Issue #5694 has been updated by akr (Akira Tanaka). Assignee changed from matz (Yukihiro Matsumoto) to nobu (Nobuyoshi Nakada) We discussed this issue at a developer meeting. We have consensus that lambda {...}.arity should behave as Method#arity. Note that we should not change proc {...}.arity. ---------------------------------------- Bug #5694: Proc#arity doesn't take optional arguments into account. https://bugs.ruby-lang.org/issues/5694#change-28044 Author: marcandre (Marc-Andre Lafortune) Status: Assigned Priority: Normal Assignee: nobu (Nobuyoshi Nakada) Category: core Target version: ruby -v: - Currently: ->(foo = 42){}.arity # => 0, should be -1 This is contrary to the documentation and to what we should expect from the equivalent method definition. Fixed in trunk, requesting backport for the 1.9 line. -- http://bugs.ruby-lang.org/