Host Association vs Use Association in Fortran

Question

Are there any "general rules" as to when one is preferable to the other?

The context of this question is: I asked a different question regarding host association yesterday (link) and in the comments, I was advised to use host association with caution. The reason being that through host association, it is easy to inadvertently modify variables since the subroutines have unrestricted access to all variables that are declared in the module.

To illustrate this, I will use the following code example:

module mod
implicit none

real :: x

contains

subroutine sub(y)
  use other_mod, only: a

  real, intent(out) :: y

  y = a + x
  a = a + 1.
  x = x + 1.
end subroutine sub

end module mod

Both aand x are modified in sub. But for x, I need to go through all the code to see this. That a is used in sub (and possibly modified) can be seen easily by looking at the declaration part of sub.

In this sense, it seems preferable to have two kinds of modules:

A module or modules only containing variable declarations (which are then used when needed)
Modules that only contain procedures and possibly parameter declarations but no variable declarations

This gets rid of host association for variables altogether.

But this doesn't seem practical for a number of reasons:

I might have a dozen subroutines using (and modifying) the same variables in one module. Having to use these variables everytime clutters the code, especially if there are a lot of them (say a few hundred).
Seperating the declaration of a variable from where it is actually used seems to make the code less comprehensible:
- Either, one creates one giant control file containing all the declarations. This could be quite confusing if the code is large and uses many variables.
- Or, one creates a seperate control file for every module (or group of modules, if they depend on the same content). This would make the code itself better comprehensible, since using the variables immediately shows where they are coming from. But it would complicate the structure of the code, creating a vastly more complicated file structure (and accompanying dependency structure).

In the end, all of this boils down to: When is it more sensible to put the declaration of variables in the same module in which they are used (so that they are used by host association) and when is it more sensible to outsource the declaration to a seperate module (so that the variables will be used via use association when they are needed)?

Are there any general guidelines or should this be decided on a case by case basis? And if it is case by case, what are the reasons to go for one over the other?

Host Association vs Use Association in Fortran

1 Answers