Issue #21701 has been updated by Eregon (Benoit Daloze). Ah, I missed that `Enumerator.produce(...).size` returns `Infinity`. I think that's the problem, it should return `nil` because it doesn't know if infinite or not, as you say in many cases it is not infinite. If that's done, I think adding an optional `size` is fine, although probably not so useful, because people can just do `def enum.size = ...` (though that creates a singleton class so not ideal). ---------------------------------------- Feature #21701: Enumerator.produce accepts an optional `size` keyword argument https://bugs.ruby-lang.org/issues/21701#change-115446 * 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 -- https://bugs.ruby-lang.org/