[swift-evolution] [pitch] CopyInitializable for value-type semantics

[Gor Gyolchanyan]

Hello, swift community!

Recently I’ve come across a dilemma regarding value-type semantics when dealing with generic types.
Consider a protocol that has a mutating in-place function and a non-mutating returning variant of that function:

protocol Transmogrifier {

    mutating func transmogrify()

    func transmogrified() -> Self

}

One of these methods has to have a default implementation in terms of the other.

One way doing it is to implement the mutating version in terms of non-mutating because it doesn’t depend on additional conditions to work, since assigning to `self` causes a complete copy of the internal state of the object regardless of whether it’s a value type or a reference type. However, this approach has a big downside: in many cases mutating functions mutate only part of the instance, which means that an efficient implementation will have to implement the mutating version and because of the way the default implementation works, the non-mutating version would also need to be manually implemented, which makes the default implementation useless in those cases.

Implementing the non-mutating version in terms of mutating version solves this problem nicely, allowing one to focus on mutating only the necessary parts of the instance, while leaving the need to return a separate instance to the default implementation, which would be perfectly adequate in most cases. This approach has its own problem that this pitch seeks to solve. The problem becomes apparent when you consider this naive implementation:

extension Transmogrifier {

    public func transmogrified() -> Self {
        var result = self
	result.transmogrify()
	return result
    }

}

The above implementation is only correct for value types, because assignment is a deep copy. If the instance is of a reference type, the assignment will do nothing and the call to the mutating version will apply to the original object, violating the postcondition of the function (which states that the function shall not modify the instance in any way).

The most straight-forward way of solving this problem is to introduce a new protocol for making sure the original instance is always copied:

protocol CopyInitializable {

    init(copying other: Self)

}

In which case the default implementation becomes fully correct:

// The `CopyInitializable` conformance can also be moved to the protocol itself
// if the protocol conformance requires value-type semantics.
extension Transmogrifier where Self: CopyInitializable {
    
    public func transmogrified() -> Self {
        var result = Self(copying: self)
	result.transmogrify()
	return result
    }

}

The downside of this approach is the need to manage CopyInitializable conformance of the types  that becomes extra hassle that seems to conflict with the behavior of value types.

This pitch proposes adding CopyInitializable protocol to the swift standard library and having the compiler automatically generate conformance to it for all value types.
This would immediately solve all problems of correct convenient implementations of non-mutaiting variants of in-place functions as well as remove the hassle of having to manage conformance to CopyInitializable for all value types that are guaranteed to have this behavior in the first place.

An good use case would be the NSNumber class, which would conform to CopyInitializable and make use of a single obvious mutating-to-nonmutating implementation of arithmetic operations that would work equally well on all standard numeric types.

I’d like to hear opinions regarding this pitch and in case of consensus, I’d write an official proposal and offer it for review.

Regards,
Gor Gyolchanyan.

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