From: Gavin Kistner <gavin.kistner@...>
Date: 2006-09-12T01:08:21+09:00
Subject: Re: Newbie syntax questions

> 1. Why is there an equals sign in the code "def 
> self.run=(flag)" and what
> does it do?

Methods can have an equals sign at the end of their name. Ruby calls
these methods when you write something that LOOKS like you're assigning
to a property:

class Foo
  def bar=( new_value )
    puts "I'm going to set a new value of #{new_value}!"
    @some_var = new_value
  end
end

f = Foo.new
f.bar = "Hello World"
#=> I'm going to set a new value of Hello World!


> 2. Why is there an equals sign in the code "@run ||= false" 
> and what does
> it do?

a ||= b
is shorthand for
a = a || b
just like a += b is shorthand for a = a + b

Because the || operator does not return a boolean value, but rather the
first operand that isn't a non-truth value, the above code is equivalent
to:

@run = if @run
  @run
else
  false
end

In practical application, what it's saying is:
"If the @run variable is nil or false, set the value to false".
or precisely:
"If the @run variable is nil (probably because it hasn't been assigned
to a value yet), please set it to false."


> 5. What does "$0 != "-e" && $0" mean and do?

$0 is a global variable that holds the name of the currently-running
script.
If you run "ruby foo.rb" then $0 will be the string "foo.rb".
If you run ruby with the -e option, it will evaluate the string you
supply as ruby code. For example, "ruby -e 'p 1+1'" will print 2.

When run like this, $0 is set to "-e", because there is no file name.

SO, the above code is saying "If $0 isn't -e, then the name of the
currently-running file." Like ||, the && operator doesn't actually
return true or false, it returns the value of the left side if the left
side is a non-truth value (false or nil), or the value of the right
side.

The $0!="-e"&&$0 bit is a terse (obfuscated?) way of saying "if the
script being run is off of the command line, use a value of false;
otherwise, use the value of the currently-running file name"