Basic Suspension Facility

## 14.5 Basic Suspension Facility

For the more complex constraint behaviours (beyond those waiting for instantiations), we need to employ lower-level primitives of the ECLiPSe kernel (suspensions and priorities). If we want to add a new constraint to an existing solver, we also need to use the lower-level interface that the particular solver provides.
Apart from the delay clauses used above, ECLiPSe also provides a more powerful (though less declarative) way of causing a goal to delay. The following is another implementation of the constraint checking behaviour, this time using the suspend/3 built-in predicate to create a delayed goal for capacity/2:

>     capacity(T,N) :- (var(T);var(N)), !,
>             suspend(capacity(T,N), 0, [T,N]->inst).
>     capacity(1, N) :- N>=0.0, N=<350.0.
>     capacity(2, N) :- N>=0.0, N=<180.0.
>     capacity(3, N) :- N>=0.0, N=<50.0.

> suspend(Goal, Priority, Triggers) Creates Goal as a delayed goal with a given waking priority and triggering conditions. Triggers is a list of Variables->Conditions terms, specifying under which conditions the goal will be woken up. The priority specifies with which priority the goal will be scheduled after it has been triggered. A priority of 0 selects the default for the predicate. Otherwise, valid priorities range are from 1 (most urgent, reserved for debugging purposes) to 12 (least urgent). Some valid triggers: X->inst wake when the variable becomes instantiated (most specific) X->constrained wake when the variable becomes constrained somehow (most general) X->ic:min wake when the lower bound of an ic-variable changes X->ic:max wake when the upper bound of an ic-variable changes X->ic:hole wake an internal domain value gets removed

> Figure 14.3: The Basic Suspension Facilities