Equality for generic container and Void

Is it possible now (with conditional conformances) to write a generic container, say Result<T>, that would be Equatable both when T: Equatable and T == Void? The obvious solution leads to overlapping conformances:

enum Result<T> {
    case success(T)
    case failure

extension Result: Equatable where T: Equatable { … }
extension Result: Equatable where T == Void { … }

I know I can make my own “placeholder” type like enum Unit: Equatable { case unit }, but that feels like a hack and has to be explained to other people reading the code.

Wouldn’t it make sense for Void (alias ()) to be Equatable? (Does this require solving the issue of protocol conformances for non-nominal types?)

Yes, it’s been a longstanding goal to have tuples of Equatable values conform to Equatable (currently, there’s a hack to implement == up to arity 6). We are held back because tuples cannot conform to protocols. Likewise, () would naturally conform to Equatable if it could. With time, I think, it’s certain that this will be addressed.


I remember adding my own unit type to a project too. I later removed it for the same reason, it’s just confusing. I wonder why Void in Swift is an empty tuple instead of say an empty struct or an enum with a single case. Anybody knows?

IIRC in ancient times, before Swift went public and open source, tuples where more fundamential to the language. Function arguments were tuples and an empty argument list was therefore an empty tuple. A function was a named closure that takes in a tuple as its argument and returns a tuple foo () -> (). An empty tuple then is naturaly the Void type.

Perhaps some of the ‘old’ gurus can clarify.

Has there been any updates or further discussion on this?

I asked a similar question some time ago. One of the last responses from the core team suggest a way to implement this, but I guess someone will have to pick it up.

I don't think we have to solve the general case of allowing tuples to conform to protocols to get this into Swift. Based on the reception from members of the core team on my thread, I'm sure a proposal will go through like a breeze as long as someone is capable of producing a working implementation.

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