Modules and Derived Types

Code

module geometry
  implicit none
  private
  public :: point_type, distance

  type :: point_type
    real :: x = 0.0, y = 0.0
  contains
    procedure :: dist_to_origin
  end type

contains

  function distance(p1, p2) result(d)
    type(point_type), intent(in) :: p1, p2
    real :: d
    d = sqrt((p1%x - p2%x)**2 + (p1%y - p2%y)**2)
  end function

  function dist_to_origin(self) result(d)
    class(point_type), intent(in) :: self
    real :: d
    d = sqrt(self%x**2 + self%y**2)
  end function

end module

! Usage
program use_geom
  use geometry
  type(point_type) :: a, b
  a%x = 0; a%y = 0
  b%x = 3; b%y = 4
  print *, distance(a, b)    ! 5.0
  print *, b%dist_to_origin()! 5.0
end program

Explanation

Modules encapsulate types, procedures, and variables. private/public controls visibility. Derived types with contains support type-bound procedures (OOP). Use % to access components (like . in C++). class() (polymorphic) allows subclasses; type() is static. Always use implicit none to catch typos.

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