From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2008-10-29T02:56:43+09:00
Subject: Re: memoization example the ruby programming language question

On Tue, Oct 28, 2008 at 5:32 PM, Hiro Protagonist <credulity@gmail.com> wrote:

> I marked the line that I don't understand.  I want an explanation in
> terms of syntax.  If cache has data why is method memoize called first?

This:

class Proc; include Functional; end

adds the method "memoize" to the Proc class.

This:

lambda {|x| return 1 if x==0; x*factorial[x-1]; }

creates a Proc object, which, by means of the above extension
has a method memoize. With this:

lambda {|x| return 1 if x==0; x*factorial[x-1]; }.memoize

you call the method memoize of the created Proc object.
Maybe this is more clear:

temp_proc = lambda {|x| return 1 if x==0; x*factorial[x-1]; }
factorial = temp_proc.memoize()

The method memoize returns a Proc itself. This new Proc
maintains a cache of calls. When there's a cache miss, it calls
the original Proc (check uses of "self" inside the memoize method).
When there's a cache hit, the original proc is not called, and the cached
value is returned instead.


> I understand the initially recursive calls when cache is empty. In empty
> cache scenario the factorial lambda Proc keeps being called recursively
> until x == 0.  Once this happens memoize is called up the stack for each
> factorial call and data is cached for each value of x.  But when cache
> is full and momoization is used.  Why does it go to method memoize
> first?  I just don't see it syntactically.  This example may use some
> ruby 1.9 syntax.  I think factorial[x-1] is equivalent to
> factorial.call(x-1).

Yes, but factorial here is not your original proc. It's the proc returned
but the memoize method.

> I need an explanation of syntax.  Also I have never seen the syntax:
> factorial = lambda {|x| return 1 if x==0; x*factorial[x-1]; }.memoize
> where the memoize method is attached to end of lambda {}.  Does
> factorial PROC object contain lambda block plus the appended memoize
> method?

I think you are confusing the terms here. "memoize" is a method of proc
objects that returns a proc.

Let me know if this sheds some light...

Jesus.