Bug #16414: Incompatible behavior of Proc/lambda with single argument when using `Enumerator::Lazy#with_index`

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.0dev (2019-12-10T10:12:21Z master af11efd377) [x86_64-darwin18]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN

[tomog105 (Tomohiro Ogoke)]

The following code raised an error wrong number of arguments (given 1, expected 2) (ArgumentError) in master.

$ ruby -e 'lambda = -> (s, i) { "#{i}:#{s}" }; p %w(a b c).each.lazy.with_index.map(&lambda).first(2)'
# expected result => ["0:a", "1:b"]

This code is valid up till Ruby 2.6.5 and 2.7.0-preview1, but it raised the error in Ruby 2.7.0-preview2 or later. Maybe, this behavior has been there since the implementation of Enumerator::Lazy#with_index.

Is this behaviour intended?

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[jeremyevans0 (Jeremy Evans)]

I agree this is a bug. The with_index block should be called with two arguments, not an array with one argument.

The reason this happens is that Enumerator::Lazy#with_index ends up calling Enumerator::Yielder#<< (yielder_yield_push C function) on the yielder with an array with two entries. That in turn calls rb_proc_call_with_block(ptr->proc, 1, &arg, Qnil);. This is why the block gets called with a single array argument.

After experimenting more, Enumerator::Lazy#with_index has other issues. The most critical is it ignores the block passed to it. I'm fairly sure it needs to be rewritten. I'll see if I can fix the problems with it. If the problems can't be fixed, we should probably back out the changes, so that Enumerator::Lazy#with_index is no longer lazy.

[jeremyevans0 (Jeremy Evans)]

I have a possible fix for this: https://github.com/ruby/ruby/pull/2742

After CI completes, I'll try to merge it so it makes 2.7.0-rc1.

• Status changed from Open to Closed

Applied in changeset git|85e43e1dfecef69b935c48c235cc20f21bd4f0d4.

---

Fix Enumerator::Lazy#with_index

• Make it correctly handle lambdas
• Make it iterate over the block if block is given

The original implementation was flawed, based on lazy_set_method
instead of lazy_add_method.

Note that there is no implicit map when passing a block, the return
value of the block passed to with_index is ignored, just as it
is for Enumerator#with_index. Also like Enumerator#with_index,
when called with a block, the return value is an enumerator without
the index.

Fixes [Bug #16414]