infinite lists, lazy evaluation and length

Question

Haskell noob here: I\'m still trying to understand the mechanics of the language, so if my question is plain stupid, forgive me and point me to some link which I can learn from

Answer 1

is length actually iterating the whole list to give a response?

Yes, absolutely.

length is to be avoided every time I try to implement something working well with the lazy evaluation

Yes, absolutely.

so there is some alternative?

Yes: solve the problem without referencing length. There are no general methods of problem solving so you need to work out each specific case.

how can I improve my example to work with infinite lists

You are a railroad worker. A huge train if cars begins where you are standing and stretches over the horizon. You have no idea where it ends, if ever. Your job is to separate it into small trains of three cars each. How do you proceed?