From: "knu (Akinori MUSHA) via ruby-core" <ruby-core@...>
Date: 2025-11-21T06:26:50+00:00
Subject: [ruby-core:123875] [Ruby Feature#21701] Enumerator.produce accepts an optional `size` keyword argument

Issue #21701 has been reported by knu (Akinori MUSHA).

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Feature #21701: Enumerator.produce accepts an optional `size` keyword argument
https://bugs.ruby-lang.org/issues/21701

* Author: knu (Akinori MUSHA)
* Status: Open
* Assignee: knu (Akinori MUSHA)
* Target version: 4.0
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Enumerator::Producer#size currently always returns `Float::INFINITY`, and it is not specifiable.

However, a produce sequence is known to be at least finite in many cases, and you can even tell or compute the exact size in some cases.

```ruby
# Infinite enumerator
enum = Enumerator.produce(1, size: Float::INFINITY, &:succ)
enum.size  # => Float::INFINITY

# Finite enumerator with known/computable size
abs_dir = File.expand_path("./baz") # => "/foo/bar/baz"
traverser = Enumerator.produce(abs_dir, size: -> { abs_dir.count("/") + 1 }) {
  raise StopIteration if it == "/"
  File.dirname(it)
}
traverser.size  # => 4
```

As a background, Enumerator#to_set was changed to refuse to work against an infinite enumerator in https://bugs.ruby-lang.org/issues/21513.
That made #to_set always fail against produce sequences.  This enhancement will fix the problem (kind of) safely, and we will also be able to add the same size check to #to_a, which is currently missing and inconsistent with #to_set.

Here's the implementation: https://github.com/ruby/ruby/pull/15277




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