Modules again

Last updated on 2026-07-10 | Edit this page

Overview

Questions

  • How to organise large Fortran programs?
  • How to minimise recompilation in large Fortran programs?

Objectives

  • Understand how to separate modules into modules and submodules
  • Understand the difference between host and use association

Modules

The introductory course encouraged the use of modules to provide structure, and to separate public interface from private implementation. Schematically, we have seen:

module schematic

  ! Broadly, public "interface"

contains

  ! Broadly, private "implementation"

end module schematic

However, …

We also have a situation in which the (public) interface can remain unchanged, while a change to the (private) implementation requires recompilation of the module. This may have knock-on effects requiring re-compilation of any dependencies. Is this really necessary if we have not changed the interface?

The answer should be no. However, a typical makefile dependency might cause re-compilation of all units use-ing the module schematic. Some (rather baroque) mechanisms have been used to avoid this, e.g., making a copy of the .mod file, and interrogating whether this changes on compilation.

The separation of large modules into parts may be desirable, but can necessitate the proliferation of public entities which might not really be wanted. Cross-dependencies can also be a problem (the situation where module a wants to use module b and vice-versa).

Callout

Separate interface and implementation

This is not quite satisfactory. A clearer separation between interface and implementation (cf. C header files) is required.

Submodules

Submodules may be organised in a tree-like hierarchy, with the ancestor module as the root.

module example_module

end module example_module

The module should define only the interface (there should probably be no contains statement).

The implementation can then be placed in a submodule:

submodule (example_module) example_submodule

contains

end submodule example_submodule
Callout

Host association

The submodule automatically has access to the declarations in the example module via host association. There is no need to use the ancestor module.

Formally, we have:

submodule (parent-identifier) submodule-name
  [ specification part ]
  [ module-subprogram-part]
end [submodule [ submodule-name] ]

where the parent-identifier is the name of the parent module.

Submodule procedures

Suppose in our module we define a new data type:

  type, public :: example_t
    ...
  end type example_t

We can define an interface block using this type (also in the module) to specify the contract:

   interface example_t
     module function example_int_t(ival) result(e)
       integer, intent(in) :: ival
       type (example_t)    :: e
     end function example_int_t
   end interface example_t

Note the module at the start of the function declaration, and that there is no import statement required in the interface block.

This would then be implemented in the submodule subprogram part:

  module function example_int_t(ival) result(e)
    integer, intent(in) :: ival
    type (example_t)    :: e
    ...
  end function example_t
Callout

Matching text

The interface and the function declaration must match precisely (only the name of the result variable is not formally part of the interface and need not match).

Exercise (2 minutes)

Discussion

Compiling modules and submodules

Compile (do not link) the module and submodule files example.f90 and example_a.f90 in the current directory. E.g.,

$ ftn -c example_module.f90
$ ftn -c example_submodule.f90

Check what additional files have been generated (the exact details will depend on the compiler: some produce .smod files, others just additional .mod files).

Abbreviated form

It’s possible to omit the interface details in the submodule. E.g.,

  module procedure example_t
    ...
  end procedure example_t

Note the use of procedure here. Again, the details must match the interface declared in the module.

Callout

A matter of taste

While this provides some saving in verbosity, it may be preferable to keep the full form.

Host association and use association

A submodule has access to entities in the parent module automatically by host association, but may also use other modules in the standard way.

Callout

Breaking circular dependencies

A submodule of module a can access module b by use association. In addition, a submodule of module b can access module a. This breaks the circular dependency problem in standard modules.

Submodule declarations

Note that submodules do not contain public or private declarations. However, additional variables, types, or named constants may be defined in a submodule specification part. These are neither public nor private, but can only be accessed in the submodule and any descendants by host association.

Callout

Comment on the use of submodules

Submodules may only really come into their own if you have an extremely large software project for which the overheads of re-compilation are high, or the organisation of extremely large single modules becomes a problem. For small exercises, such as those presented in this course, the additional files involved are probably an unnecessary distraction. So we will only use them on one occasion.

Exercise (15 minutes)

Discussion

Splitting a module into submodules

We will take the single module developed for the file_writer_t and split it into a module containing the abstract definition, and one or two implementations. A client program should then only depend on the top-level (abstract) interface.

It’s probably easiest to make a number of copies of a working solution module, and then delete unwanted parts from each. Suggested procedure:

  1. Keep the abstract type file_writer_t definition and its related interface block in file_module.f90. Here we will also need an interface for the file_writer_from_string() function. Check the new file_module.f90 compiles on its own.
  2. Make a new (ordinary) module file_unformatted for the unformatted implementation in a separate file, and a new module file_formatted for the formatted implementation.
  3. Add a submodule to file_module to hold the implementation of the file_writer_from_string() function in a separate file.

An example program which only has use file_module will now depend only on the public definition of the abstract type.

All the Fortran sources should be compiled together to produce an executable.

What further decomposition into submodules could we make at this point?

Key Points
  • Submodules provide the mechanism to separate interface and implementation in Fortran