! 例題 8-9  ４次元時空距離 ［オプション引数 ］
FUNCTION space_time( time, x_pos, y_pos, z_pos ) RESULT ( ss )

  IMPLICIT NONE
  REAL, INTENT ( IN ), OPTIONAL :: time
  REAL, INTENT ( IN ) :: x_pos, y_pos, z_pos
  REAL :: ss

  ss = x_pos ** 2 + y_pos ** 2 + z_pos ** 2
  IF ( PRESENT( time ) ) ss = ss - time * time

END FUNCTION space_time

! main
PROGRAM ex8_9

  IMPLICIT NONE
  REAL :: a, b, c, t

  INTERFACE
    FUNCTION space_time( time, x_pos, y_pos, z_pos )  &
             RESULT ( ss )
      REAL, INTENT ( IN ), OPTIONAL :: time
      REAL, INTENT ( IN ) :: x_pos, y_pos, z_pos
      REAL :: ss
    END FUNCTION space_time
  END INTERFACE

  a = 1.0E0
  b = 2.0E0
  c = 1.0E0
  PRINT '( F7.4 )', space_time( x_pos = a, y_pos = b, z_pos = c )

  t = 1.5E0
  PRINT '( F7.4 )', space_time( t, a, b, c )

END PROGRAM ex8_9