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Ruby 2.1

RubyArchived

Named arguments with defaults

# length is required
def pad(num, length:, char: "0")
  num.to_s.rjust(length, char)
end

pad(42, length: 6) #=> "000042"
pad(42) #=> #<ArgumentError: missing keyword: length>

Module.prepend

prepend(
  Module.new do
    define_method ...
  end
)

References