[swift-evolution] Make generics covariant and add generics to protocols

[Howard Lovatt]

Currently you generics are invariant whereas function arguments etc. are covariant. I am suggesting that if the way generics are implemented is changed then they can be made covariant and that this will add considerable utility to Swift generics.

1st a demonstration of the current situation of invariant generics:

    // Current system
    class Top {}
    class Bottom: Top {}

    struct Box<T: AnyObject> {
        var value: T
        init(_ initialValue: T) {
            value = initialValue;
        }
    }

    let boxB = Box(Bottom())
    // let boxT: Box<Top> = boxB // Covariance currently not allowed

The key point is although `Bottom` 'is a’ `Top`, `Box<Bottom>` *is not* a `Box<Top>`.

I am suggesting:

1. That `Box<Bottom>` should be a `Box<Top>` (covariance).
2. An implementation that allows the above covariance.
3. That protocols are made generic, i.e. `protocol Box<T> { var value: T { get set } }` and that this mechanism replaces associated types for protocols.

    // Proposal:
    // 1. No change to Box, i.e. programmer would just write Box as before
    // 2. Code transformed by comiler with write check for each specific, generic type instance
    // Best approximation of resulting code in current Swift to demonstrate spirit of idea:

    // Compiler writes a universal form using the upper bound (it writes the underlyting representation).
    // In practice this would be called `Box` but used `BoxAnyObject` to indicate that it has a generic argument bounded by `AnyObject`.
    struct BoxAnyObject {
        // Generated from generic argument `<T: AnyObject>`.
        let T: AnyObject.Type // Store the actual type.
        
        // Generated from stored property `var value: T` and noting that `T`'s upper bound is `AnyObject`.
        private var _value: AnyObject // Access the stored property through a setter so that type can be checked
        var value: AnyObject {
            get {
                return _value
            }
            set {
                // In all functions check that args declared as `T` are actually a `T` or a sub-type.
                // Note: `is` only works with type literal and there is no `>=` operator for types :(.
                // `is` would need changing or `>=` for types adding, nearest at moment `==`.
                precondition(T == /* >= */ newValue.dynamicType, "Type of newValue, \(newValue.dynamicType), is not a sub-type of generic type T, \(T)")
                _value = newValue
            }
        }
        
        // Generated from `init(_ initialValue: T)` and noting that `T`'s upper bound is `AnyObject`.
        init(_ lowestCommonDeclaredT: AnyObject.Type, _ initialValue: AnyObject) {
            T = lowestCommonDeclaredT
            _value = initialValue
        }
    }

    // Demonstrate that all `Box`es are the same size and therefore can be bitwise copied
    // Compiler supplies lowest-common, declared, generic type for all the `T`s in the `init` call.
    var bT = BoxAnyObject(Top.self, Top()) // In practice user would write `let bT = Box(Top())`.
    bT.T // Top.Type
    sizeofValue(bT) // 16

    var bB = BoxAnyObject(Bottom.self, Bottom()) // In practice user would write `let bB = Box(Bottom())`.
    bB.T // Bottom.Type
    sizeofValue(bB) // 16

    // Demonstration covariance.
    bT = bB // Compiler would check covariance of declared generic types.
    bT.T // Bottom.Type

    // Demonstrate generic returned type
    // Compiler would add cast to declared, generic type.
    bB.value as! Bottom // In practice user would write `bB.value`.

    // Demonstrate type safety
    bT = BoxAnyObject(Top.self, Top()) // In practice user would write `bT = Box(Top())`.
    bT.value = Top() // OK
    // bT.value = Bottom() // Doesn't work at present because need `>=` for types, but would work in practice
    // bB.value = Top() // Runtime error - wrong type

The implications of this proposal are:

1. The compiler can statically type check a read from a stored property.
2. A write to a stored property is type checked at runtime.
3. Protocols can be made generic instead of having an associated type and then they become a proper type with dynamic dispatch.
4. Generic protocols can be a type just like non-generic protocols, structs, and classes and unlike associated type protocols that can only be a generic constraint.
5. The awkwardness of dealing with associated type generics is replaced by a more powerful and easier to understand semantic of a type, just like the other types.
6. There is a lot of ‘non-obvoius’, long code, for example `inits`, that use a `where` clause to constrain an associated type protocol, this would be unnecessary.
7. There are whole types, `AnySequence`, `AnyGenerator`, etc., that would be replaced by a generic protocols, `Sequence`, `Generator`, etc.

Advantages:

1. Covariant generics are a powerful addition to the language.
2. Generics’ invariance are inconsistent with the rest of the language.
3. Generic protocols would become a ‘proper’ type and you could have arrays and fields of a generic protocol.
4. There are many threads on swift-evolution looking at how protocols can be made into a ‘proper’ type or at least a concept that is easier to understand.

Compatibility:

1. This would be a major change since associated types in protocols would be replaced by generics.
2. The new implementation of generics might break some existing `struct` and `class` code, for example if it is dependent on the exact size of an object because the class will have extra fields, one for each generic type, and therefore will be larger.

Disadvantages:

1. Major change.
2. Object size increases.

Thanks in advance for any comments,

  — Howard.

PS This is part of a collection of proposals previously presented as “Protocols on Steroids”.

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