On Sunday, 9 July 2017 at 04:23:15 UTC, Meta wrote:
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.
In C++/D terms, the unit type could be expressed as `enum Unit {
unit }` while the bottom type would be `enum Bottom {}` (although
only conceptually because in C++/D you can still do `Bottom b;`).
