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:
1: primeFactors x = unfoldr findFactor x
3: first (a,b,c) = a
4: findFactor 1 = Nothing
5: findFactor b = (\(_,d,p)-> Just (p, d))
6: $ head $ filter ((==0).first)
7: $ 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:
1: prop_factors num = num > 1 ==> num == (foldr1 (*) $ primeFactors num)
When running quickcheck on this property you see the following:
OK, passed 100 tests.