[swift-evolution] [swift-evolution-announce] [Rejected] SE-0097: Normalizing naming for "negative" attributes

[Brent Royal-Gordon]

> 1) For noreturn, the core team prefers to explore a solution where a function can be declared as returning an non-constructable “bottom” type (e.g. an enum with zero cases).  This would lead to something like:
> 
> 	func abort() -> NoReturn { … }
> 
> This will require some new support in the compiler, but should flow better through the type system than @noreturn in function composition and other applications.  Joe Groff offered to write a proposal for this.

Are you thinking in terms of a *real* bottom type—that is, a type which is the subtype of all types—or a fake bottom type which is simply an empty enum?

If you're thinking about a real bottom type, I wouldn't want to call it `NoReturn`, because the bottom type may end up playing a larger role in the language. Given our use of `Any`, the natural names for a bottom type are probably `All` (as the subtype of all types) or `None` (as a type with no instances). I do worry that those names are a little too short and attractive, though. `None` might be mistaken for `Void`; `All` might be mistaken for `Any`, and wouldn't make much sense when read as the return value of a function.

My best suggestion is `Never`. A function with a `Never` return type would read as "never returns":

	func abort() -> Never { … }

If it appeared in, say, a generic type, it would mean "never occurs":

	let result: Result<String, Never>

Flowing from that, we can end up with functions taking a `Never` parameter, which are never called:

	result.flatMapError { (_: Never) in fatalError("can't happen") }

Or `Never?` values, which are never `some`:

	let _: Never? = Result<String, Never>.error

(By the way, the return type of the force unwrap operator on a `Never?` is `Never`, which is just right: if you force unwrap a `Never?`, it will always trap, never return.)

The main issue I see with `Never` is that it's an adverb, not a noun. But the nouns all seem to have problems. And besides, the bottom type isn't so much a thing as a lack of a thing, isn't it? That's bound to have a slightly funky name.

-- 
Brent Royal-Gordon
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