From: matt@... (matt neuburg)
Date: 2006-10-28T06:30:12+09:00
Subject: Re: What are closures, continuations?

Joe Ruby MUDCRAP-CE <joeat303@yahoo.com> wrote:

> - Ruby has closures -- which seem to be returning procs from functions?
> - Continuations may be planned for a future release of Ruby.

Continuations are hard, so I won't touch them. Very good examples here:

<http://www.rubycentral.com/faq/rubyfaq-11.html>

Closures are something I can handle (I discuss them in my AppleScript
book, for example).

* A binding is an association of a name and a value.

* A free variable is a variable defined in a surrounding scope.

* A closure is block of code where free variables are bound in
accordance with the surrounding scope and (this is important) retain
that binding.

So, for example:

var = 7
p = Proc.new {puts var}
def hereGoesNothing
  var = "yoho"
  yield
end
hereGoesNothing &p # => 7
var = 200
hereGoesNothing &p # => 200

The block "{puts var}" is a closure. Its "var" is a free variable and is
therefore bound to the "var" in the surrounding scope, which is the
"var" set to 7 in the first line. So what we see is that the "var"
inside hereGoesNothing makes no difference. As we change the top-level
"var", its new value is used each time we execute the block. It doesn't
matter what we do with p; it will retain its bindings.

Here's another example (more like your procs from functions idea):

def proc_maker
  var = 7
  Proc.new {puts var}
end
p = proc_maker
p.call # => 7

Here, "var" exists only inside the method "proc_maker". When we come to
call p, there is no "var"! But that doesn't matter; the proc "{puts
var}" has retained the binding of "var" with 7 from the place where it
was bound to start with, inside the method.

m.

-- 
matt neuburg, phd = matt@tidbits.com, http://www.tidbits.com/matt/
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