Gfortran warning

Makefile

all:
	gfortran -Wextra -Wall -o w.o -c w.f90

w.f90

module mixings

implicit none
private
public mixing_linear

integer, parameter :: dp=kind(0.d0)

contains

function mixing_linear(R, x0, max_iter, alpha)
! Finds "x" so that R(x) = 0, uses x0 as the initial estimate
real(dp), intent(in) :: x0(:), alpha
integer, intent(in) :: max_iter
real(dp) :: mixing_linear(size(x0))
interface
    function R(x, i)
    implicit none
    integer, parameter :: dp=kind(0.d0)
    real(dp), intent(in) :: x(:)
    integer, intent(in) :: i ! iteration #
    real(dp) :: R(size(x))
    end function
end interface

real(dp), dimension(size(x0)) :: x_i
integer :: i
x_i = x0
do i = 1, max_iter
    x_i = x_i + alpha * R(x_i, i)
end do
mixing_linear = x_i
end function

end module

If you compile this, it will produce the following warning:

$ make
gfortran -W -Wall -o w.o -c w.f90
w.f90:11:0: warning: unused parameter ‘r’