mixins-draft.md

Mixins

• Proposal: SE-NNNN
• Author(s): Anton3
• Status: Awaiting review
• Review manager: TBD

Introduction

Add mixins, which contain both protocol requirements and stored properties. Mixins will subsume abstract class functionality and allow similar functionality on structs. They will also allow for safe multiple inheritance, defining standard "bricks" carrying interface, implementation and state, which can be reused in multiple types.

Swift-evolution thread: link (the threads are broken, search for "Mixins")

Motivation

Fixing drawbacks of abstract classes

A popular request for Swift is to add abstract classes to couple interface (requirements) with implementation of methods that use them and stored properties needed for their operation. Consider two such abstract classes:

class CachingSerializable {
  private var cache: String? = nil
  
  abstract func toString() -> String
  
  func serialize() -> String {
    cache = cache ?? toString()
    return cache
  }
  
  func invalidateCache() {
    cache = nil
  }
}

class SignalSender {
  private var slots: [String: [() -> ()]] = []
  
  final func connect(signal: String, slot: () -> ()) {
    slots[signal] = slot
  }
  
  final func notify(signal: String) {
    for slot in slots[signal] {
      slot()
    }
  }
}

Firstly, only classes can inherit from abstract classes, while it can be easily seen that some subclasses of CachingSerializable or SignalSender would be better represesnted as value types.

Secondly, multiple inheritance is forbidden for classes. No object can inherit both the ability to be serializable and to send signals, in the form shown. We would need to make CachingSerializable a subclass of SignalSender or the reverse, but in practice there could be both "serializable, not signaling" and "signaling, not serializable" classes. Abstract classes on themselves provide no solution to this problem.

As a result of such semantics of classes, many framewords have "master classes", from which every abstract class inherits. In Qt, for example, every object must inherit from a base class similar to SignalSender. Many of those classes don't ever send signals, but this overhead is required to imitate double inheritance by inheriting every other class from that one.

Example with logging

Consider a logging library, which has a Logger class and a convenience base class LoggingObject:

class Logger {
  init(logLevel: Int) { /*...*/ }
  func log(message: String, logLevel: Int) { /*...*/ }
}

class LoggingObject {
  var logger: Logger
  init(logger: Logger) { self.logger = logger }
}

class SomeComputer: LoggingObject {
  init(logger: Logger) {
    super.init(logger)
  }
  func compute() {
    logger.log("Starting", 1)
    if someCondition {
      logger.log("Error", 3)
    }
    logger.log("Ending", 1)
  }
}

The problem with such design is that it prohibits all classes using logging from participating in any other class hierarchy.

One of "solutions" would be to inherit all base classes from LoggingObject. It leads to a situation, where most to all classes, even not actually using logging, contain logger property with corresponding runtime overhead.

Proposed solution

Introduce mixins, which are basically protocols with ability to contain members of classes, including, most importantly, stored properties.

Mixins can contain:

• Method, property, subscript, initializer requirements, analogous to protocols
• Method, property, subscript, initializer definitions, analogous to classes

Methods, computed properties, subscripts, initializer definitions can be declared in their extensions.

Mixins can be inherited from, or mixed in, in the body declaration of a struct or class. This type must satisfy all the requirements, and will get all defined members of the mixin.

Example

The problem with abstract classes can be solved using mixins. We just need to modify headers of the declarations:

mixin CachingSerializable {
  // the same
}
mixin SignalSender {
  // the same
}

struct BestOfBothWorlds: CachingSerializable, SignalSender {
  // ...
}

Multiple inheritance is allowed for mixins.

Example with logging

LoggingObject is not a class of objects in the original meaning. It is more like a feature, or behavior of a data type. Using mixins, we can free the road for other parent classes (or other mixins) in the hierarchy:

class Logger { /*...*/ }

mixin LoggingObject {
  var logger: Logger
  init(logger: Logger) { self.logger = logger }
}

class GenericComputer {
  var result: String? = nil
  init(options: Options) { /*...*/ }
  // ...
}

class SomeComputer: GenericComputer, LoggingObject {
  init(logger: Logger, options: Options) {
    LoggingObject.super.init(logger)
    super.init(logger)
  }
  
  override func compute() {
    logger.log("Starting", 1)
    if someCondition {
      logger.log("Error", 3)
    }
    logger.log("Ending", 1)
  }
}

Detailed design

Mixin members

Property requirements:

var name: Type { get set }

Property definitions:

var name: Type

Method requirements:

func name(arg: Type) -> Result

Method definitions:

func name(arg: Type) -> Result { }

There is no notion of default methods in mixins. Methods of mixins require override to redefine them in types and can be final.

Mixins consider themselves as value types, as protocols and structs. Every member and requirement, which needs to mutate self, needs to be declared as mutating.

Subscript requirements and definitions.

Initializer requirements:

init(name: Type)

Initializer definitions:

init(name: Type) { }

Properties can be immediately initialized: var name: Type = initializer

Initialization of these initialized properties is prepended to all initializers. If no initializers are defined, an implicit default initializer is created.

