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suspend(+Goal, +Prio, +CondList, -Susp)

Suspend the Goal and wake it with priority Prio as soon as one of the conditions in CondList occurs.
Goal
A callable term.
Prio
An integer.
CondList
A term of the form Vars->Cond or trigger(Atom) or a list of such terms.
Susp
A free variable used to return the created suspension.

Description

The specified goal Goal is suspended (a suspension is created as with make_suspension/3) and attached to the trigger conditions given in CondList. When any of the trigger conditions arise in the future, the goal is going to be woken up.

The Prio argument determines the priority with which the Goal will be scheduled when woken. It can be a positive number between 1 and 12, or zero, in which case the priority defaults to the priority setting of the predicate which is called in Goal.

CondList is a list of terms (or a single term) of the form Vars->Cond or trigger(Atom). It specifies the conditions that will lead to the goal being woken up. It is enough for one of the specified conditions to arise in order for the goal to be woken.

Vars->Cond is used to specify trigger conditions on variables. Vars is an arbitrary term, and the suspension will be attached to all the variables that occur in Vars.

The condition Cond is either the name of a predefined suspension list or the specification of a suspension list in one of the variable's attributes. The predefined suspension lists are: 'inst' (for instantiation), 'bound' (instantiation, including aliasing to another variable) and 'constrained' (any constraining attribute modification). The general specification has the form

	Vars->moduleName:(suspListName of attrStruct)
which can be abbreviated (if moduleName and attrStruct are identical) to
	Vars->moduleName:suspListName
The following are examples for valid conditions:
	Vars->inst
	Vars->constrained
	Vars->fd:min
	Vars->fd:(min of fd)
A specification of the form trigger(Atom) states that the goal should be woken by a symbolic trigger, ie. by a matching invocation of the built-in schedule_suspensions/1. The name of the trigger can be an arbitrary atom.

The Susp argument returns a handle for the created suspension. This handle can, for example, be passed as an argument to the suspended goal itself, which is useful for demon goals which need to kill their own suspension under certain circumstances.

NOTE: it is possible to create a suspension that can never be woken, e.g. by specifying a Vars-term without variables.

Modes and Determinism

Modules

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

Exceptions

(6) out of range
CondList is ill-formed.

Examples

[eclipse]: suspend(writeln(hello), 2, X->inst, Susp),
        get_suspension_data(Susp, goal, Goal),
        get_suspension_data(Susp, module, Module).

X = X
Goal = writeln(hello)
Susp = 'SUSP-_321-susp'
Module = eclipse

Delayed goals:
        writeln(hello)
yes.


[eclipse]: suspend(writeln(hello), 2, X->inst, Susp),
        kill_suspension(Susp),     % killed before woken
	X=1.

Susp = 'SUSP-_308-dead'
X = 1
yes.



% A demon that wakes whenever X becomes more constrained
report(X) :-
      suspend(report(X, Susp), 1, X->constrained, Susp).

:- demon(report/2).
report(X, Susp) :-
      ( var(X) ->
          writeln(constrained(X))   % implicitly re-suspend
      ;
          writeln(instantiated(X)),
          kill_suspension(Susp)     % remove from the resolvent
      ).

See Also

demon / 1, make_suspension / 3, insert_suspension / 3, suspend / 3, attach_suspensions / 2