Enum Case KeyPaths

[Joe_Groff]

@stephencelis

On top of the groundwork laid out so far, a new subclass in the key path hierarchy could potentially be introduced with the following functionality:

class CasePath<Root, Value>: KeyPath<Root, Value?> {
  func embed(_ value: Value) -> Root
}

With this in place, we could sunset our library, and downstream libraries could embrace native support, instead.

I think there's also at least one intermediate subclass between read-only KeyPath and CasePath. It is true that enum case components have three key operations:

• project the value (if it exists), which this proposal provides
• attempt to rewrite the payload in-place in an existing value
• construct an entire new value of the enum, using the given case and a payload

However, if you have a composition of a case component followed by a struct property or tuple element access on the payload, such as \Enum.casePayload?.foo, then you can only do the first two; since the key path only refers to a partial payload, you can't create an entire new value without an existing value to update. So that suggests to me that you'd have:

// names chosen arbitrarily, not being seriously proposed as is
class ConditionallyWritableKeyPath<Root, Value>: KeyPath<Root, Value?> {
  func trySet(in: inout Root, to: Value) throws
}

class CasePath<Root, Value>: ConditionallyWritableKeyPath<Root, Value> {
  func embed(_ value: Value) -> Root
}