A shelf is an object with multiple slots whose contents survive backtracking. The content of each slot can be set and retrieved individually, or the whole shelf can be retrieved as a term.
Shelves are referred to by handle, not by name, so they make it easy to write robust, reentrant code. A shelf disappears when the system backtracks over its creation, when the shelf handle gets garbage collected, or when it is explicitly destroyed.
When using shelf_create/3, ShelfSpec determines the number of slots on the shelf, and all slots get initialized identically with the value SlotInit.
% finding the sum and maximum of data/1 facts: data(2). data(9). data(3). data(5). data(7). sum_and_max(Sum, Max) :- shelf_create(agg/2, 0, Shelf), ( data(X), shelf_get(Shelf, 1, OldMax), ( X > OldMax -> shelf_set(Shelf, 1, X) ; true ), shelf_get(Shelf, 2, OldSum), NewSum is OldSum + X, shelf_set(Shelf, 2, NewSum), fail ; shelf_get(Shelf, 0, agg(Max, Sum)) ), shelf_abolish(Shelf).