Anton3/NNNN-remove-protocol-metatype.md

## NNNN-remove-protocol-metatype.md

      
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              NNNN-remove-protocol-metatype.md
            
          
    Remove .Protocol metatype


Proposal: SE-0126
Authors: Adrian Zubarev, Anton Zhilin
Status: Revision
Review manager: Chris Lattner
Revision: 2
Previous Revisions: 1

Introduction

This proposal removes P.Protocol metatypes.
Swift-evolution threads:

[Revision] [Pitch] Rename T.Type
[Review] SE-0126: Refactor Metatypes, repurpose T[dot]self and Mirror
[Proposal] Refactor Metatypes, repurpose T[dot]self and Mirror
[Discussion] Seal T.Type into Type<T>

Motivation

Explanation of metatypes
For every type T in Swift, there is an associated metatype T.Type.
Basics: function specialization

Let's try to write a generic function like staticSizeof. We will only consider its declaration; implementation is trivial and unimportant here.
Out first try would be:
func staticSizeof<T>() -> Int
staticSizeof<Float>()  // error :(
Unfortunately, it's an error. We can't explicitly specialize generic functions in Swift. Second try: we pass a parameter to our function and get generic type parameter from it:
func staticSizeof<T>(_: T) -> Int
staticSizeof(1)  //=> should be 8
But what if our type T was a bit more complex and hard to obtain? For example, think of struct Properties that loads a file on initialization:
let complexValue = Properties("the_file.txt")  // we have to load a file
staticSizeof(complexValue)                     // just to specialize a function
Isn't that weird? But we can work around that limitation by passing instance of a dummy generic type:
struct Dummy<T> { }
func staticSizeof<T>(_: Dummy<T>) -> Int
staticSizeof(Dummy<Properties>())
This is the main detail to understand: we can explicitly specialize generic types, and we can infer generic type parameter of function from generic type parameter of passed instance. Now, surprise! We've already got Dummy<T> in the language: it's called T.Type and created using T.self:
func staticSizeof<T>(_: T.Type) -> Int
staticSizeof(Float.self)
But there's a lot more to T.Type. Sit tight.
Subtyping

Internally, T.Type stores identifier of a type. Specifically, T.Type can refer to any subtype of T. With enough luck, we can also cast instances of metatypes to other metatypes. For example, Int : CustomStringConvertible, so we can do this:
let subtype = Int.self
metaInt    //=> Int
let supertype = subtype as CustomStringConvertible.Type
supertype  //=> Int
Here, supertype : CustomStringConvertible.Type can refer to Int, to String or to any other T : CustomStringConvertible.
We can also use as?, as! and is to check subtyping relationships. We'll only show examples with is:
Int.self is CustomStringConvertible.Type  //=> true
protocol Base { }
protocol Derived: Base { }
Derived.self is Base.Type  //=> true
protocol Base { }
struct Derived: Base { }
let someBase = Derived.self as Base.Type
// ...
someBase is Derived.Type  //=> true
A common practise is to store metatypes as Any.Type. When needed, we can check all required conformances.
Dynamic dispatch of static methods

If we have an instance of T.Type, we can call static methods of T on it:
struct MyStruct {
    static func staticMethod() -> String { return "Hello metatypes!" }
}
let meta = MyStruct.self
meta.staticMethod()  //=> Hello metatypes!
What is especially useful, if our T.self actually stores some U : T, then static method of U will be called:
protocol HasStatic { static func staticMethod() -> String }
struct A: HasStatic { static func staticMethod() -> String { return "A" } }
struct B: HasStatic { static func staticMethod() -> String { return "B" } }

var meta: HasStatic.Type
meta = A.self
meta.staticMethod()  //=> A
meta = B.self
meta.staticMethod()  //=> B
Summing that up, metatypes have far deeper semantics than a tool for specialization of generic functions. They combine dynamic information about a type with static information "contained type is a subtype of this". They can also dynamically dispatch static methods the same way as normal methods are dynamically dispatched.

Current behavior of .Protocol

For protocols P, besides normal P.Type, there is also a "restricting metatype" P.Protocol that is the same as P.Type, except that it can only reflect P itself and not any of its subtypes:
Int.self is CustomStringConvertible.Type      //=> true
Int.self is CustomStringConvertible.Protocol  //=> false
Even without P.Protocol, we can test for equality:
Int.self is CustomStringConvertible.Type  //=> true
Int.self == CustomStringConvertible.self  //=> false
For protocols P, P.self returns a P.Protocol, not P.Type:
let metatype = CustomStringConvertible.self
print(type(of: metatype))  //=> CustomStringConvertible.Protocol
In practise, the existence of P.Protocol creates problems. If T is a generic parameter, then T.Type turns into P.Protocol if a protocol P is passed:
func printMetatype<T>(_ meta: T.Type) {
    print(dynamicType(meta))
    let copy = T.self
    print(dynamicType(copy))
}

printMetatype(CustomStringConvertible.self)  //=> CustomStringConvertible.Protocol x2
Lets review the following situation:
func isIntSubtype<T>(of: T.Type) -> Bool {
    return Int.self is T.Type
}

isIntSubtype(of: CustomStringConvertible.self)  //=> false
Now we understand that because T is a protocol P, T.Type turns into a P.Protocol, and we get the confusing behaviour.
Summing up issues with P.Protocol, it does not bring any additional functionality (we can test .Types for is and for ==),
but tends to appear unexpectedly and break subtyping with metatypes.
Even more issues with .Protocol


[1] When T is a protocol P, T.Type is the metatype of the protocol type itself, P.Protocol. Int.self is not P.self.
[2] There isn't a way to generically expression P.Type yet.
[3] The syntax would have to be changed in the compiler to get something that behaves like .Type today.
Written by Joe Groff: [1] [2]  [3]

There is a workaround for isIntSubtype example above. If we pass a P.Type.Type, then it turns into P.Type.Protocol, but it is still represented with .Type in generic contexts. If we manage to drop outer .Type, then we get P.Type:
func isIntSubtype<T>(of _: T.Type) -> Bool {
  return Int.self is T   // not T.Type here anymore
}

isIntSubtype(of: CustomStringConvertible.Type.self) //=> true
In this call, T = CustomStringConvertible.Type. We can extend this issue and find the second problem by checking against the metatype of Any:
func isIntSubtype<T>(of _: T.Type) -> Bool {
	return Int.self is T
}

isIntSubtype(of: Any.Type.self) //=> true

isIntSubtype(of: Any.self)      //=> true
When using Any, the compiler does not require .Type and returns true for both variations.
The third issue shows itself when we try to check protocol relationship with another protocol. Currently, there is no way (that we know of) to solve this problem:
protocol Parent {}
protocol Child : Parent {}

func isChildSubtype<T>(of _: T.Type) -> Bool {
	return Child.self is T
}

isChildSubtype(of: Parent.Type.self) //=> false
We also believe that this issue is the reason why the current global functions sizeof, strideof and alignof make use of generic <T>(_: T.Type) declaration notation instead of (_: Any.Type).
Proposed solution

Remove P.Protocol type without a replacement. P.self will never return a P.Protocol.
Impact on existing code

This is a source-breaking change that can be automated by a migrator.
All occurrences of T.Protocol will be changed to T.Type.
Alternatives considered

Leave .Protocol, but use a separate syntax like .protocol for its creation.