An example for Nathanael7 on #ruby IRC

sig.rb

def sig(method_name)
  meth = method(method_name)
  parameters = meth.parameters.map do |type, name|
    case type
    when :req
      name
    when :rest
      "*#{name}"
    when :opt
      # Unknown default argument.
      "#{name} = nil"
    when :key
      # Unknown default argument.
      "#{name}: nil"
    when :keyreq
      "#{name}:"
    when :keyrest
      "**#{name}"
    when :block
      "&#{name}"
    end
  end

  "def #{meth.name}(#{parameters.join(', ')})"
end

def foo(a, b = :oops, *c, d: :oops, e:, **f, &g) end

sig :foo
#=> "def foo(a, b = nil, *c, e:, d: nil, **f, &g)"