Ruby vs Haskell: Collatz Sequence

collatz.hs

-- Collatz Sequence (http://en.wikipedia.org/wiki/Collatz_conjecture)
chain :: (Integral a) => a -> [a]
chain 0 = error "Zero will recurse forever"
chain 1 = [1]
chain x
  | even x = x:chain (x `div` 2)
  | odd x = x:chain (x*3 + 1)

collatz.rb

def chain(num)
  raise "Zero is invalid" if num == 0
  return [1] if num == 1
  results = []
  if num.even?
    results << chain(num / 2).insert(0, num)
  else
    results << chain(num*3 + 1).insert(0, num)
  end
  results.flatten
end