From: Brian Candler <b.candler@...>
Date: 2008-10-29T21:36:33+09:00
Subject: Re: Local variables in lambda

List Rb wrote:
> Hi.  Is there a way to create a lambda that references local variables
> not yet defined ( with out pawing them to the prod) ?
> 
> I.e.
> x = lambda {a+b}
> a = 1
> b = 2
> c = x.call

I don't know what "pawing them to the prod" means, but you can do

  a, b = nil, nil
  x = lambda {a+b}
  a = 1
  b = 2
  c = x.call

This is because Ruby makes a parse-time decision as to whether 'a' is a 
local variable or a method call. If no assignment to 'a' has been seen 
earlier in the scope, then it is interpreted as a method call instead: 
self.a()

Since this is done while the parse tree is built, prior to execution, it 
doesn't make any difference whether the assignment is actually executed 
or not. For example:

  def x
    "hello"
  end

  if false
    x = 1
  end

  puts x      # shows 'nil' since x is a local variable

This is necessary to have any sort of efficiency. Otherwise, a simple 
statement like "b = a + 1" inside a loop would have to decide at 
run-time, every time it is executed, whether 'a' is a local variable or 
a method.
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