Erlang modules and functions

1. module
2. export
3. function
4. macro
5. macro-usage

% the first attribute to declare in a module is the name
-module(hello_world). 

% all functions that interface to the outside world must be declared
% the number present after the function name is the number of arguments in the function     
-export([add/2, helloWorld/0, callOthers/2, multiply/2]). 
 
% function to add 2 numbers
add(Var1, Var2) ->
  Var1 + Var2. 
 
% function to say the usual - Hello World
helloWorld() ->
  io:format("Hello World~n"). 
 
% function to show how to call other functions
% also shows multi-line functions
callOthers(X,Y) ->
  helloWorld(),
  add(X,Y). 
  

% Erlang supports macros like #define in c
-define(multiplyMacro(A,B), (A*B)). 
-define(divide(A,B), A/B). 

% Only difference from c is that the macro needs to be called with a ?
multiply(A,B) ->
  ?divide(?multiplyMacro(A,B), ?multiplyMacro(A,B)). 

% compile step
erl> c(hello_world). 
{ok,hello_world}


% call some functions
erl> hello_world:add(2,3). 
5

erl> hello_world:helloWorld().
Hello World
ok

erl> hello_world:callOthers(3,4).
Hello World
7

erl> hello_world:multiply(3,4).
1.0


% see some information about the module just compiled  
erl> hello_world:module_info(). 
[{exports,[{multiply,2},
           {add,2},
           {helloWorld,0},
           {callOthers,2},
           {module_info,0},
           {module_info,1}]},
 {imports,[]},
 {attributes,[{vsn,[311445890858620593432522823934882247552]}]},
 {compile,[{options,[]},
           {version,"4.9.2"},
           {time,{2013,8,9,4,51,54}},
           {source,"c:/Work/Erlang_code/hello_world.erl"}]}]