[swift-evolution] [Draft] Change @noreturn to unconstructible return type

Damien Sorresso dsorresso at apple.com
Tue Jun 7 13:11:10 CDT 2016

On 7 Jun, 2016, at 09:47, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>> I disagree. We are discussing how to annotate a function in some way so that the compiler knows that the code following it will never be executed *and* so a human who reads the declaration knows that it does not return. “Never" is a poor choice for that. Never what? Never return? Never use this function? Never say never again? 
> "Never return". That's why it's in the return type slot, right after the `->`. If you read it out loud, you'll read "returns Never", which is exactly correct.

A return type makes a lot of sense linguistically but does not cover all practical cases because you might not be the one deciding what the function's signature is. For example, you might implement a protocol method that is non-optional, but you never expect to be called. The only way to indicate that to the compiler is with an attribute.

> NoReturn, on the other hand, does *not* read well in that slot: "returns NoReturn". Huh? I mean, I suppose you won't misunderstand it, but it makes no sense whatsoever *as a type name*.
>> If you want bottom types for other uses, give them their own appropriate and self documenting names. 
> The problem is, types flow through the type system. Use a NoReturn method with optional chaining, now you have an Optional<NoReturn>. flatMap over that Optional<NoReturn>, now you have a parameter of type NoReturn. What's a *parameter* of type NoReturn? You'd want it to be, say, a different bottom type named NoPass, but you can't—the type came from a NoReturn, and it's stuck being a NoReturn.
> Never works pretty well—honestly, surprisingly well—in all of these contexts. The method returns Never, so optional chaining gives you an Optional<Never>, so flatMap has a Never parameter. I have yet to discover a case where it isn't easily interpreted to mean exactly what it really does mean.
> -- 
> Brent Royal-Gordon
> Architechies
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