In
Type Theory, the
type that has exactly
one element. This
element is called, not
surprisingly, the
unit element. We usually
represent this type as
1, and the unit element as
() or
<>. We
write () : 1.
Since it is the only element of type 1, the unit element carries no information. When we don't want a function to return anything useful (perhaps it has useful side effects, such as I/O or messing with ephemeral structures), we have it return (). This is somewhat analogous to functions in C returning void, but isn't exactly the same thing.
Similarly, if we want a function to take no useful input (say it returns the same thing every time or returns something dependent on ephemeral structures), we have it take () as its only argument.