I randomly yesterday started thinking about the unfoldr function in Haskell while working out at the gym (how nerdy is that, I am lifting iron but thinking of functional programming). Unfoldr take a single and an unfolding function and turns it into a list (the opposite of fold).  At the gym I was thinking about an application where I can use this and I decided that when I got home I would use it to write a prime factorization function.  This is a method that when given a number returns the list of its prime factors.

It was easy to write the only part I am not pleased about is the code I used to deal with tuples.  It seems clumsy and I am still looking for a way to clean that up.

One note: The code below references a list of prime numbers called primes , which is not shown.

Here is the code:

primeFactors x = unfoldr findFactor x
                where
                  first (a,b,c) = a
                  findFactor 1 = Nothing
                  findFactor b = (\(_,d,p)-> Just (p, d))
                                 $ head $ filter ((==0).first)
                                 $  map (\p -> (b `mod` p, b `div` p, p))  primes

This function will take any number which is greater than 1 and return a list of its prime factors.  But don’t take my word for it, I wrote a quickcheck property to ensure the prime factors multiply back to the original number:

prop_factors num =  num > 1 ==> num == (foldr1 (*) $ primeFactors num)

When running quickcheck on this property you see the following:

quickCheck prop_factors
OK, passed 100 tests.