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shelf_dec(+ShelfHandle, +Index)

Decrement an integer slot within a shelf object but fail if is zero

ShelfHandle: A shelf handle or shelf name
Index: An integer

Description

This looks up the entry in the Index'th slot of the shelf object denoted by ShelfHandle, and if such an entry exists, and is of integer type, it is decremented by one. If the old value is already zero (or less), the predicate fails and the shelf remains unchanged. This predicate is a shorthand for:

	shelf_dec(ShelfHandle, Index) :-
	    shelf_get(ShelfHandle, Index, C0),
	    C0 > 0,
	    C1 is C0 - 1,
	    shelf_set(ShelfHandle, Index, C1).

The slots are numbered from 1 to the maximum which was determined during shelf creation (but note that ECLiPSe's struct-syntax can be used to give the slots symbolic names, see struct/1).

Note: If ShelfHandle is not a handle, then it must be an atom or a compound term, and the shelf is identified by this term's toplevel functor together with the context module.

Modes and Determinism

shelf_dec(+, +) is semidet

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Fail Conditions

Fails if decrementing the value would result in a negative number

Exceptions

(4) instantiation fault: ShelfHandle is not instantiated
(5) type error: Index is not instantiated
(5) type error: ShelfHandle is not a shelf
(5) type error: Index is not an integer
(6) out of range: Index is less than 1 or greater than the number of slots on the shelf
(6) out of range: The old counter value exceeds an implementation-defined limit (at least 2^31)
(40) stale object handle: ShelfHandle refers to an already destroyed shelf

Examples

    :- local shelf(backtrack_limit, count(100)).

    nat(0).
    nat(N) :-
	shelf_dec(backtrack_limit, 1),   % fail when limit reached
	nat(N0),
    	N is N0+1.

    ?- shelf_set(backtrack_limit, 1, 5),
       findall(X, nat(X), L).

    X = X
    L = [0, 1, 2, 3, 4, 5]
    Yes (0.00s cpu)