We have tried to keep the predicate descriptions clear and concise. First, the predicate name is printed in bold face, followed by the arguments in italics. Arguments are preceded by a mode indicator.
An argument mode
indicator gives information about the intended direction in which
information carried by a predicate argument is supposed to flow. Mode
indicators (and types) are not a formal part of the Prolog language but
help in explaining intended semantics to the programmer. There is no
complete agreement on argument mode indicators in the Prolog community.
We use the following definitions:48These
definitions are taken from the PlDoc markup language
description. PldDoc markup is used for source code markup (as
well as for the commenting tool). The current manual has only one mode
declaration per predicate and therefore predicates with mode (+--+??@-mode is often replaced by
chr+.
| ++ | At call time, the argument must be ground, i.e., the argument may not contain any variables that are still unbound. |
| + | At call time, the argument must be instantiated to a
term satisfying some (informal) type specification. The argument need
not necessarily be ground. For example, the term [_] is a
list, although its only member is the anonymous variable, which is
always unbound (and thus nonground). |
| - | Argument is an output argument. It may or may
not be bound at call-time. If the argument is bound at call time, the
goal behaves as if the argument were unbound, and then unified with that
term after the goal succeeds. This is what is called being steadfast:
instantiation of output arguments at call-time does not change the
semantics of the predicate, although optimizations may be performed. For
example, the goal findall(X, Goal, [T]) is good style and
equivalent to findall(X, Goal, Xs), Xs = [T]49The
ISO standard dictates that findall(X, Goal, 1) raise a type_error
exception, breaking steadfastness. SWI-Prolog does not follow the
standard here. Note that any determinism
specification, e.g., det, only applies if the argument is
unbound. For the case where the argument is bound or involved in
constraints, det effectively becomes
semidet, and multi effectively becomes
nondet. |
| -- | At call time, the argument must be unbound. This is typically used by predicates that create‘something' and return a handle to the created object, such as open/3, which creates a stream. |
| ? | At call time, the argument must be bound to a partial
term (a term which may or may not be ground) satisfying some
(informal) type specification. Note that an unbound variable is
a partial term. Think of the argument as either providing input or
accepting output or being used for both input and output. For example,
in stream_property(S, reposition(Bool)), the
reposition part of the term provides input and the
unbound-at-call-time Bool variable accepts output. |
| : | Argument is a meta-argument, for example a
term that can be called as goal. The predicate is thus a meta-predicate.
This flag implies |
| @ | Argument will not be further instantiated than it is at call-time. Typically used for type tests. |
| ! | Argument contains a mutable structure that may be modified using setarg/3 or nb_setarg/3. |
See also section 4.8 for examples of meta-predicates, and section 6.5 for mode flags to label meta-predicate arguments in module export declarations.
Referring to a predicate in
running text is done using a
predicate indicator. The canonical and most generic form of a
predicate indicator is a term [<module>:]<name>/<arity>.
The module is generally omitted if it is irrelevant (case of a built-in
predicate) or if it can be inferred from context.
Compliant to the ISO
standard draft on Definite Clause Grammars (see
section 4.13), SWI-Prolog
also allows for the non-terminal indicator to refer to a DCG
grammar rule. The non-terminal indicator is written as [<module>]:<name>//<arity>.
A non-terminal indicator <name>//<arity>
is understood to be equivalent to <name>/<arity>+2,
regardless of whether or not the referenced predicate is defined or can
be used as a grammar rule.50This,
however, makes a specific assumption about the implementation of DCG
rules, namely that DCG rules are preprocessed into standard Prolog rules
taking two additional arguments, the input list and the output list, in
accumulator style. This need not be true in all
implementations. The //
To describe the general behaviour of a predicate, the following vocabulary is employed. In source code, structured comments contain the corresponding keywords:
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 an choicepoint on the last solution. |
multi | As nondet, but succeeds
at least once. |
undefined | Well founded semantics third value. See undefined/0. |