Feature #12210: Add IdentitySet class that compares members by identity

• Status: Closed
• Assignee: knu (Akinori MUSHA)
• Target version: -

[tjwp (Tim Perkins)]

This subclass of Set handles a use case that we ran into where we needed to track instances of objects that might compare as equal.

I was surprised that there was not a core way to do this. IdentitySet allows you to do the following (trivial example using strings):

  a_str = "a"
  s = IdentitySet.new([a_str, a_str, "b", "b"])

  p s # => #<IdentitySet: {"a", "b", "b"}>

[duerst (Martin Dürst)]

I think that rather than creating a new class, adding an option on Set.new (e.g. Set.new([a_str, a_str, "b", "b"], compare_by: :identity)) is more Ruby-like.

[shyouhei (Shyouhei Urabe)]

If you want a new class, why not start as a separate gem for it?

[tjwp (Tim Perkins)]

I submitted this as a patch because the std lib already contains one subclass of Set (SortedSet) and a commented-out implementation of another (RestrictedSet).

I don’t expect that this class will change beyond the initial implementation. Following the precedent of SortedSet, I think that including IdentitySet in the std lib improves discoverability over providing it via a gem, which seems like a more heavyweight solution for a simple, useful variation on a Set.

I was surprised that there wasn't already a standard way to do this.

[shyouhei (Shyouhei Urabe)]

• Status changed from Open to Assigned
• Assignee set to knu (Akinori MUSHA)

[knu (Akinori MUSHA)]

• Tracker changed from Misc to Feature

I'd rather add Set#compare_by_identity.

[knu (Akinori MUSHA)]

• Status changed from Assigned to Closed

Applied in changeset r56589.

---

Add Set#compare_by_identity and Set#compare_by_identity?

• lib/set.rb (Set#compare_by_identity, Set#compare_by_identity?):
  New methods. [Feature #12210]

[knu (Akinori MUSHA)]

Note that SortedSet#compare_by_identity is unimplemented when SortedSet uses rbtree.

rbtree.readjust { |a, b| a.object_id <=> b.object_id } comes pretty close, but rbtree treats string keys specially by storing a copy of each string key given, so it does not work as expected for string keys. There needs some way provided on the rbtree side.