Hierarchical Planning

Hierarchical Planning is an Artificial Intelligence (AI) problem solving approach for a certain kind of planning problems -- the kind focusing on problem decomposition, where problems are step-wise refined into smaller and smaller ones until the problem is finally solved. A solution hereby is a sequence of actions that's executable in a given initial state (and a refinement of the initial compound tasks that needed to be refined). This form of hierarchical planning is usually referred to as Hierarchical Task Network (HTN) planning, but many variants and extensions exist. This website provides some resources and links related to hierarchical planning that are hopefully useful to some.

Also in the age of impressively powerful LLMs like ChatGPT, planning is not obsolete! Quite the opposite, it might be more demanded than ever as LLMs can't plan (or only to an extremely limited extent), so on top of all the applications we had for automated planning in the first place, it has now also become relevant for LLMs. See below a short clip on the role of hierarchical planning for LLMs/AI by Yann LeCun and Lex Fridman:

Stay Connected!

There's also a mailing list with currently >90 subscribers (date: April 2024) that's dedicated specifically to hierarchical planning. It is moderated (thus SPAM-free) and very low traffic (almost zero, actually) as only content is allowed that's directly related to hierarchical planning. If you'd like to subscribe, please contact Pascal Bercher.

There's also an annual workshop on hierarchical planning (HPlan), in which planning researchers interesting in hierarchical planning come together to discuss novel ideas and developments. Even without a submission, everybody is welcome to join! (Though since it's part of the ICAPS conference, registration fees for the conference had to be paid.)

Quotes on Planning

Although not specific towards Hierarchical Planning, please find below a number of quotes related to Planning. Enjoy! :)

  1. "A goal without a plan is just a wish" (Antoine de Saint Exupéry)
    I learned about this quote in the community meeting at ICAPS 2020, organized by Olivier Buffet (INRIA) and Jörg Hoffmann (Saarland University), though of course I can't tell who contributed it. Re the author: ChatGPT 4 said the following: "The source of this quote is often not cited in detail, and its direct connection to Saint-Exupéry may be more of an attribution of wisdom in line with his known philosophies and writings rather than a quote from his specific texts."
  2. "Adventure is just bad planning" (Roald Amundsen)
    Also from the ICAPS 2020 community meeting. In case you don't know the author, this is a summary by ChatGPT 4: He was a Norwegian explorer of polar regions. He is best known for leading the first expedition to reach the South Pole in 1911. This quote reflects his meticulous approach to exploration, emphasizing the importance of thorough planning in undertaking such dangerous ventures.
  3. "I love it when a plan comes together." (John "Hannibal" Smith)
    Also from the ICAPS 2020 community meeting. John "Hannibal" Smith is a fictional character ("the Colonal") from the famous children television series The A-Team. According to Chat GPT 4, the character frequently stated this after the successful completion of team missions, reflecting his satisfaction with the execution of their often unconventional and intricate plans.
  4. "Just because you made a good plan doesn't mean that's what's gonna happen." (Taylor Swift)
    Also from the ICAPS 2020 community meeting. I can't add any further background, so you'll have to search yourself.
  5. "Plans are of little importance, but planning is essential." (Winston Churchill)
    Also from the ICAPS 2020 community meeting. Probably not much to add to the context of this quote.
  6. "Plans are worthless, but planning is everything." (Dwight D. Eisenhower)
    I just heard ot if at some point; and fits in great -- and also seems to be pretty much the same as the other; funnily, both by great politicians.
  7. "Planning is the art and practice of thinking before acting." (Patrik Haslum)
    This is a famous quote in the planning community, made by Patrik Haslum in his 2006 dissertation (on AI planning), first sentence of Chapter 1.
  8. "HTN planning is short for Harder Than Normal planning." (ANU student)
    HTN planning is actually short for Hierarchical Task Network planning, and the most famous formalism for hierarchical planning. This "alternative meaning" (by an unknown ANU undergraduate student, 2024) of the abbreviation is quite funny because the most famous "normal" (non-hierarchical) planning approach is only PSPACE-complete to decide (whether it has a solution), whereas the similar HTN framework (just by adding a hierarchy on top of it) is already undecidable.
  9. "Plan execution is the process of throwing away the file that contains the plan." (Jörg Hoffmann)
    Conveyed in person over a beer! :)