From: Nikodemus Siivola <tsiivola@...>
Date: 2002-10-17T06:54:27+09:00
Subject: Re: Precompiling eval expressions


On Thu, 17 Oct 2002, Brian Candler wrote:

> (Can anyone point me to a good explanation of what is going on underneath?)

Procs are closures: they contain their enclosing environment (ie. as a
pointer to a stack frame). This is cool because it enables you to create
the kinds of functions you need, dynamically:

def adder n
  proc { |x| x + n } # The n is part of the environment, see?
end

a = adder 1          # But is actually 1 here
b = adder 2          # and 2 here

a.call(1)            # => 1   ...hence the different behaviour
b.call(1)            # => 3

This storage of context is also why they are expensive, and why you should
pass blocks, instead of procs to methods when you have the choise:

def one   # Better, provided that the outside context is not needed
  yield
end

def two   # Useless proc creation unless you are going to store it somewere
  Proc.new.call
end

one { puts "No context" }
two { puts "context" }

If you really want to grok them, I recommend the "Structure and
Interpretation of Computer Programs", esp. the metacircular evaluator part.
It's a bloody good book, and it's online.

Hope this helps,

  -- Nikodemus