Module privacy

Module entities are public by default. Write private on its own line to flip the default, then list the API: public :: stack_push, stack_pop.

protected module variables can be read by users but changed only inside the module, which gives read-only global state.

use m, only: a => alpha renames on import to avoid clashes between modules.

Keep helper procedures private so you can change them freely; only the public interface is a contract.

Private derived-type components (integer, private :: n) hide representation even when the type itself is public.

module counter_mod
  implicit none
  private
  public :: bump, count_value
  integer, protected, public :: total = 0
contains
  subroutine bump()
    total = total + step()
  end subroutine bump
  integer function count_value()
    count_value = total
  end function count_value
  integer function step()
    step = 1
  end function step
end module counter_mod

program main
  use counter_mod
  implicit none
  call bump(); call bump()
  print '(i0,1x,i0)', count_value(), total
end program main