Are Erlang -callbacks to be invoked only through MFA functions (apply/3, spawn/3, ...)? (Custom Behaviors HOWTO)

Question

That's my suspicion as this simple code

-module(simple_server).
-export( [sayHello/0] ).

-callback say(Num :: term()) -> term().

sayHello() ->
    io:fwrite( "Hello 1: ~p\n", [ say(1) ]) ,
    io:fwrite( "Hello 2: ~p\n", [ say(2) ]) .

fails to be compiled:

$ erlc simple_server.erl
simple_server.erl:7: function say/1 undefined
simple_server.erl:8: function say/1 undefined

If that is the case, then this is not explicitly commented elsewhere: official docs, "learn erlang", this answer.

Answer 1

You need to provide the name of the callback module, which can be done through apply and spawn, but you can also make a simple call using a variable as the module name, e.g. CallbackModule:say(1).

So you could do either:

sayHello(CallbackModule) ->
    io:fwrite( "Hello 1: ~p\n", [ CallbackModule:say(1) ]) ,
    io:fwrite( "Hello 2: ~p\n", [ CallbackModule:say(2) ]) .

or

sayHello(CallbackModule) ->
    io:fwrite( "Hello 1: ~p\n", [ apply(CallbackModule, say, [1]) ]) ,
    io:fwrite( "Hello 2: ~p\n", [ apply(CallbackModule, say, [2]) ]) .

Those two versions are equivalent.

Let's create a callback module implementing the simple_server behaviour:

-module(my_callback).
-behaviour(simple_server).
-export([say/1]).

say(N) ->
    {N, is, the, loneliest, number}.

Now we can call simple_server:sayHello with the module name as the argument:

> simple_server:sayHello(my_callback).
Hello 1: {1,is,the,loneliest,number}
Hello 2: {2,is,the,loneliest,number}