Issue #7715 has been updated by marcandre (Marc-Andre Lafortune).


I believe I have found the key to resolve this issue, Lazy.new issue [#7248] and others.

We simply need to specialize `to_enum/enum_for` for lazy enumerators.

In the same way, RETURN_SIZED_ENUMERATOR should return a lazy enumerator, when called for a lazy enumerator.

With this in mind:
* Lazy.each_with_object, etc..., will correctly return lazy enumerators [#7715] without being overriden.
* Lazy#cycle can be removed. It no longer needs to be overriden.
* Lazy.new really has no need to accept (method, *args) and can be modified as proposed in [#7248]
* Any user method of Enumerable that returns an Enumerator using `to_enum` will conserve laziness.

None of this could create a regression, since Lazy & RETURN_SIZED_ENUMERATOR are both new to 2.0.0

I'm working on a patch...
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Bug #7715: Lazy enumerators should want to stay lazy.
https://bugs.ruby-lang.org/issues/7715#change-35814

Author: marcandre (Marc-Andre Lafortune)
Status: Assigned
Priority: Normal
Assignee: marcandre (Marc-Andre Lafortune)
Category: core
Target version: 2.0.0
ruby -v: r38825


I'm just waking up to the fact that many methods turn a lazy enumerator in a non-lazy one.

Here's an example from Benoit Daloze in [ruby-core:44151]:

lines = File.foreach('a_very_large_file').lazy
            .select {|line| line.length < 10 }
            .map {|line| line.chomp!; line }
            .each_slice(3)
            .map {|lines| lines.join(';').downcase }
            .take_while {|line| line.length > 20 }

That code will produce the right result but *will read the whole file*, which is not what is desired

Indeed, `each_slice` currently does not return a lazy enumerator :-(

To make the above code as intended, one must call `.lazy` right after the `each_slice(3)`. I feel this is dangerous and counter intuitive.

Is there a valid reason for this behavior? Otherwise, I would like us to consider returning a lazy enumerator for the following methods:
  (when called without a block)
    each_with_object
    each_with_index
    each_slice
    each_entry
    each_cons
  (always)
    chunk
    slice_before

The arguments are:
* fail early (much easier to realize one needs to call a final `force`, `to_a` or `each` than realizing that a lazy enumerator chain isn't actually lazy)
* easier to remember (every method normally returning an enumerator returns a lazy enumerator). basically this makes Lazy covariant
* I'd expect that if you get lazy at some point, you typically want to remain lazy until the very end


-- 
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