However, asserting new clauses is rather slow in some Prolog systems. So instead, we can pass the newly-generated results around in a list:
fib( N, F ) :- fib( N, F, [ done(0,1), done(1,1) ], _ ). fib( N, F, L, L ) :- member( done(N,F), L ). fib( N, F, L, [ done(N,F) | L2 ] ) :- N1 is N - 1, N2 is N - 2, fib( N1, F1, L, L1 ), fib( N2, F2, L1, L2 ), F is F1 + F2.
fib(N,F,L,L1) as the following relation between ,
, , and
This technique is often used in processing graphs, where we need to record the points we have already processed, in order to avoid processing them again.
There are more efficient ways to represent sets than as unordered lists. Other possibilities: ordered lists, binary trees. See books on data structures in the computing science literature.