gistfile1.rb

meta = Class.new
meta.class_eval {define_method(:foo, Proc.new {||})}
meta.class_eval {define_method(:bar, Proc.new {|x|})}
meta.class_eval {define_method(:baz, Proc.new {|x, y|})}

p meta.new.method(:foo).arity == 0
p meta.new.method(:bar).arity == 1
p meta.new.method(:baz).arity == 2

p proc{|*|}.arity == -1