Issue #19971 has been updated by shan (Shannon Skipper). Docs say it's intended, since #arity "returns -n-1, where n is the number of mandatory arguments, with the exception for blocks that are not lambdas and have only a finite number of optional arguments; in this latter case, returns n." A Proc in this case isn't a lambda, so it returns `n` rather than `-n-1`. Procs have loose arity, like blocks, unlike lambdas. ---------------------------------------- Misc #19971: Confusing arity of a Proc with implicit rest parameter https://bugs.ruby-lang.org/issues/19971#change-105062 * Author: andrykonchin (Andrew Konchin) * Status: Open * Priority: Normal ---------------------------------------- I've noticed that such proc `proc { |a,| }` has arity `1` that means 1 required parameter, but behaves like there is a rest parameter (`proc { |a, *| }`) that has arity `-2` (that means 1 required parameter + rest parameter). So I am wondering whether it's intentional behaviour and what the rational is behind it. -- https://bugs.ruby-lang.org/