Initializers

Mixins can have implemented initializers. They act like initializers of classes. super initializer calls are prefixed with parent protocol name for disambiguation purposes. super initializer must be explicitly called in initializer of subtype. The syntax for that is SuperMixinName.super.init()

The only exception of that rule is that default initializer of parent mixin is implicitly prepended to initializers of subtype, if only a single mixin and possibly multiple protocols are being inherited from.

mixin P {
  var path: String
  init(newPath: String) { path = newPath }
}

mixin S : P {
  init(id: Int) {
    P.super.init("base/path/\(id)")
  }
}

Mixin inheritance

Problem of members with same signatures inherited from two different places results in compile error.

mixin A { var x: Int = 0 }
mixin B { var x: Int = 1 }
mixin C : A, B { }  // error

Diamond problem is solved by keeping only one copy of mixins mixed-in in the final struct/class. Example:

mixin A { var x: Int = 1 }
mixin B: A { }
mixin C: A { }
struct D: B, C { }  // contains a single copy of x

It works as if bodies of all mixins indirectly mixed-in into D were merged directly into D:

mixin ASelf { var x: Int = 1 }
mixin BSelf { }
mixin CSelf { }
struct D: ASelf, BSelf, CSelf { }

Default super initializers need to be explicitly called in presence of multiple inheritance:

mixin A { }
mixin B { }

mixin C: A, B {
  init() {
    A.super.init()
    B.super.init()
  }
}

It is a responsibility of mixin C to ensure that initialization order matches semantics of A and B.

Multiple inheritance support summarized

• Protocols can inherit from multiple protocols
• Enums can inherit from multiple prototols
• Mixins can inherit from multiple protocols and mixins
• Structs can inherit from multiple protocols and mixins
• Classes can inherit from multiple protocols and mixins, and a single class

Support for associated types

Mixins support genericity through associated types the same way protocols do.

Support for dynamic dispatch

Mixins without Self or associated types support dynamic dispatch the same way protocols do.

That said, I want to underline that the primary goal of mixins is to aid in constructing new types.

When designing an API, it is a good pattern to not require conformance to a mixin, but to require conformance to a protocol and provide a mixin to aid in implementing a common case. This will provide user of your API as much freedom as possible.

// Create a protocol with what you require from the mixin
protocol Interface {
  var propertyRequirement: Int { get set }
  func methodRequirement()
}

// Adopt the protocol in the mixin
mixin PartialImplementation: Interface {
  var propertyRequirement: Int = 0
  func methodRequirement() { /*...*/ }
  
  // Requirement used to implement Interface requirements
  func requiredPiece()
}

// Code smell: Relying of implementation
func apiFunc(obj: PartialImplementation) { /*...*/ }

// Much better
func apiFunc(obj: Interface) { /*...*/ }

This pattern helps preserve UAC.

Limitations of mixins, compared to protocols

• Mixins must be declared in the declaration of a subtype. Mixins cannot be mixed-in in extensions, retroactively.
• Subtypes must add override to be reimplement methods of a mixin, while default methods of protocols do not require that.
• Multiple inheritance of mixins is limited, by comparison with protocols. Duplicate member definitions cannot occur.

Impact on OOP and POP

Currently, POP has strictly less abilities at its disposal than OOP. Mixins are intended to extend the power of POP to be equal (or greater due to multiple inheritance) than power of OOP. The only significant difference between abilities of structs and classes in Swift will be that classes have reference semantics and structs have value semantics.

Impact on existing code

mixin will be taken as a keyword or a local keyword, so mixin names will no longer be available in some to all places.

Future directions

Conflict resolving

When including mixins containing the same property or function, we could allow to disambiguate the implementation by some means:

mixin A {
  func f() { print(1) }
  var x: Int = 1
}
mixin B {
  func f() { print(2) }
  var x: Int = 2
}
mixin C: A, B {
  func f() { A.f() }
  var x: Int = B.x
}

class mixins and deinit

In analogy with protocols, give mixins the ability to become applicable for classes only via "inheriting" from class keyword. Such mixins would be able to define deinit.

mixin HelloOnDeinit: class {
  deinit { print("Hello") }
}

In case of multiple inheritance, the order of deinit of neighbor mixins is undefined, but submixins are deinited first:

mixin A { deinit { print(0) } }
mixin B: A { deinit { print(1) } }
mixin C: A { deinit { print(2) } }
struct D: B, C { }

do {
  _ = D()
}  // 120 and 210 are valid outputs

@objc mixins

Mixins can be bridged to Objective-C the same way Swift @objc classes do. @objc on mixin would imply class. Such mixins could be used to define Objective-C classes.

protected access level

Classes in other languages often use protected keyword which allows abstract base class to provide members that only subclasses can use. In Swift, this access level would allow classes and mixins to define or require members, visible only to itself and subclasses.

Alternatives considered

Add described features directly to protocols

Mixins are not just protocols with stored properties, this difference is described under "Limitations of mixins". If mixins and protocols are defined using the same keyword, then these "protocols" will create errors because this kind of protocols can't be used where usual protocols can. This would aggravate the situation currently present with dynamically dispatched protocols and protocols with Self or associated types.

Allow implicit overriding in multiple inheritance

That is, the following code:

mixin A { var x: Int = 1 }
mixin B { var x: Int = 2 }
struct C: A, B { }
let c = C()
print(c.x)

would print 2, using definition B.x, because B goes later in the multiple inheritance list.

This implicit behavior is natural in script languages, but would look alien in Swift, which intends to be safe where possible.

Prepend method requirements with abstract

Mixins lean more towards protocols and not classes. Therefore, it is logical to inherit syntax for defining requirements from protocols.

Use term trait instead of mixin

Mixins allow implicit overriding in multiple inheritance, while traits do not. Swift's mixins will actually be closer to traits in this regard.

On the other hand, traits originally do not carry state, but mixins do.

"Mixins", as suggested, will not be purely mixins or purely traits, but rather a combination of both.

Historically, the name "mixins" was used throughout the discussion on swift-evolution.

Add abstract classes

Although abstract classes case is covered by mixins, they can still be added in a separate proposal, duplicating a subset of functionality of mixins with a different syntax.