“Never” is a proposition that an event doesn’t occur at any time in the past or future. It’s logical impossibility with a time axis; nothingness stretching out in all directions, forever.

…which is why it’s especially worrisome to encounter this comment in code:

  this will never happen  

Every compiler textbook will tell you that a comment like this one can’t and won’t affect the behavior of compiled code. Murphy’s Law says otherwise.

How does Swift keep us safe in the unpredictable chaos that is programming? You’ll never believe the answer: nothing and crashing.

[…]

Never was proposed as a replacement for the @noreturn attribute in SE-0102: “Remove @noreturn attribute and introduce an empty Never type”, by Joe Groff.

Really great article by Mattt over at NSHipster¹ on the proposed Never type for Swift. The article also goes over the concept of Uninhabited Types, which I’ve been using in my own code for a while without realising they had a name.

The idea of expressing unreachable states through the type system is a powerful one, and it’s already in use today in libraries like Result, which let you define a type as Result<Value, NoError to indicate a result that can never contain an error and have the compiler enforce that.