[swift-evolution] [pitch] composition as part of the language

[Jay Abbott]

Let's take a quick look at how we can achieve very simple compile-time
composition in Swift today.

I want to define a `Doorman` behaviour, and I want to compose it from other
behaviours that are shared by some of my other staff types:

```swift
protocol Greeter {
    func greet()
}
protocol Fareweller {
    func farewell()
}
protocol Doorman: Greeter, Fareweller {}
```

Great - that's my interface defined, now some implementations that I can
compose my staff from:

```swift
protocol FriendlyGreeter: Greeter {}
extension FriendlyGreeter {
    func greet() {
        print("Hello and welcome")
    }
}

protocol FriendlyFareweller: Fareweller {}
extension FriendlyFareweller {
    func farewell() {
        print("I bid thee farewell")
    }
}

protocol InsultingGreeter: Greeter {}
extension InsultingGreeter {
    func greet() {
        print("You make me sick")
    }
}

protocol InsultingFareweller: Fareweller {}
extension InsultingFareweller {
    func farewell() {
        print("Get lost")
    }
}
```

Now we have two kinds of `Greeter` and two kinds of `Fareweller` that can
be used to compose different `Doorman` types (and potentially other staff
types). Here's two examples:

```swift
struct FriendlyDoorman: Doorman, FriendlyGreeter, FriendlyFareweller {}
struct TrickingDoorman: Doorman, FriendlyGreeter, InsultingFareweller {}
```

I can instantiate and make use of these to perform their defined behaviours:

```swift
let friendly: Doorman = FriendlyDoorman()
let tricking: Doorman = TrickingDoorman()
friendly.greet() // Hello and welcome
friendly.farewell() // I bid thee farewell
tricking.greet() // Hello and welcome
tricking.farewell() // Get lost
```

It works! But there are some things that could be nicer:
* I don't really want `***Greeter` or `***Fareweller` to be sub-protocols
at all, these are supposed to be implementations - the only reason they are
protocols is so I can extend them with a default implementation and then
use more than one of them to compose my actual `Doorman` types. This
clutters up the namespace with unnecessary protocols, that have the same
interface as their parent.
* Since the `***Doorman` types need to be instantiable, they are structs. I
couldn't compose a `LobbyMultiTasker` from a `FriendlyDoorman` and a
`GrumpyPorter` at compile-time, the same easy way I composed the
`***Doorman` types. The manual solution would be to add properties for
`doormanDelegate` and `porterDelegate` and assign appropriate instances
(run-time composition), then add the boiler-plate to pass on the
`LobbyMultiTasker` behaviour to these delegates. This is also how the
compiler could implement this feature.
* Actually providing the implementations is optional for the protocols (the
extensions can happily be missing), meaning any error messages will appear
in the types that fail to fully implement the protocols, instead of here in
my `***Greeter` implementation.

So I'd like to discuss the possibility of a new category of types called
`component`, to allow compile-time composition; composition as part of the
language. The `***Greeter` and `***Fareweller` might look like this:

```swift
component FriendlyGreeter: Greeter {
    func greet() {
        print("Hello and welcome")
    }
}

component FriendlyFareweller: Fareweller {
    func farewell() {
        print("I bid thee farewell")
    }
}

component InsultingGreeter: Greeter {
    func greet() {
        print("You make me sick")
    }
}

component InsultingFareweller: Fareweller {
    func farewell() {
        print("Get lost")
    }
}
```

And the `TrickingDoorman` might look like this:

```swift
component TrickingDoorman: Doorman⎄(FriendlyGreeter, InsultingFareweller) {
    // optionally override any Doorman-defined functions
}


```

Here's how I think they would work:

* Components must conform to at least one protocol.
* Components must provide an implementation for all the things from the
protocols - no part of it is 'abstract' - and they can't provide extra
things (although it might be useful to allow some kind of config-values,
but I'd say initially keep it simple).
* Components can be composed of other components and override some/all of
the borrowed behaviour. This should provide well defined
multiple-inheritence semantics (not sure of details), some syntax would be
required to allow the compiler to totally flatten the component, selecting
which "parent" implementations to use if needed to satisfy all the
protocols. Only the functions from the explicit protocols are pulled in,
not everything implemented in the "parent" components.
* Components can be instantiated and have value-semantics, like structs
(they are basically structs with extra features/checks). They can therefore
be used at compile-time but also at run-time for example `delegate =
MyComponent()` - so composed behaviour can be either static or dynamic,
also existing protocol-based `delegate` variables can have a component
instance assigned.
* Classes, structs, and enums can NOT override functions implemented in a
component, when they compose themselves from those components. To do this,
create a sub-component and override the method, then compose your type from
that instead.

Other benefits:
* I think this would encourage more single-responsibility by default.
Because when designing a protocol, people tend to forget composition.
Enforcing must-implement-everything would remind/encourage API designers to
split protocols up, especially if they want to provide a default
implementation for some but not all of the functions.
* Tidier code, with much clearer intention (component vs extension in
particular).
* Easy re-use without having pass-through boilerplate code.
* Brings well-defined-ness to default implementations, which can sometimes
be unclear whether it's an actual default implementation or an empty
placeholder

 I haven't thought of everything here, obviously - and I'm tired, so please
poke holes and supply constructive corrections :)

Any suggestions for the "composed-of" syntax for when a type wants to
utilise a component would be welcome. I used `Doorman⎄(FriendlyGreeter,
InsultingFareweller)` in the example above, where ⎄ is the composition
symbol that I just discovered, but this is just intended to be a
placeholder.
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