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

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}

Answer 2

HOWTO Erlang Custom Behaviors (template method pattern).
-1. Write a generic module. Here, an algorithm is defined, but some steps (callback functions in Erlang nomenclature) are left for a future specific definition.
%% generic.erl
-module(generic).
-export( [sayHello/1] ).
-callback say(Num :: term()) -> term(). %% future definition
%% generic algorithm: needs the reference to the provider module
sayHello(ProviderModule) ->
io:fwrite( "Hello 1: ~p\n", [ ProviderModule:say(1) ]) ,
io:fwrite( "Hello 2: ~p\n", [ ProviderModule:say(2) ]) .
-2. Compile generic.erl
erlc generic.erl
-(3.) Try to write a provider (callback) module
%% callbacks1.erl (fails to implement say/1 callback)
-module( callbacks1 ).
-behaviour( generic ).
-(4.) Compile callbacks1.erl: use -pa . to say where generic.beam is. (Therefore, the expected warning about say/1 is issued).
erlc -pa . callbacks1.erl
callbacks1.erl:2: Warning: undefined callback function say/1 (behaviour 'generic')
(Note: If -pa is not given, you'll got this: "callbacks1.erl:2: Warning: behaviour generic undefined")
-3. Write a correct provider (callback) module.
%% callbacks2.erl
-module( callbacks2 ).
-behaviour( generic ).
-export( [say/1] ).
say(1) -> "good morning";
say(2) -> "bon jour";
say(_) -> "hi".
-4. Compile it
erlc -pa . callbacks2.erl
(Ok now).
-5. Write a main.erl to gather generic module with callback module.
%% main.erl
-module( main ).
-export( [main/0] ).
main() ->
%% call the generic algorithm telling it what callback module to use
generic:sayHello( callbacks2 )
. % ()
-6. Compile and run main
erlc main.erl
erl -noshell -s main main -s init stop
We get:
Hello 1: "good morning"
Hello 2: "bon jour"