Introduction to Programming Languages/List Cost Model
A Cost Model for Functional Lists[edit | edit source]
List is possibly the most well-known data structure in functional programming languages. A functional list provides users with two well known operations: head and tail. The former returns the first element of the list. The latter returns a reference to the tail of the list, that is, the sublist that contains every element of the original sequence, but the first. With only these two operations, we cannot have random access, i.e., the ability to read any element in list paying the same fee in time. To illustrate this point, let's consider the implementation of the predicate below, which lets us concatenate lists:
@([], L, L).
@([H|T], L, [H|Tt]) :- @(T, L, Tt).
This predicate is formed by two clauses. The first is clearly O(1), because it is always true. The second, on the other hand, involves recursion. The predicate will be invoked recursively once, for each element on the first list. Each recursive call performs an O(1) amount of computation. Thus, this predicate is linear on the number of elements of the first list. Notice that we are talking about the number of elements, not the size of the elements. In other words, regardless of what the first list contains, the predicate should run always on the same time. As another example, let's this time analyze a naive implementation of a predicate to reverse lists:
reverse([],[]).
reverse([Head|Tail],Rev) :- reverse(Tail,TailRev), @(TailRev,[Head],Rev).
One could be tempted to believe that this predicate is linear on the size of the list that we want to invert: the first clause runs in O(1), and the second calls reverse once, recursively. However, the cost of each invocation of reverse is not O(1). Each of these calls, but the last, uses our append, which we already know to be O(N), N being the number of elements of the first list. Therefore, our current implementation of reverse is quadratic on the number of elements of the list. Given that is it so easy to invert an array in C, Python or Java in linear time, we could wonder if it is possible to achieve the same complexity on the cost model of functional lists. Indeed, that is possible, and we shall demonstrate it soon. For now, we shall push this question onto our stack.