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 <[email protected]> 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 > > -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); > ``` > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
_______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
