スタートHaskell 2 第2回目演習　コラッツ数列

collatz.hs

import Data.List
import Data.Maybe

collatz :: Int -> [Int]
collatz 1 = [1]
collatz x
	| even x = [x] ++ (collatz $ x `div` 2)
	| otherwise = [x] ++ (collatz $ 3 * x + 1)

longestLengthOfCollatz :: Int
longestLengthOfCollatz = fromJust (elemIndex (maximum lengthList) lengthList) + 1
	where
		lengthList = [length $ collatz x | x <- [1..100]]