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

[Lukas Larsson]

Hello Thomas,

I agree that there is a need for better encapsulating possibilities in
Erlang. However in order to allow calls only within the current application
you have to introduce the concept of an application into the language and
not only in OTP. Personally I think having language support for a
collection of modules (like packages in Java, or namespace in C++) is a
necessity for the language to grow and mature. I do however not know what
that would look like and what properties that would bring. Have you thought
anything about this?

I also do not really understand the purpose of private. How does it differ
from not-exported? Is it an exported function which can only be called from
a specific behaviour module?

Lukas


On Thu, Apr 25, 2013 at 9:21 AM, Thomas Järvstrand <tjarvstrand@REDACTED>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
>
>
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