fib.rb

# Also see: http://timeless.judofyr.net/tailin-ruby

# Requires Ruby 1.9:

fib = lambda do |i, n = 1, result = 0|
  if i == -1
    result
  else
    i, n, result = i - 1, n + result, n
    redo
  end
end

puts fib.call 10000