Fortran elemental functions vs elemental subroutines

Fortan allows elemental subroutines to have intent(inout) and intent(out) arguments, but elemental functions are only allowed intent(in).

Why is that? Is it just a stylistic convention, or is there something generically different about invoking functions and calling subroutines?

In other words,

Elemental Integer Function FOO(i)
  Integer, intent(in) :: i
    ...
  FOO=something
End Function

and

Elemental Subro开发者_Python百科utine FOO(i, v)
  Integer, intent(in)  :: i
  Integer, intent(out) :: v
    ...
  v=something
End Subroutine

— are these implementations of FOO equivalently efficient?

There is no point in having an elemental subroutine without at least one argument marked as intent(out) or intent(inout), because you have to pass the result somehow. A function has its return value, a subroutine must use its arguments. In Fortran 2008 AFAIK elemental procedures doesn't have to be pure, but it's hard to imagine a useful elemental subroutine only through its side effects.