Using @optional_callbacks I love the idea of this. I think it simplifies a lot of the work and is still explicit. If this is in place can a callback still have an implementation? I ask because sometimes it is nice to have the exception give some larger context of information when an undefined callback is used. Many times I think a default error is okay, but sometimes I want to provide the developer with a more in depth explanation.
defoverridable with behaviours I like the simplification, but I'm not sure of the implementation. The original way is more explicit about what is overridable and what is not. Would it be possible to be explicit by changing defoverridable to be used the same way as def, defmodule, and defimple? The defoverridable feels like it goes along with the other "def" tokens. defmodule GenServer do defmacro __using__(_) do quote do defoverridable init(...) do ... end defoverridable terminate(..., ...) do ... end defoverridable code_change(..., ..., ...) do ... end end end end If this isn't possible it might be nice to look at a new name altogether. The @impl annotation I like the notation here when it includes that name of the behavior, but I'm not a fan of the true. I understand that it appears directly above the header in question, but I like the explicit version. Would this only go over one clause?, or would it have to be over top of each clause? Cheers, Amos Amos King Owner Binary Noggin http://binarynoggin.com #business http://thisagilelife.com #podcast ======================================================= I welcome VSRE emails. Learn more at http://vsre.info/ ======================================================= On Thu, Jan 19, 2017 at 10:39 AM, Iuri Machado <imet....@gmail.com> wrote: > Hi José, > > I really like the idea of changing the behaviour of defoverridable, > although I would rather go with the check on the implementation to raise > warnings as the default instead of setting @impl true. > > Em quinta-feira, 19 de janeiro de 2017 09:52:42 UTC-2, José Valim escreveu: >> >> Hi everyone, >> >> One of the features added to Elixir early on to help integration with >> Erlang code was the idea of overridable function definitions. This is what >> allowed our GenServer definition to be as simple as: >> >> defmodule MyServer do >> use GenServerend >> >> Implementation-wise, use GenServer defines functions such as: >> >> def terminate(reason, state) do >> :okend >> >> and then mark them as overridable: >> >> defoverridable terminate: 2 >> >> As the community grew, defoverridable/1 started to show some flaws in >> its implementation. Furthermore, the community did not always follow up on >> best practices, often times marking functions as overridable but without >> defining a proper Behaviour behind the scenes. >> >> The goal of this proposal is to clarify the existing functionality and >> propose extensions that will push the community towards best practices. >> Using @optional_callbacks >> >> In the example above, we have used defoverridable terminate: 2 to make >> the definition of the terminate/2 function optional. >> >> However, in some cases, the use of defoverridable seems to be >> unnecessary. For instance, we provide a default implementation for >> handle_call/3 and mark it as overridable, but the default implementation >> simply raises when invoked. That's counter-intuitive as it would be best to >> simply not define a default implementation in the first place, truly making >> the handle_call/3 callback optional. >> >> Luckily, Erlang 18 added support for marking callbacks as optional, which >> we support on Elixir v1.4. We propose Elixir and libraries to leverage this >> feature and no longer define default implementations for the handle_* >> functions >> and instead mark them as optional. >> >> Instead of the version we have today: >> >> defmodule GenServer do >> @callback handle_call(message, from, state) >> >> defmacro __using__(_) do >> quote do >> @behaviour GenServer >> >> def handle_call(_message, _from, _state) do >> raise "handle_call/3 not implemented" >> end >> >> # ... >> >> defoverridable handle_call: 3 >> end >> endend >> >> We propose: >> >> defmodule GenServer do >> @callback handle_call(message, from, state) >> @optional_callbacks handle_call: 3 >> >> defmacro __using__(_) do >> quote do >> @behaviour GenServer >> >> # ... >> end >> endend >> >> The proposed code is much simpler conceptually since we are using the >> @optional_callbacks feature instead of defoverridable to correctly mark >> optional callbacks as optional. defoverridable will still be used for >> functions such as terminate/2, which are truly required. >> >> For developers using GenServer, no change will be necessary to their code >> base. The goal is that, by removing unnecessary