[ library(eplex) | The ECLiPSe Libraries | Reference Manual | Alphabetic Index ]
lp_demon_setup(+Objective, ?Cost, ++ListOfOptions, ++TriggerModes, -Handle)
Setup the external solver as a simplex demon.
- Objective function: min(CostExpr) or max(CostExpr)
- Variable bounded by the optimal solution
- List of solver options
- List of conditions for re-triggering solver
- handle to solver state
Setup the external solver as a simplex demon. A simplex demon collects
linear constraints and re-solves the problem whenever the triggering
conditions in TriggerModes are met.
Declaratively, this can be seen as a compound constraint representing all
the individual linear constraints that have been set so far and are going
to be set up later. Operationally, the delayed constraints are collected
and an external solver is set up (as with lp_setup/4). Then the problem
is solved once initially (if initial_solve option is yes) and a
delayed goal lp_demon is set up which will re-trigger the solver when
certain conditions are met.
CostExpr is a linear cost expression (or quadratic, if supported
by the external solver).
Handle refers to the created solver state (as in lp_setup/4 or
lp_read/3 described below). It can be used to access and modify the state
of the solver, retrieve solution information etc.
Unlike with lp_solve/2, Cost will not be instantiated to a
solution's cost, but only be bounded by the best-bound on cost:
For a minimisation problem, each solution's best-bound becomes a lower bound,
for maximisation an upper bound on Cost. This technique allows for
repeated re-solving with reduced bounds or added constraints. Note that
Cost is not automatically made a problem variable (it can be a problem
variable if there are constraints that involve it), and thus may not have
bounds associated with in. In order for the bounds information not to be
lost, some bounds should be given to Cost (e.g. making it a
problem variable (but this might introduce unnecessary self-waking on
bounds change), or via another solver with bounds (e.g. ic)).
ListOfOptions is a list of solver options as described for
lp_setup/4. In addition, the following extra options are also available:
- Specifies if this solver state should be associated with an eplex
instance. If Pool is none, then the solver is not associated
with an eplex instance. If Pool is pool(Instance), where
Instance is the (atomic) name of an existing eplex instance, then this
eplex instance would be associated with the solver state, i.e. when the
solver is invoked, it will collect constraints posted to
Instance. Note that Instance must not be associated
with any other solver state already.
The default value for Pool is pool(eplex) (for backward
- Specifies if an initial solve (call to the external solver) should
be performed immediately after problem setup.
YesNo is one of the atoms yes or no, the default is
yes if any trigger condition is specified in TriggerModes,
no if no trigger condition is specified.
TriggerModes specifies under which conditions the solver demon
will be re-triggered. It can be a list of the following specifiers
The tolerances mentioned can be specified in lp_setup/4 or lp_set/3
- re-trigger if a problem variable gets instantiated.
- re-trigger when the suspension list given by ModuleName:Index is woken
for any of the problem variables.
The format for ModuleName:Index is the same as for specifying
the suspension list in suspend/3,4.
- re-trigger if a problem variable gets instantiated
to a value that differs from its lp-solution more than a tolerance.
- re-trigger each time a variable bound for the solver instance changes.
- re-trigger each time a variable's solver instance bound changes
such that its lp-solution gets excluded more than a tolerance.
- re-trigger each time a new (arithmetic or integral) constraint is
added to the solver instance. Note that adding integral constraint
on new problem variables, or adding bounds constraint, or adding
constraints to the cutpool will *not* re-trigger.
- re-trigger each time the symbolic trigger Atom is pulled by invoking
- an additional condition to be used together with other triggers. When
the demon is triggered, it first executes PreGoal. Only if
that succeeds, does the appropriate external solver get invoked.
This provides a way of reducing the number of (possibly expensive)
solver invocations when given preconditions are not met.
- this is not a trigger condition, but specifies a goal to be executed
after solver success, but before the Cost variable gets
constrained. It is intended as a hook for exporting solution
information, e.g. copying solutions from the solver state into
variable attributes (eg. tentative value), or computing weights for
labelling heuristics from the solver state.
- this is not a trigger condition, but instead is used to access the
demon used to trigger the solver. Susp is instantiated to
the suspension that triggers the solver: by waking Susp, the solver
is triggered. Susp is a demon in that it stays around after being
woken. Accessing Susp allows the user to specify orbitally conditions
for triggering the solver.
If several trigger conditions are specified, then any of them will trigger
When a solver demon runs frequently on relatively small problems,
it can be important for efficiency to switch off the presolve option
to reduce overheads.
The solver demon calls lp_solve/2 when it wakes up. See the description
of lp_solve/2 for the handling of exceptions.
Note: Some external solvers need to write temporary files when they
are solving a problem. These are written to the temporary directory
specified in ECLiPSe's tmp_dir setting (get_flag/2, set_flag/2).
Some common invocations patterns for this predicate are the following.
The first triggers the solver only on instantiation of variables to
values that don't fit with the simplex solution:
lp_demon_setup(min(Expr), C, , [deviating_inst], H)
lp_solve / 2, lp_set / 3, lp_setup / 4, solution_out_of_range / 1, schedule_suspensions / 1, library(constraint_pools)