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


Patch updated, rdoc improved too.

Makes for a clean API for Lazy#new also, and there's even less code (~20 loc).

I'll review the patch one last time before committing it (in about 5 hours).
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Bug #7715: Lazy enumerators should want to stay lazy.
https://bugs.ruby-lang.org/issues/7715#change-35836

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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