From: Bill Atkins <batkins57@...>
Date: 2004-10-20T03:31:35+09:00
Subject: Re: Question about closure

Methods aren't closures in Ruby - only blocks are.  In that example,
the makeCounter method is returning a lambda, which is a block (and
thus a closure).  A closure is (from what I understand) just a block
that remembers its context.  Consider this:

  a = 34
  b = proc { a * 2 }

  # in a different scope
  b.call                  # = > 68

Even though a has gone out of scope by the time b is actually called,
Ruby saves the value of a with b so that when you call it, b can
return the proper value.  In other words, a closure is just a block
that refers to its environment.

Bill

On Wed, 20 Oct 2004 03:04:20 +0900, Sam Sungshik Kong
<ssk@chol.nospam.net> wrote:
> Hi, group!
> 
> I am trying to understand what exactly a closure is.
> I'm kinda familiar with C#.
> In C# 2.0 (the next version that's coming out), there's a anonymous
> delegate.
> People say that it's a closure (See
> http://martinfowler.com/bliki/Closures.html).
> They even compare it with Ruby codes saying that closure is block.
> Is it true?
> 
> When I saw the closure example in Ruby, it was not just a block.
> 
> Here's a typical example of Ruby closure.
> 
> <snip>
> def makeCounter
>   var = 0
>   lambda do
>     var +=1
>   end
> end
> 
> c1 = makeCounter
> c1.call
> c1.call
> c1.call
> 
> c2 = makeCounter
> 
> puts "c1 = #{c1.call}, c2 = #{c2.call}"
> </snip>
> 
> Here is what the above page(http://martinfowler.com/bliki/Closures.html)
> says...
> 
> <snip>
> Closures have been around for a long time. I ran into them properly for the
> first time in Smalltalk where they're called Blocks. Lisp uses them heavily.
> They're also present in the Ruby scripting language - and are a major reason
> why many rubyists like using Ruby for scripting.
> 
> Essentially a closure is a block of code that can be passed as an argument
> to a function call.
> 
> ....
> 
> In a language that has Closures, in this case Ruby, I'd write this.
> 
> def managers(emps)
>  return emps.select {|e| e.isManager}
> end
> </snip>
> 
> This is just a block, right?
> Is it also a closure?
> 
> I'm confused.
> Can somebody enlighten me?
> 
> 
> Sam
> 
>