From: Motohiro KOSAKI <kosaki.motohiro@...>
Date: 2012-02-08T03:09:21+09:00
Subject: [ruby-core:42406] [ruby-trunk - Feature #708] Lazy Enumerator#select, Enumerator#map etc.


Issue #708 has been updated by Motohiro KOSAKI.


Can anyone take a feedback? If nothing, I have to close this ticket sadly.

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Feature #708: Lazy Enumerator#select, Enumerator#map etc.
https://bugs.ruby-lang.org/issues/708

Author: Brian Candler
Status: Assigned
Priority: Low
Assignee: Yukihiro Matsumoto
Category: core
Target version: 2.0.0


=begin
 There are a number of methods in Enumerable which build an Array of results from the entire collection - e.g. map, select, take etc.
 
 I propose that the Enumerator class have its own implementations of these methods, which return another Enumerator. Enumerators can then be chained:
 
   seq.to_enum.map { ... }.select { ... }.take(...).each { |x| puts x }
 
 This runs "horizontally": that is, each element is processed left to right. No intermediate arrays are created, and it works happily with infinite sequences.
 
 There are precendents for SomeClass#select behaving differently to Enumerable#select. For example, Hash#select now returns a Hash. So I believe it would be reasonable for Enumerator to return another Enumerator.
 
 You can then choose between array-building or lazy evaluation, depending on whether there is an Enumerator in the chain. Of course, the last Enumerator has to be turned into something useful, e.g. by calling to_a or each { ... }.
 
    # Normal
    res = (1..1_000_000).map { |x| x * 2 }.take(100)
    # Lazy
    res = (1..1_000_000).to_enum.map { |x| x * 2 }.take(100).to_a
 
 I have attached a simple implementation of this for select, map, take and a new method skip. There are further methods like take_while, zip and so on which would also need to be implemented.
=end



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