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?

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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.