Lazy Trees
So far, we've seen that the "laziness" of lazy sequences is that they can point to future computations that will only be performed if they become necessary. There is another important advantage that is equally important, and that is what we are going to explore now. Remember from Chapter 6, Recursion and Looping, how recursive functions in Clojure need to use recur to avoid blowing up the stack? And remember how recur only works with a specific kind of recursion, tail recursion, where the next call to the recursive function can totally replace the previous call? The problem, you'll recall, is that only a limited number of stack frames are available. The function call on the root node of the tree needs to wait until all the calls have completed on all the child and grandchild and great-grandchild nodes, and so on. Stack frames are a limited resource but the data we need to operate on is often vast. This mismatch is a problem.
This is where lazy evaluation...