The
keywords in table 4
may appear in the manual's predicate descriptions and in pldoc
structured comments in source code. They describe the general behaviour
of a predicate.
det | A deterministic predicate
always succeeds exactly once and does not leave a choicepoint. |
semidet | A semi-deterministic
predicate succeeds at most once. If it succeeds, it does not leave a
choicepoint. |
nondet | A non-deterministic
predicate is the most general case and no claims are made on the number
of solutions (which may be zero, i.e., the predicate may fail)
and whether or not the predicate leaves a choicepoint on its last
solution. |
multi | As nondet, but succeeds
at least once. |
failure | Always fails. |
undefined | Well-founded semantics “third
value” . See undefined/0. |
Table 4 : Determinism indicators
“Leaving no choicepoint” means that the system knows
that redoing the predicate will not yield any additional solutions. For
example, member/2
is non-deterministic, but deterministic if the solution is the last
element of the list. The predicate member/2
may not know immediately whether there are more solutions after the
first one, so a choicepoint remains, even if it eventually turns out to
yield nothing:
?- member(1, [2,1,3]).
true ; % there may be more solutions
false. % actually not
If a solution is the last element of the list, there is enough
information to leave no choicepoint:
?- member(1, [2,3,1]).
true.
Note that if the toplevel waits for input after a query this
indicates that the query succeeded with a choicepoint. If the user
enters *
