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.
A shelf is initialized from a compound term InitTerm. InitTerm's arity determines the number of slots the shelf provides, and InitTerm's arguments are used to initialize the corresponding shelf slots.
% a meta-predicate to count the number of solutions to a goal:
count_solutions(Goal, Total) :-
shelf_create(count(0), Shelf),
(
call(Goal),
shelf_inc(Shelf, 1),
fail
;
shelf_get(Shelf, 1, Total)
),
shelf_abolish(Shelf).
% 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(0,0), 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).
% if-then-else with backtracking over the condition:
if(Cond, Then, Else) :-
shelf_create(sol(no), SolFlag),
(
call(Cond),
shelf_set(SolFlag, 1, yes), % remember there was a solution
call(Then)
;
shelf_get(SolFlag, 1, no), % fail if there was a solution
call(Else)
).