Project euler utility functinos

project-euler-utils.hs

-- Convert number to list of digits
tods n = if n < 10 then [n] else reverse $ unfoldr (\i -> if i == 0 then Nothing else Just (i `mod` 10, i `div` 10)) n

-- Convert list of digits to number
ton = foldl' (\a b -> a * 10 + b) 0

-- Make list of numbers from digits
numbers ns = concat $ iterate (\xs -> concatMap (\n -> map (\xs' -> n : xs') xs) ns) (map (\c -> [c]) ns)

-- Primality test
isPrime n = 2 <= n && all (\k -> n `mod` k /= 0) [2..ceiling (sqrt (fromIntegral n))]