default_param_hack.rb

def output name=((default=true); "caius")
  puts "name: #{name.inspect}"
  puts "default: #{default.inspect}"
end

output
# >> name: "caius"
# >> default: true

output "avdi"
# >> name: "avdi"
# >> default: nil

also

def x(y=(raise 'i <3 params')); y; end

Don't forget:

def foo(something = (def foo(*); 'bar'; end)) 'qux' end

puts foo(10)  # qux
puts foo      # qux
puts foo(10)  # bar
puts foo      # bar