How to make some generic programming in fortran 90/95 working with intrinsic types

Question

I would like to program some procedure that will work with different types. I am planning to use the "include" method used in flibs described here and here. I give here a simple exemple.

  ! -------------------------------------------------------------- ! 
  module data_type

  type ivalue
  integer :: v
  end type

  type rvalue
  real(8) :: v
  end type

  end module data_type
  ! -------------------------------------------------------------- ! 
  module imod

  use data_type, only: T => ivalue 

  include "template.f90"

  end module imod
  ! -------------------------------------------------------------- ! 
  module rmod

  use data_type, only: T => rvalue 

  include "template.f90"

  end module rmod
  ! -------------------------------------------------------------- ! 
  module mod

  use imod, only:
 &     ivalue => T,
 &     iprintme => printme

  use rmod, only:
 &     rvalue => T,
 &     rprintme => printme

  private
  public :: ivalue, rvalue
  public :: printme

  interface printme
  module procedure iprintme
  module procedure rprintme
  end interface printme

  end module mod
  ! -------------------------------------------------------------- !
  program hello

  use mod

  implicit none

  type(ivalue) :: iv
  type(rvalue) :: rv

  iv%v=42
  rv%v=3.14

  call printme(iv)
  call printme(rv)      

  end program hello

with the included file:

  contains

  subroutine printme(a)

  implicit none

  type(T) :: a

  print *,a

  end subroutine printme

What bothers me is that it seems only to work with derived type, and not with intrinsic types. If the user of the module mod want to use the printme routine on an simple integer, it is really annoying for him to encapsulate it in a ivalue type and cannot doing:

integer :: a=42
call printme(a)

Is there any way to extend this method to intrinsic types, or another method that would do it in strict f90/f95 (I don't want to use the "transfer" method because of the data copy)

Tanks!

Answer 1

You can use the C preprocessor (CPP) in all major Fortran compilers. Usually there is a flag for invoking it (-cpp in gfortran) or it is invoked automatically if the file suffix contains capital F (.F90, .F). The preprocessor allows more powerful inclusion of sources with the usage of macros.

module imod

  use data_type, only: ivalue 

#define T type(ivalue)
#include "template.f90"
#undef T

end module imod


module intmod

#define T integer
#include "template.f90"
#undef T

end module intmod

and template.f90

contains

subroutine printme(a)

  implicit none

  T :: a

  print *,a

end subroutine printme

This is not strict f90 / f95, but it uses a preprocessor, included in the compilers, which produces another (strict f95) source file and it automatically compiles it instead of the original source that contains the macros.

The compilation is then straightforward

gfortran -cpp main.f90

--Edit--

For non-believers, if you want to see some real code using this, check https://github.com/LadaF/fortran-list (disclaimer: my own code). You can use the parametric linked list there as:

list of len(20) character strings:

module str_list

#define TYPEPARAM character(20)

#include "list-inc-def.f90"
contains
#include "list-inc-proc.f90"
#undef TYPEPARAM
end module

list of integers

module int_list

#define TYPEPARAM integer

#include "list-inc-def.f90"
contains
#include "list-inc-proc.f90"
#undef TYPEPARAM
end module

list of some derived type

module new_type_list
  use, new_type_module, only: new_type

#define TYPEPARAM type(newtype)

#include "list-inc-def.f90"
contains
#include "list-inc-proc.f90"
#undef TYPEPARAM
end module

Answer 2

You can use implicit typing. Make sure you wash your hands though - as this opens the possibility for the usual errors associated with implicit typing.

Consider a replacement for your module imod.

module imod
  use data_type    ! oops - I forgot this.
  implicit type(itype) (q)
contains
  include 'template.f90'
end module imod

(I've moved the contains statement into the including module - as it then allows you to have more than one templated include file.)

and then a procedure in included file that looks like:

 ! Assume anything starting with q is the type to be templated.
 subroutine printme(q_arg)
   print *, q_arg
 end subroutine printme

If you wanted to template printme for an intrinsic type, then you just change the implicit statement in the parent module appropriately.

---

It's debatable, but there's also the view that you can use the module renaming facility to introduce new names for intrinsic types. If so, and if you have a F2008 compiler (so we aren't talking strict F95) then your current approach should still be able to work - using an intermediate module to allow renaming of the intrinsic integer type to have a name T.

But this confuses most (all?) compilers I've used in some way. Given that, plus the debatable legality, plus it requiring F2008, it's not a real solution.