Interface mismatch - higher order functions

Question

I\'m trying to \'reproduce\' higher order functions in fortran.

module rk4

contains



 pure function f(t,x) result (fx)
    real, dimension(1), intent(in)         


        

Answer 1

The error message is complaining about the function f being incompatible with the dummy argument f.

You declare it as

   real, external :: f

which means it should return a scalar whereas in reality the function f returns an array.

The same names do not really help the understanding here. I changed the name of the dummy argument in the following code to g.

The easiest way to solve this is

 pure function f(t,x) result (fx)
    real, dimension(1), intent(in) :: x
    real, intent(in) :: t
    real, dimension(1) ::  fx

    fx = x
  end function f

  function step(x,g,dt) result(xn)
    real, intent(in) :: dt
    real, intent(in),  dimension(:) :: x
    real, dimension(:), allocatable :: xn
    procedure(f) :: g

    !here call g, not f!!!

The procedure statement comes from Fortran 2003 and causes the dummy argument procedure g to have the same interface as the procedure f.

Otherwise you can use an interface block:

  function step(x,g,dt) result(xn)
    real, intent(in) :: dt
    real, intent(in),  dimension(:) :: x
    real, dimension(:), allocatable :: xn

    interface
      pure function g(t,x) result (fx)
        real, dimension(1), intent(in) :: x
        real, intent(in) :: t
        real, dimension(1) ::  fx
      end function g
    end interface

The external statement should be used in modern code only in some exceptional cases.