program main
  implicit none

  real :: n

  print*, 'Give N'
  read*, n

  print*, square_a(n)
  print*, square_b(n)
  print*, square_c(n)
  print*, square_d(n)

contains

  ! --- First way
  real function square_a(x) result(sol) 
    implicit none
    real :: x
    sol=x**2 
  end function square_a

  ! --- Second way
  real function square_b(x)
    implicit none
    real, intent(in) :: x  ! The 'intent' is optional (providing additional checking &
                           ! optimization) but IS necessary for elemental procedures
    square_b=x**2
  end function square_b

  ! --- Third way
  function square_c(x)
    real, intent(in) :: x
    real :: square_c
    square_c=x**2
  end function square_c

  ! --- Fourth way
  pure function square_d(x)  ! 'pure' here means that the function is free from side
                             ! effects and the compiler can optimize it in ways that
                             ! it otherwise cannot
    real, intent(in) :: x
    real :: square_d
    square_d=x**2
  end function square_d

end program main