5 Records and Macros

Larger programs are usually written as a collection of files with a well defined interface between the various parts.

5.1  The Larger Example Divided into Several Files

To illustrate this, we will divide the messenger example from the previous chapter into five files.

mess_config.hrl
header file for configuration data
mess_interface.hrl
interface definitions between the client and the messenger
user_interface.erl
functions for the user interface
mess_client.erl
functions for the client side of the messenger
mess_server.erl
functions for the server side of the messenger

While doing this we will also clean up the message passing interface between the shell, the client and the server and define it using records, we will also introduce macros.

%%%----FILE mess_config.hrl----
%%% Configure the location of the server node,
-define(server_node, messenger@super).
%%%----END FILE----
%%%----FILE mess_interface.hrl----
%%% Message interface between client and server and client shell for
%%% messenger program 
%%%Messages from Client to server received in server/1 function.
-record(logon,{client_pid, username}).
-record(message,{client_pid, to_name, message}).
%%% {'EXIT', ClientPid, Reason}  (client terminated or unreachable.
%%% Messages from Server to Client, received in await_result/0 function 
-record(abort_client,{message}).
%%% Messages are: user_exists_at_other_node, 
%%%               you_are_not_logged_on
-record(server_reply,{message}).
%%% Messages are: logged_on
%%%               receiver_not_found
%%%               sent  (Message has been sent (no guarantee)
%%% Messages from Server to Client received in client/1 function
-record(message_from,{from_name, message}).
%%% Messages from shell to Client received in client/1 function
%%% spawn(mess_client, client, [server_node(), Name])
-record(message_to,{to_name, message}).
%%% logoff
%%%----END FILE----
%%%----FILE user_interface.erl----
%%% User interface to the messenger program
%%% login(Name)
%%%     One user at a time can log in from each Erlang node in the
%%%     system messenger: and choose a suitable Name. If the Name
%%%     is already logged in at another node or if someone else is
%%%     already logged in at the same node, login will be rejected
%%%     with a suitable error message.
%%% logoff()
%%%     Logs off anybody at at node
%%% message(ToName, Message)
%%%     sends Message to ToName. Error messages if the user of this 
%%%     function is not logged on or if ToName is not logged on at
%%%     any node.
-module(user_interface).
-export([logon/1, logoff/0, message/2]).
-include("mess_interface.hrl").
-include("mess_config.hrl").
logon(Name) ->
    case whereis(mess_client) of 
        undefined ->
            register(mess_client, 
                     spawn(mess_client, client, [?server_node, Name]));
        _ -> already_logged_on
    end.
logoff() ->
    mess_client ! logoff.
message(ToName, Message) ->
    case whereis(mess_client) of % Test if the client is running
        undefined ->
            not_logged_on;
        _ -> mess_client ! #message_to{to_name=ToName, message=Message},
             ok
end.
%%%----END FILE----
%%%----FILE mess_client.erl----
%%% The client process which runs on each user node
-module(mess_client).
-export([client/2]).
-include("mess_interface.hrl").
client(Server_Node, Name) ->
    {messenger, Server_Node} ! #logon{client_pid=self(), username=Name},
    await_result(),
    client(Server_Node).
client(Server_Node) ->
    receive
        logoff ->
            exit(normal);
        #message_to{to_name=ToName, message=Message} ->
            {messenger, Server_Node} ! 
                #message{client_pid=self(), to_name=ToName, message=Message},
            await_result();
        {message_from, FromName, Message} ->
            io:format("Message from ~p: ~p~n", [FromName, Message])
    end,
    client(Server_Node).
%%% wait for a response from the server
await_result() ->
    receive
        #abort_client{message=Why} ->
            io:format("~p~n", [Why]),
            exit(normal);
        #server_reply{message=What} ->
            io:format("~p~n", [What])
    after 5000 ->
            io:format("No response from server~n", []),
            exit(timeout)
    end.
%%%----END FILE---
%%%----FILE mess_server.erl----
%%% This is the server process of the messenger service
-module(mess_server).
-export([start_server/0, server/0]).
-include("mess_interface.hrl").
server() ->
    process_flag(trap_exit, true),
    server([]).
%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server(User_List) ->
    io:format("User list = ~p~n", [User_List]),
    receive
        #logon{client_pid=From, username=Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
        {'EXIT', From, _} ->
            New_User_List = server_logoff(From, User_List),
            server(New_User_List);
        #message{client_pid=From, to_name=To, message=Message} ->
            server_transfer(From, To, Message, User_List),
            server(User_List)
    end.
%%% Start the server
start_server() ->
    register(messenger, spawn(?MODULE, server, [])).
%%% Server adds a new user to the user list
server_logon(From, Name, User_List) ->
    %% check if logged on anywhere else
    case lists:keymember(Name, 2, User_List) of
        true ->
            From ! #abort_client{message=user_exists_at_other_node},
            User_List;
        false ->
            From ! #server_reply{message=logged_on},
            link(From),
            [{From, Name} | User_List]        %add user to the list
    end.
%%% Server deletes a user from the user list
server_logoff(From, User_List) ->
    lists:keydelete(From, 1, User_List).
%%% Server transfers a message between user
server_transfer(From, To, Message, User_List) ->
    %% check that the user is logged on and who he is
    case lists:keysearch(From, 1, User_List) of
        false ->
            From ! #abort_client{message=you_are_not_logged_on};
        {value, {_, Name}} ->
            server_transfer(From, Name, To, Message, User_List)
    end.
%%% If the user exists, send the message
server_transfer(From, Name, To, Message, User_List) ->
    %% Find the receiver and send the message
    case lists:keysearch(To, 2, User_List) of
        false ->
            From ! #server_reply{message=receiver_not_found};
        {value, {ToPid, To}} ->
            ToPid ! #message_from{from_name=Name, message=Message}, 
            From !  #server_reply{message=sent} 
    end.
%%%----END FILE---

