouldn't be too
>> difficult to implement the `lazy name: expression` syntax. In addition,
>> I'm not too keen on your `lazy name() { return expression; }` syntax
>> because:
>>
>>
>>    1. It's more verbose.
>>    2. It seems to me that it's no different than creating a regular
>>    getter:
>>
>>    const take = (n, xs) => n === 0 ? null : xs && {
>>        head: xs.head,
>>        get tail() {
>>            const value = take(n - 1, xs.tail);
>>            Object.defineProperty(this, "tail", {
>>                configurable: false,
>>                get: () => value
>>            });
>>            return value;
>>        }
>>    };
>>
>>
>> Regarding the second bullet point, I've probably misunderstood what you
>> were trying to convey. Perhaps you could elucidate.
>>
>> Anyway, making the property non-configurable after accessing it seems
>> like a reasonable thing to do.
>>
>>
>> On Tue, Jun 12, 2018, at 3:44 AM, Andrea Giammarchi wrote:
>>
>> My 2 cents,
>>   I use lazy getters since about ever and I'd love to have such syntax in
>> place but I think there is room for some improvement / simplification in
>> terms of syntax.
>>
>> *## Keep it getish*
>>
>> From parsing perspective, introducing `lazy tail()` seems way simpler
>> than introducing `lazy tail:` for the simple reason that everything that
>> can parse `get tail()` and `set tail()` is in place already in every
>> engine. I don't write them but I'm sure having an extra keyboard to catch
>> shouldn't be crazy complicated.
>>
>> *## class compatible*
>>
>> because you used `delete this.tail` and mentioned functional programming,
>> I'd like to underline ES doesn't force anyone to one programming style or
>> another. That means new syntax should play nicely with classes too, and in
>> this case the proposal doesn't seem to address that because of the direct
>> value mutation, as generic property, and the removal of that property from
>> the object, something not needed if inherited.
>>
>> My variant would do the same, except it would keep the value an accessor:
>>
>> ```js
>> const take = (n, xs) => n === 0 ? null : xs && {
>>     head: xs.head,
>>     lazy tail() {
>>       return Object.defineProperty(this, 'tail', {
>>         configurable: false,
>>         get: (value =>
>>           // still a getter
>>           () => value
>>         )(
>>           // executed once
>>           take(n - 1, xs.tail)
>>         )
>>       }).tail;
>>     }
>> };
>> ```
>>
>> This would keep initial accessor configuration, in terms of
>> enumerability, but it will freeze its value forever and, on top of that,
>> this will play already well with current valid ES2015 classes syntax.
>>
>> I also believe myself proposed something similar a while ago (or somebody
>> else and I agreed with that proposal) but for some reason it never landed.
>>
>> Hopefully this time the outcome would be different.
>>
>> Best Regards
>>
>>
>>
>>
>> On Tue, Jun 12, 2018 at 9:13 AM, Aadit M Shah <aaditmshah at fastmail.fm>
>> wrote:
>>
>>
>> Hello TC39,
>>
>> I recently opened an issue <https://github.com/tc39/ecma262/issues/1223> in
>> the tc39/ecma262 <https://github.com/tc39/ecma262> repository, proposing
>> a new syntax for lazy getters, and I was directed to the CONTRIBUTING
>> <https://github.com/tc39/ecma262/blob/master/CONTRIBUTING.md> page which
>> stated that I should start a conversation on this mailing list.
>>
>> So, my feature proposal is to have syntactic sugar for creating lazy
>> getters
>> <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get#Smart_self-overwriting_lazy_getters>.
>> To summarize my original proposal (which you can read by following the very
>> first link above), I find that creating lazy getters is very verbose. For
>> example, consider:
>>
>> const take = (n, xs) => n === 0 ? null : xs && {
>>     head: xs.head,
>>     get tail() {
>>         delete this.tail;
>>         return this.tail = take(n - 1, xs.tail);
>>     }
>> };
>>
>> My proposed solution is to add a new keyword lazy to the language. This
>> keyword can only be used as a prefix to longhand property names in object
>> initializers, and it defers the execution of the value expression until the
>> property is accessed. In short, it's just syntactic sugar for lazy getters:
>>
>> const take = (n, xs) => n === 0 ? null : xs && {
>>     head: xs.head,
>>     lazy tail: take(n - 1, xs.tail)
>> };
>>
>> This is purely syntactic sugar. The semantics of this new syntax would
>> remain the same as that of the desugared syntax. In particular, calling
>> Object.getOwnPropertyDescriptor(list, "tail") would return an accessor
>> descriptor before accessing list.tail and a data descriptor afterwards.
>>
>> Furthermore, there are other advantages of having this syntactic sugar.
>> For example, creating cyclic data structures becomes much easier. Examples
>> are provided in my original proposal which is linked above. Hope to hear
>> your thoughts on this.
>>
>> Regards,
>> Aadit M Shah
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
>>
>
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