[erlang-questions] Request for comment: A proposal to introduce the concept of function domains

[Michael Truog]

On 04/25/2013 12:21 AM, Thomas Järvstrand wrote:
> Hi all,
>
> I write this email to get the community's opinion on an idea to extend Erlang with the concept of function domains. The intention is for the end result to be submitted as an EEP.
>
> The rationale is that encapsulation is a good thing and that code should not be allowed to depend on library functions that the library's author did not intend to expose to the outside world. Because of this I would like to introduce the idea of exporting functions into different domains.
>
> There will be three predefined function domains: one will allow the function to be called from anywhere, one will allow the function to be called from within its own application and one to  disallow the function from being explicitly referenced anywhere outside its module (this is for behaviour callbacks, functions passed/returned as funs etc.)
>
> To allow for extension in the future, compilation will allow any atom to be given as the domain name (warnings could be added for non-predefined domains), but xref will only be extended with processing of the predefined domains.
>
> Suggestions for what to call the predefined domains are:
>
> *public, restricted, private*
> The rationale is that due to how the language works, a domain-declaration will only specify where we allow the function to be referenced with its fully qualified name we can't detect other any other references anyway.
> *
> external, internal, restricted*
> The rationale is that functions in the private domain are not really private at all since they can be called from anywhere (eg. the handle_call of a gen_server will be called from the gen_server-module). Private the becomes restricted, because that's what it really is and we use the duality of
> external/internal for allowing calls from outside/inside the application. The main issue with this suggestion is that many people associate internal with non-exported functions.
>
> I have two suggestions for how the domains should be declared in a module, they are both attributes that take two arguments: the domain and a list of <function>/<arity>:
>
> *The domain/2 attribute:
> *
> Pros: Backwards compatibilty
> Cons: "Clutters" the attribute namespace. Requires information to be duplicated in the module (need both export and domain)
>
> *The export/2 attribute:
> *
> Pros: Avoids cluttering the attribute namespace and avoids duplicated information in the code.
> Cons: Breaks backwards compatibility with earlier OTP releases for code written using the new attribute.
>
> I would appreciate some input on this, both from the community and the OTP-team.
>
> Regards
> Thomas Järvstrand
>

I think that function domains touches upon a related issue, which could hopefully be combined into one EEP.  The issue is a lack of module namespaces, which currently impacts application dependencies.  Ignoring any potential use by the developer of a namespace concept, and definitely ignoring the past packages implementation that has since been removed from Erlang, there exists an application dependency problem which limits the growth and flexibility of Erlang systems.  The problem is that if application A and application B both depend on C, but on different versions of C, it is difficult to include both A and B without modifying one of them.  The idea of function domains can help provide some access control of a module within the context of a single application, however, access control is a larger problem which impacts how applications can be combined.  To make Erlang more flexible for integrating with various open source dependencies, and various legacy code dependencies,
it is often not realistic to build the release with only a single version of a specific application dependency.  I think having the version optionally be added to all of the module names (as a suffix) within an application, which is a problematic dependency, is a way of coping with the issue (it could be based on a flag within the application file of the dependency), but I am sure there are other solutions which may be better.  Either way, both emails deal with the concept of access control for Erlang source code.

- Michael
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