4.32 Exercise 4.32

One thing you can do when cons is lazy in both arguments that you couldn’t do with the streams from earlier is do computations on the "spine" of a list – that is, ask for questions such as the length – without evaluating any of the values in that list. That means that you could have a list entirely filled with expression dividing by zero and compute the length of it anyway. This may be a contrived example, but you could also imagine a situation where the values in a list are expensive to compute, and now having a general way to handle that list without forcing any of the list values that you don’t need to use.

An obvious counterpoint is that you can already do this without lazy evaluation, as long as you manually "thunk" the list items yourself. That would allow you to control when the list items are evaluated, but naively wrapping the values in functions of no arguments would not allow you to memoize the computed values. So you’ll have to define a slightly more sophisticated representation that mutates itself to return the computed result once it has been evaluate, even though this facility could be supported directly by the language without too much trouble.