[swift-evolution] continuations - "extensions on steroids" idea

[Mike Kluev]

a general feeling that there are two very different use cases of extensions
-- one to extend own classes and another to extend other people classes; a
lengthy discussion in a nearby thread; a disparity of features of class and
it's extensions and different access right treatment depending upon whether
the extension is in the same with the main class or a different file lead
me to the following idea. hopefully you will find this idea useful and we
may even see it one day in swift.

introducing class / struct / enum continuations.

in a few words the are:

1) "extensions on steroids". in fact very similar to Objective-C "class
extensions" but can be more than one and in more than one file.

2) as good as the classes they continue (or other continuations they
continue)

3) can declare variables

4) have full access to "private" members of the class or other continuations
regardless whether they are in the same or in a different files. no need
for "public/internal/module" fallbacks to pass though files boundaries.
similarly the class itself can use private members of its continuations.

5) "by invitation only" system. can only be written if declared by the
class or some continuation of it.

6) A particular continuation only defined once (of course). There can be an
arbitrary number of different continuations to a class similar to
extensions.

7) alternative name considered: "extending" (as a noun). shall definitely
be a different name than say, "extension" with some bracket-syntax
variation - the two use cases are very different and shall be named
differently.

8) the particular syntax is preliminary of course.

example:

============= Some.swift ==================

class Some {

    private func private_method_of_class() {

        // *** can use private methods of continuations even if they are in
a different file
        private_method_defined_in_a_continuation()
    }

    // *** "by invitation" system

    continuation Feature // *** declaring a continuation
    continuation SomethingElse // *** Capitalized names, like classes
}

============= Some-Feature.swift ==================

// *** similar naming convetion to "Some+Feature.swift"
// used for extensions, just with "-" instead

// *** this is merely a naming convention, no need to
necessarily follow it, can put more than one thing into
a file, the continuation can reside in the same file as
the main class fie, etc.

continuation Feature of Some {

// *** as good as class itself. first-class citizen
// *** same syntax can be used for structs and enums

    var x: Int // *** can declare variables

    private func private_method_defined_in_a_continuation() {
        private_method_of_class()

        // *** can use private methods of the class or of other
continuations even if they are in a different file
    }

    // *** continuations are as good as classes and can
    // *** declare other continuations if needed

    continuation CanDoThis
}

============= Any file ==================

continuation Feature of Some { // *** error, continuation is already defined
}

continuation Other of Some { // *** error: Other is not a continuation of
Some
}

thoughts?

Mike
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