Bug #7715: Lazy enumerators should want to stay lazy.

• Status: Closed
• Assignee: marcandre (Marc-Andre Lafortune)
• Target version: 2.0.0
• ruby -v: r38825

[marcandre (Marc-Andre Lafortune)]

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

[ko1 (Koichi Sasada)]

• Target version set to 2.0.0

Who's ball?

[marcandre (Marc-Andre Lafortune)]

• Status changed from Open to Assigned
• Assignee set to marcandre (Marc-Andre Lafortune)

I can do it, unless there are objections.

[shugo (Shugo Maeda)]

marcandre (Marc-Andre Lafortune) wrote:

I can do it, unless there are objections.

Your proposal sounds reasonable.
I guess these methods were forgotten to change when lazy was implemented.

[yhara (Yutaka HARA)]

shugo (Shugo Maeda) wrote:

I guess these methods were forgotten to change when lazy was implemented.

That's right :-( I thought these methods does not need to be overriden
because they return Enumerator, but they should return Enumerator::Lazy for such cases.

[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...

[marcandre (Marc-Andre Lafortune)]

Patch almost done, which also fixes #7248

https://github.com/marcandre/ruby/compare/marcandre:master...marcandre:lazy

Still missing:

• tweak inspect
• fix .lazy.size
• couple more tests

[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).

[marcandre (Marc-Andre Lafortune)]

• Status changed from Assigned to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r39058.
Marc-Andre, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

• enumerator.c: Use to_enum for Enumerable methods returning Enumerators.
  This makes Lazy#cycle no longer needed, so it was removed.
  Make Enumerator#chunk and slice_before return lazy Enumerators.
  [Bug #7715]

• internal.h: Remove ref to rb_enum_cycle_size; no longer needed

• enum.c: Make enum_cycle_size static.

• test/ruby/test_lazy_enumerator.rb: Test for above