From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@...>
Date: 2025-12-03T20:59:51+00:00
Subject: [ruby-core:124013] [Ruby Feature#21701] Enumerator.produce accepts an optional `size` keyword argument

Issue #21701 has been updated by Eregon (Benoit Daloze).


I think having to provide a size for those cases to not raise is too inconvenient.
I agree with what has been said in #21654: we should not check raise for infinite loops, unless we are absolutely certains they are infinite.
A `size` of `nil` means unknown, it does not mean infinite, so it's always incorrect to raise if the size is unknown.

----------------------------------------
Feature #21701: Enumerator.produce accepts an optional `size` keyword argument
https://bugs.ruby-lang.org/issues/21701#change-115444

* Author: knu (Akinori MUSHA)
* Status: Open
* Assignee: knu (Akinori MUSHA)
* Target version: 4.0
----------------------------------------
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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