it wouldn't work, would it ? I mean, you still have to pass through the "ugly" _db.get() thingy, right?
how do you access and trigger the lazy bit within the class? On Thu, Aug 31, 2017 at 7:56 PM, Isiah Meadows <[email protected]> wrote: > What about this (using the stage 3 class fields proposal)? > > ```js > declare function lazy<T>(init: () => T): () => T; > > class WithLazyVals { > _db = lazy(() => new Promise(...)); > } > ``` > ----- > > Isiah Meadows > [email protected] > > 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 1:34 PM, Andrea Giammarchi > <[email protected]> wrote: > >> this proposal doesn't compose well with classes > > > > to expand a little, if you were proposing > > > > ```js > > class WithLazyVals { > > lazy _db() { return new Promise(...); } > > } > > ``` > > > > I would've taken first flight to come over and hug you. > > > > Best Regards > > > > > > > > > > On Thu, Aug 31, 2017 at 6:25 PM, Andrea Giammarchi > > <[email protected]> wrote: > >> > >> > How often do you start out with a class like this ... > >> > >> Never, like I've said. This is the lazy pattern I know since ever. > >> > >> ```js > >> class Foo { > >> get _db() { > >> return Object.defineProperty(this, '_db', { > >> value: new Promise((resolve, reject) => { > >> // open a database connection > >> // set up whatever tables you need to > >> // etc. > >> }) > >> })._db; > >> } > >> } > >> ``` > >> > >> Whenever you need, you just access `this._db`, no need to create an > >> enumerable variable and a class method. > >> > >> It looks cleaner to me. > >> > >> > >> > Things you don't want to initialize right away because initialization > >> > >> You don't really have to convince me, I've written lazy properties since > >> getters and setters were introduced [1] > >> > >> All I am saying is that this proposal doesn't compose well with classes, > >> it's just yet another SuperPrimitive for the language. > >> > >> It is also something trivial to implement on user land, yet I haven't > seen > >> many writing code like the following: > >> > >> ```js > >> function Lazy(fn) { > >> let c = false, v; > >> return {get(){ return c ? v : (c = !c, v = fn()) }}; > >> } > >> > >> var o = Lazy(() => Math.random()); > >> o.get(); // ... > >> ``` > >> > >> Maybe it's me that hasn't seen this widely adopted from some library? > >> > >> Anyway, this is just my opinion, maybe others would be happy with this. > >> > >> Best Regards > >> > >> [1] Class.lazy example > >> https://github.com/WebReflection/prototypal/blob/master/Class.md# > classlazycallback > >> > >> > >> > >> On Thu, Aug 31, 2017 at 6:03 PM, Isiah Meadows <[email protected]> > >> wrote: > >>> > >>> It'd solve a problem similarly to Kotlin's `by lazy { ... }` delegate, > >>> .NET's `System.Lazy<T>`, Swift's `lazy var`, among many other > >>> languages. It's very useful for lazy initialization [1], such as > >>> lazily setting up a database, requesting a resource, among other > >>> costly things. [2] > >>> > >>> How often do you start out with a class like this, where you have an > >>> expensive resource you don't want to open right away? > >>> > >>> ```js > >>> class Foo { > >>> constructor() { > >>> this._db = undefined > >>> } > >>> > >>> _initDb() { > >>> if (this._db) return this._db > >>> return this._db = new Promise((resolve, reject) => { > >>> // open a database connection > >>> // set up whatever tables you need to > >>> // etc. > >>> }) > >>> } > >>> } > >>> ``` > >>> > >>> Or maybe, a large lookup table that takes a while to build, and might > >>> not even be used, so you don't want to do it on load? > >>> > >>> ```js > >>> var table > >>> > >>> function initTable() { > >>> if (table) return > >>> table = new Array(10000) > >>> // do some expensive calculations > >>> } > >>> ``` > >>> > >>> Things you don't want to initialize right away because initialization > >>> is expensive and/or the value might not even be used. That's the > >>> problem I'm aiming to solve, and it's something I feel would be useful > >>> in its own right in the language, about equal in importance to weak > >>> references. (Slightly specialized, but the need is not non-zero.) > >>> > >>> [1]: https://en.wikipedia.org/wiki/Lazy_initialization > >>> [2]: > >>> https://stackoverflow.com/questions/978759/what-is-lazy- > initialization-and-why-is-it-useful > >>> ----- > >>> > >>> Isiah Meadows > >>> [email protected] > >>> > >>> 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 12:23 PM, Andrea Giammarchi > >>> <[email protected]> wrote: > >>> > right ... so ... I'm not sure I understand what this proposal would > >>> > solve. > >>> > > >>> > Instead of this: > >>> > ```js > >>> > obj.val || (obj.val = getValue()) > >>> > ``` > >>> > > >>> > you want to do this > >>> > ```js > >>> > (obj.val || (obj.val = new Lazy(getValue)).get(); > >>> > ``` > >>> > > >>> > Where is the "win" and why is that? > >>> > > >>> > > >>> > > >>> > On Thu, Aug 31, 2017 at 5:18 PM, Isiah Meadows < > [email protected]> > >>> > wrote: > >>> >> > >>> >> With my proposed `Lazy` class, if you were to use an instance as a > >>> >> descriptor, the `this` value it'd receive would not be a `Lazy` > >>> >> instance like it'd expect. > >>> >> > >>> >> Consider it the difference between `a.self` and `b.get()` in your > >>> >> example. `b.get()` is what I'd be expecting. > >>> >> ----- > >>> >> > >>> >> Isiah Meadows > >>> >> [email protected] > >>> >> > >>> >> 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 12:12 PM, Andrea Giammarchi > >>> >> <[email protected]> wrote: > >>> >> >> using it in a descriptor would get it passed the wrong `this` > >>> >> > > >>> >> > sorry, what? > >>> >> > > >>> >> > ```js > >>> >> > var a = {}; > >>> >> > var b = {get() { return this; }}; > >>> >> > Object.defineProperty(a, 'self', b); > >>> >> > > >>> >> > a.self === a; // true > >>> >> > ``` > >>> >> > > >>> >> > > >>> >> > On Thu, Aug 31, 2017 at 5:09 PM, Isiah Meadows > >>> >> > <[email protected]> > >>> >> > wrote: > >>> >> >> > >>> >> >> No. `Lazy` is intended to be an object to be used directly, not a > >>> >> >> descriptor of any kind. > >>> >> >> > >>> >> >> (My `lazy.get()` is an unbound method, so using it in a > descriptor > >>> >> >> would get it passed the wrong `this`.) > >>> >> >> ----- > >>> >> >> > >>> >> >> Isiah Meadows > >>> >> >> [email protected] > >>> >> >> > >>> >> >> 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 9:39 AM, Andrea Giammarchi > >>> >> >> <[email protected]> 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 > >>> >> >> > <[email protected]> > >>> >> >> > 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 > >>> >> >> >> [email protected] > >>> >> >> >> > >>> >> >> >> Looking for web consulting? Or a new website? > >>> >> >> >> Send me an email and we can get started. > >>> >> >> >> www.isiahmeadows.com > >>> >> >> >> _______________________________________________ > >>> >> >> >> es-discuss mailing list > >>> >> >> >> [email protected] > >>> >> >> >> https://mail.mozilla.org/listinfo/es-discuss > >>> >> >> > > >>> >> >> > > >>> >> > > >>> >> > > >>> > > >>> > > >> > >> > > >
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