Bug #19569: #map on Enumerator::Lazy block arguments seem to differ from regular #map

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.0.5...3.2.0
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

[TomNaessens (Tom Naessens)]

A Ruby 2 to 3 upgrade surfaced the following issue:

Ruby 2:

irb(main):252:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map(&:last).to_a
=> [:a, :b, :c]

Ruby 3:

irb(main):020:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map(&:last).to_a
(irb):20:in `each': undefined method `last' for 1:Integer (NoMethodError)

Debugging the issue, I noticed the following:

Map with a block returns only the first argument (and seems to ignore the zipped list):

irb(main):024:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |x| x }.to_a
=> [1, 2, 3]

It's not actually ignored though, it's just passed as a second argument:

irb(main):025:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |*x| x }.to_a
=> [[1, :a], [2, :b], [3, :c]]

Thus, this looks like a workaround:

irb(main):026:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |*x| x.last }.to_a
=> [:a, :b, :c]

But it behaves differently when leaving the lazy out:

irb(main):027:0> [1, 2, 3].to_enum.zip([:a, :b, :c]).map { |*x| x.last }.to_a
=> [[1, :a], [2, :b], [3, :c]]

Not sure if I'm correct in my conclusion, but to me it looks like the argument handling of map and lazy_map differs in an unexpected manner. Is this a bug/oversight, or I am simply using the different map implementations wrong?

[TomNaessens (Tom Naessens)]

• Subject changed from #map on Enumerator::Lazy block arguments differ from regular #map to #map on Enumerator::Lazy block arguments seem to differ from regular #map

[jeremyevans0 (Jeremy Evans)]

This is because Enumerator::Lazy#zip marks the enumerator as packed when it shouldn't (introduced with the optimization in 54acb3dd52a5fe75c32c3e36a6984d3ec4314a72). I've submitted a pull request to fix this: https://github.com/ruby/ruby/pull/7967

• Status changed from Open to Closed

Applied in changeset git|a14915ca4baae40f08d288a38b8bfb595500ad90.

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Do not have Enumeratory::Lazy#zip mark result as packed

Fixes [Bug #19569]