Correct use of modules, subroutines and functions in Fortran

Modules are always the right thing to use 😉

If you have a very simple F90 program you can include functions and subroutines in the ‘contains’ block:

 program simple
   implicit none
   integer :: x, y
   x = ...
   y = myfunc(x)
 contains
   function myfunc(x) result(y)
     implicit none
     integer, intent(in)  :: x
     integer              :: y
     ...
   end function myfunc
 end program

Then the interface of the functions/subroutines will be known in the program and don’t need to be defined in an interface block.

For more complex programs you should keep all functions/subroutines in modules and load them when required. So you don’t need to define interfaces, either:

 module mymod
   implicit none
   private
   public :: myfunc
 contains
   function myfunc(x) result(y)
     implicit none
     integer, intent(in)  :: x
     integer              :: y
     ...
   end function myfunc
 end module mymod

 program advanced
   use mymod, only: myfunc
   implicit none
   integer :: x, y
   x = ...
   y = myfunc(x)
 end program advanced

The module and the program can (actually should) be in separate files, but the module has to be compiled before the actual program.