Feature #708: Lazy Enumerator#select, Enumerator#map etc.
http://redmine.ruby-lang.org/issues/show/708

Author: Brian Candler
Status: Open, Priority: Low
Category: core, Target version: 1.9.x

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.


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