Project Euler Problem 3 Solution

pop_goes_the_weasel

weasel> let x = Debug.Trace.trace "hi" True
weasel> x
hi
True
weasel> x
True
weasel> 

problem3.hs

-- factors :: (Integral a) => a -> [a]
-- factors x = l_factors x x []
import Debug.Trace

isMultiple :: (Integral a) => a -> a -> Bool
isMultiple divisor number
       | divisor > number  = False
       | otherwise         = (number `mod` divisor) == 0

factors :: (Integral a) => a -> [a]
factors x = l_factors x x 2 []

l_factors :: (Integral a) => a-> a -> a -> [a] -> [a]
l_factors _ _ 1 list = list
l_factors orig divisee current_divisor list
--     | debug = undefined
     | current_divisor > divisee = list
     | current_divisor `isMultiple` divisee = (l_factors orig (divisee `div` current_divisor) (current_divisor+1) (current_divisor:list))
     | otherwise                            = (l_factors orig (divisee) (current_divisor+1) (list        ))
     where debug = (trace ("divisee: "++show divisee++" current_divisor: "++show current_divisor++" list: "++show list) False)