[Proposal] New syntax for lazy getters

Andrea Giammarchi andrea.giammarchi at gmail.com
Tue Jun 12 07:44:31 UTC 2018

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:

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)

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>

> 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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