Emulating namespaces in Fortran 90

Question

One of the most troublesome issues with Fortran 90 is the lack of namespacing. In this previous question "How do you use Fortran 90 module data" from Pete, it has been discussed the main issue of USE behaving like a "from module import *" in python: everything that is declared public in the module is imported as-is within the scope of the importing module. No prefixing. This makes very, very hard to understand, while reading some code, where a given identifier comes from, and if a given module is still used or not.

A possible solution, discussed in the question I linked above, is to use the ONLY keyword to both limit the imported identifiers and document where they come from, although this is very, very tedious when the module is very large. Keeping the module small, and always using USE : ONLY is a potentially good strategy to work around the lack of namespacing and qualifying prefixes in Fortran 9X.

Are there other (not necessarily better) workaround strategies? Does the Fortran 2k3 standard say anything regarding namespacing support?

Answer 1

Fortran 2003 has the new ASSOCIATE construct and don't forget the possibility of renaming USE- associated entities. But I don't think that either of these is much closer to providing a good emulation of namespaces than Fortran 90 already has, just (slightly) better workarounds.

Like some of the respondents to the question you link to, I tend to think that modules with very many identifiers should probably be split into smaller modules (or, wait for Fortran 2008 and use submodules) and these days I almost always specify an ONLY clause (with renames) for USE statements.

I can't say that I miss namespaces much, but then I've never had them really.

Answer 2

For me this is the most irritating Fortran feature related to modules. The only solution is to add common prefix to procedures, variables, constants, etc. to avoid namespace collisions.

One can prefix all entities (all public entities seems to be more appropriate) right inside the module:

module constants

  implicit none

  real, parameter :: constants_pi = 3.14
  real, parameter :: constants_e = 2.71828183

end module constants

Drawback is increased code verbosity inside the module. As an alternative one can use namespace-prefix wrapper module as suggested here, for example.

module constants_internal

  implicit none

  real, parameter :: pi = 3.14
  real, parameter :: e = 2.71828183

end module constants_internal

module constants

  use constants_internal, only: &
    constants_pi => pi, &
    constants_e => e

end module constants

The last is a small modification of what you, Stefano, suggested.

Even if we accept the situation with verbosity the fact that Fortran is not case-sensitive language force us to use the same separator (_) in entities names. And it will be really difficult to distinguish module name (as a prefix) from entity name until we do not use strong naming discipline, for example, module names are one word only.