How to leave behavior callback functions unimplemented?

Question

Suppose I have a gen_server:

-module(myserver).
-behavior(gen_server).
-export([init/1, handle_call/3]).

init(Arg) -> % ...
handle_call(Request, From, State) -> % ...

Compiling this naturally produces a warning:

myserver.erl:2: Warning: undefined callback function handle_cast/2 (behaviour 'gen_server')

How do I deal with this warning if I my server isn't going to need handle_cast/2? Do I define a placeholder function just to satisfy the compiler? For example:

handle_cast(_Request, _State) -> unimplemented.

Or do I ignore/suppress the warning message?

What is the proper way to write a gen_server where not all callback functions will be used?

bxdoan · Accepted Answer · 2019-05-21 06:36:05Z

3

We should NOT ignore the warning message, this makes us difficult to trace errors/bugs in our application in future. Leaving the placeholder function is a better way, but it's should like below:

handle_cast(_Msg, State) ->
    {noreply, State}.

By default, this function can't data to the caller, so you can avoid to crash your application when you or your colleagues use gen_sever:cast/2.

I didn't have specific way to write gen_sever. In a big project that I worked for, they just leave the placeholder like below to use in the future. They don't care much about callback functions is used or not. We should take care about normal function is used or not rather than callback functions.

handle_call(_Request, _From, State) ->
    {reply, ok, State}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

answered May 21, 2019 at 6:36

bxdoan

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2 Comments

Flux Over a year ago

Is it recommended to add logging to the supposedly unused callback function so that their inadvertent use would appear in the logs?

bxdoan Over a year ago

If this warning dont bother you when compiling, so just ignore them. I just want to argue that you ignore cast and leave an unimplemented like this. You might need cast in the future.

7stud · Accepted Answer · 2019-05-21 19:00:17Z

1

How do I deal with this warning if I my server isn't going to need handle_cast/2?

Then your server isn't a gen_server, so you can remove the line:

-behavior(gen_server).

which is what causes the compiler to check that the gen_server callback functions are present.

The bottom line is that OTP requires a contract between your code and the OTP behaviors. If you don't want to adhere to the contract, then you can't use OTP behaviors. In other words, you are required to implement ALL the callback functions.

Is it recommended to add logging to the supposedly unused callback function so that their inadvertent use would appear in the logs?

It's up to you. It sounds like a good idea to me.

edited May 21, 2019 at 19:00

answered May 21, 2019 at 18:55

7stud

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