test2015.hs

import Data.Char
import Test.QuickCheck

-- count str = length [c | c <- str, isUpper c || isDigit c]
count = length . filter c
  where c x = isUpper x || isDigit x

countRec [] = 0
countRec (c:str) = (if isUpper c || isDigit c then 1 else 0) + countRec str

prop_count :: String -> Bool
prop_count str = count str == countRec str

isNext :: Int -> Int -> Bool
isNext n1 n2 | even n1   = div n1 2 == n2
             | otherwise = (n1 * 3 + 1) == n2

--collatz xs = all(==True) [isNext x (xs!!i) | (x,i) <- zip xs [1..], i < length xs]

collatz :: [Int] -> Bool
collatz xs = and (zipWith isNext xs (tail xs))

collatzRec :: [Int] -> Bool
collatzRec [] = True
collectRec [x] = True
collatzRec (y:x:xs) = isNext x y && collatzRec (y : xs)