[Haskell-beginners] fibonacci and concurrent recursion

Hi I have been looking at this fibonacci implementation: f = 0:1:(zipWith (+) f (tail f)) and I have been trying to fit it inside my head. I am not quite there yet, though. I have been tracing the steps manually like this 0:1 [0,1] [1] 0:1:1 [0,1,1] [1,1] 0:1:1:2 [0,1,1,2] [1,1,2] 0:1:1:2:3 [0,1,1,2,3] [1,1,2,3] 0:1:1:2:3:5 And it is easy to see that it works, but what I do not understand is how the two recursive applications progress. How do they evolve concurrently? For each execution of `f`, `f` will be executed twice. So how does all these results end up in the same list? I guess a more general way of asking my question is how does concurrent recursion on the same result-set work? -- Christoffer Buchholz