uses of defoverridable/1, >> the Elixir code base can lead by example and hopefully push the community >> to rely less on such tools when they are not necessary. >> The @impl annotation >> >> Even with the improvements above, the usage of defoverridable/1 and >> @optional_callbacks still have one major downside: the lack of warnings >> for implementation mismatches. For example, imagine that instead of >> defining handle_call/3, you accidentally define a non-callback >> handle_call/2. Because handle_call/3 is optional, Elixir won't emit any >> warnings, so it may take a while for developers to understand why their >> handle_call/2 callback is not being invoked. >> >> We plan to solve this issue by introducing the @impl true annotation >> that will check the following function is the implementation of a >> behaviour. Therefore, if someone writes a code like this: >> >> @impl truedef handle_call(message, state) do >> ...end >> >> The Elixir compiler will warn that the current module has no behaviour >> that requires the handle_call/2 function to be implemented, forcing the >> developer to correctly define a handle_call/3 function. This is a >> fantastic tool that will not only help the compiler to emit warnings but >> will also make the code more readable, as any developer that later uses the >> codebase will understand the purpose of such function is to be a callback >> implementation. >> >> The @impl annotation is optional. When @impl true is given, we will also >> add @doc false unless documentation has been given. We will also support >> a module name to be given. When a module name is given, Elixir will check >> the following function is an implementation of a callback in the given >> behaviour: >> >> @impl GenServerdef handle_call(message, from, state) do >> ...end >> >> defoverridable with behaviours >> >> While @impl will give more confidence and assistance to developers, it >> is only useful if developers are defining behaviours for their contracts. >> Elixir has always advocated that a behaviour must always be defined when a >> set of functions is marked as overridable but it has never provided any >> convenience or mechanism to enforce such rules. >> >> Therefore we propose the addition of defoverridable BehaviourName, which >> will make all of the callbacks in the given behaviour overridable. This >> will help reduce the duplication between behaviour and defoverridable >> definitions and push the community towards best practice. Therefore, >> instead of: >> >> defmodule GenServer do >> defmacro __using__(_) do >> quote do >> @behaviour GenServer >> >> def init(...) do ... end >> def terminate(..., ...) do ... end >> def code_change(..., ..., ...) do ... end >> >> defoverridable init: 1, terminate: 2, code_change: 3 >> end >> endend >> >> We propose: >> >> defmodule GenServer do >> defmacro __using__(_) do >> quote do >> def init(...) do ... end >> def terminate(..., ...) do ... end >> def code_change(..., ..., ...) do ... end >> defoverridable GenServer >> end >> endend >> >> By promoting new defoverridable API above, we hope library developers >> will consistently define behaviours for their overridable functions, also >> enabling developers to use the @impl true annotation to guarantee the >> proper callbacks are being implemented. >> >> PS: Notice defoverridable always comes after the function definitions, >> currently and as well as in this proposal. This is required because Elixir >> functions have multiple clauses and if the defoverridable came before, >> we would be unable to know in some cases when the overridable function >> definition ends and when the user overriding starts. By having >> defoverridable at the end, this boundary is explicit. >> Summing up >> >> This proposal promotes the use the of @optional_callbacks, which is >> already supported by Elixir, and introduces defoverridable(beha >> viour_name) which will push library developers to define proper >> behaviours and callbacks for overridable code. >> >> We also propose the addition of the @impl true or @impl >> behaviour_nameannotation, >> that will check the following function has been listed as a callback by any >> behaviour used by the current module. >> >> Feedback? >> >> >> *José Valim* >> www.plataformatec.com.br >> Skype: jv.ptec >> Founder and Director of R&D >> > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-lang-core+unsubscr...@googlegroups.com. > To view this discussion on the web visit https://groups.google.com/d/ > msgid/elixir-lang-core/85e62305-2b5f-4ae2-8e20- > 19f5cdf6e6d8%40googlegroups.com > <https://groups.google.com/d/msgid/elixir-lang-core/85e62305-2b5f-4ae2-8e20-19f5cdf6e6d8%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. 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