What Happened to Classical Planning?

Question

Classical planning is the problem of reaching a goal state from an initial state, given actions with preconditions and effects. It dates back to 1971, with STRIPS. Nowadays problems are written in PDDL ( https://planning.wiki/ ), and there's a competition for solvers. Many real-world problems from logistics to robotics can be modeled in PDDL.. it's like SMT but for plans instead of propositions.But PDDL is low-level and cumbersome, the competition problems are small and stale, the solvers haven't gotten much better in a decade and nobody seems to use it in industry. The whole field is stuck in AI winter. Why?

triska · Accepted Answer

Many features available in dedicated planning systems are now subsumed by modern Prolog systems and their constraint solvers. For example, SICStus Prolog now provides sophisticated constraints for resource planning such as cumulative/[1,2] and related global constraints:
https://sicstus.sics.se/sicstus/docs/4.3.2/html/sicstus/Sche...
The efficiency and expressiveness of global constraints in SICStus Prolog and also other Prolog systems have improved tremendously over the past few decades, and my impression is that these Prolog systems suffice to solve many planning, resource allocation and scheduling tasks that arise in practice, with the added advantage of tight integration in a full-fledged high-level programming language.
The documentation of SICStus Prolog used to include a statement that a third of all airline tickets worldwide are processed with this system.