Modules

Common blocks in Fortran 77 were the only portable means of achieving global access of data throughout a collection of subprograms. This is unsafe, error-prone, and encourages bad programming practices in general. Fortran 90 provides a new program unit, a module, that replaces the common block and also provides many other features that allow modularization and data hiding, key concepts in developing large, maintainable numerical code.

Modules consist of a set of declarations and module procedures that are grouped under a single global name available for access in any other program unit via the use statement. Interfaces to the contained module procedures are explicit and permit compile time type-checking in all program units that use the module. Visibility of items in a module may be restricted by using the private attribute. The public attribute is also available. Those identifiers not declared private in a module implicitly have the public attribute.

module TypicalModule
   private swap   ! Make swap visible only within this module.

   contains

   subroutine order( x, y )   ! Public by default.
      integer, intent( inout ) :: x, y

      if ( abs( x ) < abs( y ) )  call swap( x, y )
   end subroutine order


   subroutine swap( x, y )
      integer, intent( inout ) :: x, y
      integer tmp

      tmp = x; x = y; y = tmp ! Swap x and y.
   end subroutine swap
end module TypicalModule

program UseTypicalModule
   use TypicalModule

   ! Declare and initialize x and y.
   integer :: x = 10, y = 20   

   print *, x, y
   call order( x, y )
   print *, x, y
end program UseTypicalModule