From: Brian Candler <b.candler@...>
Date: 2009-04-15T19:17:43+09:00
Subject: Re: Proc.call with custom bindings

Adam Strzelecki wrote:
> So think this explains my problem:
> ---- cut ----
> proc = Proc.new do
>   if defined? s
>     puts "s = #{s}"
>   else
>     puts "s is undefined"
>   end
> end
> proc.call
> s = 1
> proc.call
> ---- cut ----
> 
> We got twice "s is undefined" even it is defined at second call.

The local variable/method ambiguity is resolved statically when code is 
parsed, before anything is executed. Example:

  def x
    "hello"
  end

  if false
    x = "world"
  end

  puts x    # nil

At the "puts x", within the current scope there has been a (potential) 
assignment to x, and therefore it is already decided that x is a local 
variable. When it is executed, no assignment actually took place, so it 
carries the default value nil.

Similarly:

  puts y    # NameError
  y = nil

There has been no (potential) assignment to y lexically before the puts 
statement, and therefore it is statically decided that y must be a 
method call, i.e. self.y()

This logic may be a little surprising at first, but it means that you 
don't have to declare variables, and it also means you don't have to 
provide an empty set of parentheses when invoking a method which takes 
no arguments.

Regards,

Brian.
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