shelf_create(++ShelfSpec, ?SlotInit, -ShelfHandle)

ShelfSpec

A term of the form Name/Arity

SlotInit

The value used to initialize the slots (any term)

ShelfHandle

A free variable

Description

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.

Modes and Determinism

• shelf_create(++, ?, -) is det

Exceptions

(4) instantiation fault

ShelfSpec is not fully instantiated

(5) type error

ShelfSpec is fully instantiated but not to a term of the form Atom/Integer

(5) type error

ShelfHandle is not a variable

Examples

% 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).
    

See Also