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Bug #19569
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#map on Enumerator::Lazy block arguments seem to differ from regular #map
Description
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?
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Updated by jeremyevans (Jeremy Evans)
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