From: Brian Candler <b.candler@...>
Date: 2008-11-03T06:24:47+09:00
Subject: Re: ruby1.9: lazy versions of Enumerator#select and friends?

Thomas Sawyer wrote:
> Enumerator.new() wants an enumerable object as an argument --what is
> 1.9 doing without it?

1.9 supports two completely different types of Enumerator objects. The 
first  is the simple 1.8.6 one which just maps method :each to arbitrary 
method :foo on the upstream object. The second is the clever one.

-------------------------------------------------------- Enumerator::new
     Enumerator.new(obj, method = :each, *args)
     Enumerator.new { |y| ... }

     From Ruby 1.9.1
------------------------------------------------------------------------
     Creates a new Enumerator object, which is to be used as an
     Enumerable object iterating in a given way.

     In the first form, a generated Enumerator iterates over the given
     object using the given method with the given arguments passed. Use
     of this form is discouraged. Use Kernel#enum_for(), alias to_enum,
     instead.

       e = Enumerator.new(ObjectSpace, :each_object)
           #-> ObjectSpace.enum_for(:each_object)

       e.select { |obj| obj.is_a?(Class) }  #=> array of all classes

     In the second form, iteration is defined by the given block, in
     which a "yielder" object given as block parameter can be used to
     yield a value by calling the +yield+ method, alias +<<+.

       fib = Enumerator.new { |y|
         a = b = 1
         loop {
           y << a
           a, b = b, a + b
         }
       }

       p fib.take(10) #=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
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