Why is this Haskell expression so slow?

Question

I\'m working on Project Euler Problem 14. Here\'s my solution.

import Data.List

collatzLength :: Int->Int
collatzLength 1 = 1
collatzLength n | odd n = 1         


        

Answer 1

For being able to find a maximum of a list, the whole list needs to be evaluated.

So it will calculate collatzLength from 1 to 1000000 and collatzLength is recursive. The worst thing is, that your definition of collatzLength is even not tail-recursive.