Ruby: All letter combinations of length n

letter_combos.rb

LETTERS = (65..90).collect{|char_code| char_code.chr}

def letter_combos length, accum = nil
  return accum if length == 0
  return accum || LETTERS if length == 1
  combos = (accum || LETTERS).product(LETTERS).inject([]) {|combos, product| 
    combos << product.first + product.last}     
  letter_combos(length - 1, combos)
end

#ex:
# letter_combos(2) => ["AA", "AB", "AC", ... "ZZ"]
# letter_combos(3) => ["AAA", "AAB", "AAC", ... "ZZZ"]

Haskell?

letter_combos :: Int -> [[Char]]
letter_combos n = (iterate combos [[]]) !! n
                  where combos xs = [l:x | l <- ['a'..'z'], x <- xs]