5.2  Header Files

You will see some files above with extension .hrl. These are header files which are included in the .erl files by:

-include("File_Name").

for example:

-include("mess_interface.hrl").

In our case above the file is fetched from the same directory as all the other files in the messenger example. (*manual*).

.hrl files can contain any valid Erlang code but are most often used for record and macro definitions.

5.3  Records

A record is defined as:

-record(name_of_record,{field_name1, field_name2, field_name3, ......}).

For example:

-record(message_to,{to_name, message}).

This is exactly equivalent to:

{message_to, To_Name, Message}

Creating record, is best illustrated by an example:

#message_to{message="hello", to_name=fred)

This will create:

{message_to, fred, "hello"}

Note that you don't have to worry about the order you assign values to the various parts of the records when you create it. The advantage of using records is that by placing their definitions in header files you can conveniently define interfaces which are easy to change. For example, if you want to add a new field to the record, you will only have to change the code where the new field is used and not at every place the record is referred to. If you leave out a field when creating a record, it will get the value of the atom undefined. (*manual*)

Pattern matching with records is very similar to creating records. For example inside a case or receive:

#message_to{to_name=ToName, message=Message} ->

is the same as:

{message_to, ToName, Message}

5.4  Macros

The other thing we have added to the messenger is a macro. The file mess_config.hrl contains the definition:

%%% Configure the location of the server node,
-define(server_node, messenger@super).

We include this file in mess_server.erl:

-include("mess_config.hrl").

Every occurrence of ?server_node in mess_server.erl will now be replaced by messenger@super.

The other place a macro is used is when we spawn the server process:

spawn(?MODULE, server, [])

This is a standard macro (i.e. defined by the system, not the user). ?MODULE is always replaced by the name of current module (i.e. the -module definition near the start of the file). There are more advanced ways of using macros with, for example parameters (*manual*).

The three Erlang (.erl) files in the messenger example are individually compiled into object code file (.beam). The Erlang system loads and links these files into the system when they are referred to during execution of the code. In our case we simply have put them in the same directory which is our current working directory (i.e. the place we have done "cd" to). There are ways of putting the .beam files in other directories.

In the messenger example, no assumptions have been made about what the message being sent is. It could be any valid Erlang term.