Issue #708 has been updated by Yusuke Endoh.

Status changed from Assigned to Rejected

2012/2/8 Yukihiro Matsumoto <matz / ruby-lang.org>:
> I vote for #4890.

Thank you for your opinion.
I'll close those tickets except #4890.

-- 
Yusuke Endoh <mame / tsg.ne.jp>
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Feature #708: Lazy Enumerator#select, Enumerator#map etc.
https://bugs.ruby-lang.org/issues/708

Author: Brian Candler
Status: Rejected
Priority: Normal
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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http://bugs.ruby-lang.org/