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

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

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

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