4.1.1 The argument 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:^{47These 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 (+,-) and (-,+) are described as (?,?). The @-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]48The 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.