From: Sebastian Hungerecker <sepp2k@...>
Date: 2009-04-10T05:25:34+09:00
Subject: Re: Calling a Function Before It's Defined

Mark Bilk wrote:
> I would like to be able to call a function (or a class,
> when I get more object-oriented) earlier in the program
> file than where I define it.  Such a "top-down" layout
> seems natural to me.  In C I could use a function
> prototype for this.

The reason why you have to declare a function in C before you call it (before 
meaning the code position) is fundementally different from why you have to 
define it in ruby before you call it (here before means the runtime).

Examples:
def foo()
  bar
end
def bar()
end
foo
Works just fine (because the code in foo that references bar executes after 
bar has been defined (even if it's position in the code is earlier).

def foo()
  1
end
def bar()
  foo
end
bar #=> 1
def foo()
  2
end
bar #=> 2
You can redefine methods and ruby always executes the version of the method 
that's currently in existence.

def foo()
  def bar()
  end
end
bar #error
foo
bar # Now it works

You can define method a within method b. That will have the effect that after 
running method b, method a will be defined (although it wasn't before).
You can also redefine methods that way:
def foo() 1 end
def bar() def foo() 2 end end
foo #=> 1
bar
foo #=> 2

All this has the effect that you can't forward declare methods like you can in 
C. If you do this:
def foo() end
foo
def foo() bar end
The call in line 2 will call the empty version of foo because that is the 
version currently in existence.

HTH,
Sebastian