  We investigate the problem of cost-optimal planning in ASP. Current ASP
planners can be trivially extended to a cost-optimal one by adding weak
constraints, but only for a given makespan (number of steps). It is desirable
to have a planner that guarantees global optimality. In this paper, we present
two approaches to addressing this problem. First, we show how to engineer a
cost-optimal planner composed of two ASP programs running in parallel. Using
lessons learned from this, we then develop an entirely new approach to
cost-optimal planning, stepless planning, which is completely free of makespan.
Experiments to compare the two approaches with the only known cost-optimal
planner in SAT reveal good potentials for stepless planning in ASP. The paper
is under consideration for acceptance in TPLP.
