Lazy evaluation

[Isiah Meadows]

Promises are inherently eager, but also async - consider that `new
Promise(resolve => resolve(1))` is roughly equivalent to `var promise
= Promise.resolve(1)`.

My proposal is for a single immediate value, but created on demand
(when you call `.get()`) rather than immediately.
-----

Isiah Meadows
me at isiahmeadows.com

Looking for web consulting? Or a new website?
Send me an email and we can get started.
www.isiahmeadows.com


On Thu, Aug 31, 2017 at 10:37 AM, Naveen Chawla <naveen.chwl at gmail.com> wrote:
> Could you not do this with a promise? If not, what's missing in promise that
> you could do with "lazy"? Sorry if I've missed the whole premise
>
> On Thu, 31 Aug 2017 at 19:09 Andrea Giammarchi <andrea.giammarchi at gmail.com>
> wrote:
>>
>> the following is how I usually consider lazy values
>>
>> ```js
>> class Any {
>>   _lazy(name) {
>>     switch (name) {
>>       case 'uid': return Math.random();
>>       // others ... eventually
>>     }
>>   }
>>   get uid() {
>>     var value = this._lazy('uid');
>>     // from now on, direct access
>>     Object.defineProperty(this, 'uid', {value});
>>     return value;
>>   }
>> }
>>
>> const a = new Any;
>> a.uid === a.uid; // true
>> ```
>>
>> If I understand correctly your proposal is to use Lazy as generic
>> descriptor, is that correct ?
>>
>> ```js
>> Object.defineProperty({}, 'something', new Lazy(function (val) {
>>   return this.shakaLaka ? val : 'no shakaLaka';
>> }));
>> ```
>>
>> ???
>>
>> If that's the case I see already people confused by arrow function
>> in case they need to access the context,
>> plus no property access optimization once resolved.
>>
>> It's also not clear if such property can be set again later on (right now
>> it cannot)
>> 'cause lazy definition doesn't always necessarily mean inability to
>> reassign.
>>
>> What am I missing/misunderstanding?
>>
>> Regards
>>
>>
>>
>> On Thu, Aug 31, 2017 at 2:21 PM, Isiah Meadows <isiahmeadows at gmail.com>
>> wrote:
>>>
>>> It'd be really nice if lazy values made it into the spec somehow. I've
>>> already found myself using things like this [1] quite a bit, and I've
>>> also found myself frequently initializing properties not on first
>>> access.
>>>
>>> [1]:
>>> https://gist.github.com/isiahmeadows/4c0723bdfa555a1c2cb01341b323c3d4
>>>
>>> As for what would be a nice API, maybe something like one of these?
>>>
>>> ```js
>>> class Lazy<T> {
>>>     constructor(init: () => T);
>>>     get(): T; // or error thrown
>>> }
>>>
>>> function lazy<T>(init: () => T): () => T; // or error thrown
>>>
>>> function lazy<T>(init: () => T): {
>>>     get(): T; // or error thrown
>>> }
>>> ```
>>>
>>> Alternatively, syntax might work, with `do` expression semantics:
>>>
>>> ```js
>>> const x = lazy do { ... }
>>> // expose via `x.get()` or just `x()`
>>> ```
>>>
>>> -----
>>>
>>> Isiah Meadows
>>> me at isiahmeadows.com
>>>
>>> Looking for web consulting? Or a new website?
>>> Send me an email and we can get started.
>>> www.isiahmeadows.com
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
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