Re:Never patterns, exhaustive matching, and uninhabited type...
by kibwen on Saturday August 18, 2018 @02:43PM
(#17789900)
> execution continues in the face of impossible values
I don't know about C++, but it may be doing something different from Rust here. In Rust, the ! type (which I pronounce "Never"), isn't just a case of "I, the programmer, don't think any value will exist here"; the compiler forces you to prove (just like every other type is a proof) that a value can't exist. The Never type can't be instantiated (it has no constructor), and functions that "return" Never must prove that they, er, never return; imagine a function whose body is just `loop {}` (or the stdlib function process::exit, which cannot return by dint of killing the callstack when the process dies).
I don't know about C++, but it may be doing something different from Rust here. In Rust, the ! type (which I pronounce "Never"), isn't just a case of "I, the programmer, don't think any value will exist here"; the compiler forces you to prove (just like every other type is a proof) that a value can't exist. The Never type can't be instantiated (it has no constructor), and functions that "return" Never must prove that they, er, never return; imagine a function whose body is just `loop {}` (or the stdlib function process::exit, which cannot return by dint of killing the callstack when the process dies).