http://en.wikipedia.org/wiki/Combination#Example_of_counting_combinations

combination.rb

# http://en.wikipedia.org/wiki/Combination#Example_of_counting_combinations

module Combination
  def self.size n, k
    size_r(n, k).to_i
  end

  def self.size_r n, k
    return 1 if k.zero?
    Rational(n - (k - 1), k) * size_r(n, k - 1)
  end
end

puts Combination.size(52, 5)

Main.hs

-- http://en.wikipedia.org/wiki/Combination#Example_of_counting_combinations

module Main where

import Data.Ratio ((%), numerator)

combinationSize :: (Integral a) => a -> a -> a
combinationSize n k = numerator $ cSize n k 
    where cSize _ 0 = 1
          cSize n' k' = (n' - (k' - 1)) % k' * cSize n' (k' - 1)

main :: IO ()
main = print $ combinationSize 52 5