[Proposal] New syntax for lazy getters

[Andrea Giammarchi]

That is why I'm proposing to use a getter, you don't have to reason about
the context in `lazy fullname() { return this.firstName + ' ' +
this.lastName; }` and it still works with de-contextualized invokes such
`lazy random: () => Math.random()` as long as you transpile that as getter
calling the right context (when/if needed).

Both cases can de-sugar to same redefinition of a getter.

Regards


On Tue, Jun 12, 2018 at 10:56 AM, Aadit M Shah <aaditmshah at fastmail.fm>
wrote:

> When the getter lives on a prototype it would install the memoized value
> on this using [[Define]]. This is the same way it would work in regular
> lazy getters:
>
> const defclass = prototype => {
>     const constructor = prototype.constructor;
>     constructor.prototype = prototype;
>     return constructor;
> };
>
> const Person = defclass({
>     constructor: function Person(firstname, lastname) {
>         this.firstname = firstname;
>         this.lastname  = lastname;
>     },
>     lazy fullname: this.firstname + " " + this.lastname
> });
>
> const john = new Person("John", "Doe");
> const jane = new Person("Jane", "Doe");
>
> console.log(john.fullname); // John Doe
> console.log(jane.fullname); // Jane Doe
>
> Note that the this within the lazy property value expression (i.e. within
> the expression this.firstname + " " + this.lastname) refers to the
> instance (i.e. either john or jane). This can be confusing because
> without the lazy keyword, this would refer to the lexical context.
> Although I'm comfortable with it yet I'm open to other suggestions.
>
>
> On Tue, Jun 12, 2018, at 3:31 AM, Jordan Harband wrote:
>
> How would this work when the getter lives on a prototype? (like most
> builtin getters) Would it install the memoized value on `this`, or on the
> object that contains the getter? Would it use [[Set]] or [[Define]]?
>
> On Tue, Jun 12, 2018 at 12:13 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
>
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