proposed @noreturn attribute
Meta via Digitalmars-d
digitalmars-d at puremagic.com
Sat Jul 8 21:23:15 PDT 2017
On Sunday, 9 July 2017 at 02:25:50 UTC, Walter Bright wrote:
> (D already has a `void` type, so can't use Haskell's word.)
Just so we are all on the same page, from a type-theory
perspective void is a unit type (it has 1 value), not an
uninhabited type (it has no values, i.e. Haskell's _|_ (bottom)
type).
A function with a return type of unit means "this function
returns no useful information", because the type only has one
possible value anyway. A function with a return type of bottom
means "this function can never return", because there is no value
of type bottom that could be returned. All that can be done is
for the function to diverge (throwing an exception, ending the
program, looping forever, etc.).
We sort of have this already with `assert(0)`. The compiler knows
that no execution can take place after an `assert(0)` is
encountered (in other words, it knows that the function
diverges). We just don't have a corresponding type to represent
this (Rust uses ! but if I remember correctly it's not quite a
first class type).
If we wanted to be cute we could use `typeof()` to represent this
type as there is no value you can give to typeof such that it
returns the bottom type. It also avoids having to come up with
some special symbol or name for it.
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