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Predicate succ/2
Availability:built-in
succ(?Int1, ?Int2)
True if Int2 = Int1 + 1 and Int1 ≥. At least one of the arguments must be instantiated to a natural number. This predicate raises the domain error not_less_than_zero if called with a negative integer. E.g. succ(X, 0) fails silently and succ(X, -1) raises a domain error.123The behaviour to deal with natural numbers only was defined by Richard O'Keefe to support the common count-down-to-zero in a natural way. Up to 5.1.8, succ/2 also accepted negative integers.
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LogicalCaptain said (2020-05-29T16:45:47):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

In fact, succ/2 is made to order for natural numbers.

  • What is the predecessor of 0: succ(X,0) ... fails, "there is nothing like that"
  • What is the predecessor of -1: succ(X,-1) ... throws: "What the hell is this -1?"
  • What is the successor of -1: succ(-1,X) ... throws: "What the hell is this -1?"

There should be an

zsucc(?A,?B)

working on ℤ, not only on ℕ.

zsucc(X,Y) :-
  (nonvar(X) -> (integer(X) -> true ; type_error(integer,X)) ; true),
  (nonvar(Y) -> (integer(Y) -> true ; type_error(integer,Y)) ; true),
  ((var(X),var(Y)) -> instantiation_error("Need at least one value instantiated") ; true),
  (nonvar(X)
     -> Y is X+1
     ;  X is Y-1).