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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.
Consider the following example (from ISO/IEC TR 19767):
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_aThe 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.
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