From: David Flanagan <david@...>
Date: 2007-09-26T09:38:00+09:00
Subject: next method and for loop

With the introduction of external iterators and the python-like next 
method and StopIteration exception, I'm wondering if there is any 
thought of somehow enabling the for/in loop to work with objects that 
implement next but do not implement each.

As it stands now there is an asymmetry between internal and external 
iterators.  If I implement a class with an each method for internal 
iteration, I can use to_enum to get an Enumerator object that has a next 
method for external iteration.  But if I implement a class that natively 
uses external iteration and implements next, there is no automatic way 
to get an internal iterator for the class.  I can't use for/in on 
instances of my class.  I don't have a to_enumerable method I can call 
on an instance.  And there is no module I can include that will define 
an each method based on my next method.

I'm not sure that this is a problem, but it is an asymmetry that might 
bother programmers migrating from Python, and it might be quite easy to 
address.  I believe, for example that a general-purpose each method can 
be implemented in terms of next as follows:

   def each
     loop do # Note that loop exits cleanly on StopIteration in 1.9
       yield self.next
     end
   end

So what if we had a (built-in) module something like this:

module Iterable   # What's a better name for this? ExternallyEnumerable?
   include Enumerable
   def each
     loop { yield self.next }
   end
end

Then classes that wanted to be enumerable would have a choice.  They could

a) define each and include Enumerable
b) define next and include Iterable

Am I making any sense here?  This isn't something I'm advocating 
strongly for; but it is an idea I would like to get out there.

A simple demo program follows

	David


module Iterable
   include Enumerable
   def each
     loop { yield self.next }
   end
end

class Countdown
   include Iterable

   def initialize(x)
     @x = x
   end

   def next
     raise StopIteration if @x <= 0
     @x -= 1
   end
end

f = Countdown.new(10)
for i in f
   puts i
end