From: "David A. Black" <dblack@...>
Date: 2008-11-02T04:30:06+09:00
Subject: Re: Iterator objects and lazy evaluation

Hi --

On Sun, 2 Nov 2008, Brian Candler wrote:

> Yuh-Ruey Chen wrote:
>> 1) Are there equivalents for iteration/enumeration functions like map
>> that return iterator/enumeration objects (in a Python sense)?
>
> You can generate Enumerator objects, and in 1.9 (and I think 1.8.7), an
> Enumberable method which hasn't been given a block returns an
> Enumerator.
>
> irb(main):001:0> a = (1..10).each
> => #<Enumerator:0x83b687c>
> irb(main):002:0> a.next
> => 1
> irb(main):003:0> a.next
> => 2
> irb(main):004:0> a.next
> => 3
>
> However I don't think you can use these for chaining. It might be nice
> if you could do something like
>
>   (1..10).to_enum.map2 { |x| x * 2 }.map2 { |x| x + 1 }.map2 { |x| puts
> x }
>
> which wouldn't create any intermediate array. I guess you'd need each
> element to call 'next' on the previous one, and you'd need something at
> the end to trigger the whole process off.

It used to be that you could associate a block with an enumerator
lazily:

>> e = [1,2,3].enum_for(:map,&lambda {|x| x * 10 })
=> #<Enumerable::Enumerator:0x55b11c>
>> e.next
=> 10

and some chaining was possible. But that's gone now. Enumerators are
still able to remember regular arguments, though, so you can do:

> e = [1,2,3,4,5].each_cons(2)
=> #<Enumerator:0x42548c>
>> e.select {|a,b| b > 3 }
=> [[3, 4], [4, 5]]

(The contrast here is between a relatively old 1.9 version and 1.9.1
preview 1.)


David

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