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

Thomas Järvstrand tjarvstrand@REDACTED
Sat Apr 27 13:07:43 CEST 2013

There is actually no need need to introduce the concept of applications
into the language itself, merely support for the notion of different types
of exports. Interpreting the meaning of these exports can be left to xref,
which already has support for applications.

The point of the private type of export is for functions that have to be
externally callable, but that should never be explicitly referenced.
Examples of this are behaviour callback functions (such as the handle_call
etc. of a gen_server) and functions that are passed to or returned from
functions calls (such as spawn). Ie, they are functions that may only be
implicitly referenced (ie. in the calling code, either the module, the
function name or both would have to be a variable). Basically this means
that any calls to private functions that xref is able to detect are illegal.

Yes, I'm aware of that EEP. It has similar aims and looks cosmetically
similar but the suggestion to limit access on a module level would make
things awfully messy IMHO. With EEP 5, you have to clutter your code with
individual exports for every single caller/callee-pair inside your
application. If you want to call a new function in your module, you would
now have to edit two files instead of one. A more serious flaw though is
that if you're writing a utility library that is to be used by others, you
have no idea who will be calling your module, and so you cannot export your
functions to them.


2013/4/26 Robert Virding <robert.virding@REDACTED>

> There is already an EEP which touches on this,
> http://www.erlang.org/eeps/eep-0005.html
> Robert
> ------------------------------
> *From: *"Lukas Larsson" <lukas@REDACTED>
> *To: *"Thomas Järvstrand" <tjarvstrand@REDACTED>
> *Cc: *"Erlang" <erlang-questions@REDACTED>
> *Sent: *Friday, 26 April, 2013 8:57:03 AM
> *Subject: *Re: [erlang-questions] Request for comment: A proposal to
> introduce the concept of function domains
> 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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