Submodules in Fortran Wiki

**Submodules** are a feature of [[Fortran 2008]] which allow a module procedure to have its interface defined in a module while having the body of the procedure defined in a separate unit, a submodule.
Submodules were originally introduced as an extension to [[Fortran 2003]] and were later adopted as part of the [[Fortran 2008]] standard.
For further details, see the following technical report:
[ISO/IEC TR 19767:2005 Fortran - Enhanced Module Facilities](ftp://ftp.nag.co.uk/sc22wg5/N1601-N1650/N1602.pdf).
## Example
Consider the following example (from ISO/IEC TR 19767):
~~~~~ {: lang=fortran }
module points
 type :: point
 real :: x, y
 end type point
 interface
 module function point_dist(a, b) result(distance)
 type(point), intent(in) :: a, b
 real :: distance
 end function point_dist
 end interface
end module points
submodule (points) points_a
contains
 module function point_dist(a, b) result(distance)
 type(point), intent(in) :: a, b
 real :: distance
 distance = sqrt((a%x - b%x)**2 + (a%y - b%y)**2)
 end function point_dist
end submodule points_a
~~~~~
The repetition in the above example can be avoided by using an
alternative declaration. The following example shows that it is not
necessary to restate the properties of the `point_dist` interface.
~~~~~ {: lang=fortran }
submodule (points) points_a
contains
 module procedure point_dist
 distance = sqrt((a%x - b%x)**2 + (a%y - b%y)**2)
 end procedure point_dist
end submodule points_a
~~~~~
## External Links
* [Progress on Fortran 2003 and the Technical Report on Enhanced Module Facilities](http://www.fortran.bcs.org/2004/ukfortran04.htm)
## References
* [ISO/IEC TR 19767:2005 Fortran - Enhanced Module Facilities](ftp://ftp.nag.co.uk/sc22wg5/N1601-N1650/N1602.pdf).