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# suspend(+Goal, +Prio, +CondList)

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.

## 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. (Note that this is the scheduling priority which determines the order of execution; the priority under which the woken goal is executed may be higher, and is determined by the run_priority predicate property, see set_flag/3).

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. These trigger conditions correspond to suspension lists inside of variables's attributes, and can be written in a variety of forms:

            Vars->SUSPLISTNAME                e.g. X->inst
            Vars->ATTRNAME:SUSPLISTNAME       e.g. X->fd:min
            Vars->ATTRNAME:ARGINDEX           e.g. X->fd:(min of fd)
            Vars->ATTRNAME:ARGLIST            e.g. X->fd:[min,max]
            Vars->LISTOFCONDS                 e.g. X->[fd:min,inst]

There are 3 predefined suspension list names: 'inst' (for instantiation), 'bound' (instantiation, including aliasing to another variable) and 'constrained' (any constraining attribute modification). These all belong to an attribute called 'suspend'. Others are defined by libaries, and usually the attribute name is identical to the library name (e.g. 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.

A variant suspend/4 of this predicate is available. It returns a handle for the created suspension, which is useful for suspending demons.

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

- suspend(+, +, +) is det

### Modules

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

### Exceptions

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

## Examples

    [eclipse 1]: suspend(writeln(hello), 2, X->inst).

    X = X

    Delayed goals:
            writeln(hello)
    yes.
    [eclipse 2]: suspend(writeln(hello), 2, X->inst),
            writeln(one),
            X=1,            % causes waking
            writeln(two).
    one
    hello
    two

    X = 1
    yes.
    [eclipse 3]: suspend(writeln(X), 2, [X,Y]->bound), X=Y.
    X

    X = X
    Y = X
    yes.
    [eclipse 4]: suspend(writeln(world), 2, trigger(hello)), trigger(hello).
    world

    yes.

## See Also

suspend / 4, make_suspension / 3, insert_suspension / 3, attach_suspensions / 2, schedule_suspensions / 1, meta_attribute / 2, set_flag / 3