Proposal: Default object method

Brasten Sager brasten at brasten.me
Mon Jan 28 16:04:44 UTC 2019


Yeah! The static version of this is definitely something I see as a natural next step.

From my observation, tc39 proposals seem to take on the instance and static versions of a feature separately. So for that reason it wasn’t mentioned.

But yes, if this proposal somehow made it through the process it would be almost tragic if a static version did not immediately follow. :-)

> On Jan 27, 2019, at 11:36 PM, Ron Buckton <Ron.Buckton at microsoft.com> wrote:
> 
> There’s nothing in that proposal says that an object with a `Symbol.apply` has to have a different `typeof`. It *would* mean that any Call might require additional dispatch which could have performance implications. It could also be an approach to support “callable” classes:
>  
> ```js
> class Foo {
>   constructor() { /* constructor behavior */ }
>   static [Symbol.apply]() { /* call behavior */ }
> }
> ```
>  
> From: es-discuss <es-discuss-bounces at mozilla.org> On Behalf Of Jordan Harband
> Sent: Sunday, January 27, 2019 9:35 PM
> To: Brasten Sager <brasten at brasten.me>
> Cc: es-discuss <es-discuss at mozilla.org>
> Subject: Re: Proposal: Default object method
>  
> Something that can be invoked has a `[[Call]]` slot, and is `typeof` "function".
>  
> Adding a Symbol that makes something callable would have a number of effects - it would make `typeof` (one of the most robust operations in the language) unsafe, because it would have to access the Symbol method, which could be a throwing getter (or even one that just logs how many typeofs are called on it). Additionally, it would mean any object could become callable, and any function could be made *un* callable.
>  
> This seems like a pretty large change, solely to avoid "classes with a single method", which arguably should just be a function in the first place.
>  
> On Sun, Jan 27, 2019 at 4:05 PM Brasten Sager <brasten at brasten.me <mailto:brasten at brasten.me>> wrote:
> Apologies if this has been raised before. I was unable to locate anything similar.
>  
> Any thoughts or ideas on this proposal would be appreciated!
>  
> Original: https://gist.github.com/brasten/f87b9bb470973dd5ee9de0760f1c81c7 <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fbrasten%2Ff87b9bb470973dd5ee9de0760f1c81c7&data=02%7C01%7Cron.buckton%40microsoft.com%7C8c644033cebc48cdb58708d684e27a3d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636842505603533257&sdata=qHxOcUdMF1i1QPc%2BZKC96Qq4%2BjQY1cgo7GBwDQ5f64Y%3D&reserved=0>
>  
> -Brasten
>  
>>  
> # Proposal: Default object method #
>  
> Objects w/ default method can be invoked like a function.
>  
> ## Problem ##
>  
> Objects that are well constrained (single responsibility)
> can tend to end up with a single method, or at least a single method
> that is important to most consumers. These methods tend to be named
> by either verbing the class name (eg. `UserCreator.create()`) or with
> some generic `handle` / `perform` / `doTheObviousThing`.
>  
> Whatever the name, downstream consumers of the object end up coupled to
> two implementation details: 
>  
>    1) this thing-doer is an object and not a function
>    2) this thing-doer's doing method is called `X`
>  
> ### Example ###
>  
> Here we are going to create an object that can be used to
> create a user later. Note that downstream consumers will only
> care that this object does one thing: create a user. While it
> make have other methods eventually for use in some limited
> contexts, creating a user is its primary (and often sole-)
> responsibility.
>  
> ```js
> class UserCreator {
>   constructor(repository) {
>     this.repository = repository; 
>   }
>  
>   create(name) {
>      return this.repository.createUser(name);
>   }  
> }
>  
> const userCreator = new UserCreator(userRepository);
> ```
>  
> At this point, the `userCreator` is just a single-method object.
> It is useful for injecting into other objects that may need to 
> create a user. But the fact that the `userCreator` is an object
> with a single useful method is an implementation detail to which
> consumers become coupled.
>  
> ```js
>  
> // Consumer of `userCreator`. Although this could itself be a
> // good example of a "UserCreator"-like object (due to `.handle()`).
> //
> class UserSignupHandler {
>   constructor(userCreator) {
>     this.userCreator = userCreator;
>   }
>   
>   handle(userName) {
>     // UserSignupHandler is aware of ".create" when it really doesn't have to be.
>     //
>     return this.userCreator.create(userName);
>   }
> }
>  
> const handler = new UserSignupHandler(userCreator);
> ```
>  
> Notably, if we were to change the implementation of UserCreator later to be 
> a pure function, we would have to change all consumers of UserCreator when
> conceptually it shouldn't be needed. There is still a thing-doer that has
> the same input/output.
>  
>  
> ## Proposed Solution ##
>  
> An object instance can have a default method. This would allow an
> object to be "invoked" exactly like a function, hiding the implementation
> detail from consumers.
>  
> Note that there are several ways to define how the default method is
> determined, and this proposal is less concerned with this aspect than with
> what it looks like to invoke the object. We will demonstrate an option here,
> but alternatives are welcome.
>  
> ```js
> // This particular implementataion would use a Symbol.
> //
>  
> class UserCreator {
>   constructor(repository) {
>     this.repository = repository; 
>   }
>  
>   [Symbol.apply](name) {
>      return this.repository.createUser(name);
>   }  
> }
>  
> const userCreator = new UserCreator(userRepository);
>  
> class UserSignupHandler {
>   constructor(userCreator) {
>     // NOTE: at the consumer, it almost makes more sense to
>     // name these with action verbs, as is done here.
>     //
>     this.createUser = userCreator;
>   }
>   
>   handle(userName) {
>     // UserSignupHandler is no longer coupled to the implementation details it doesn't need.
>     //
>     return this.createUser(userName);
>   }
> }
>  
> const handler = new UserSignupHandler(userCreator);
> ```
>  
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