Planner (programming language)
|Designed by||Carl Hewitt|
Planner (often seen in publications as "PLANNER" although it is not an acronym) is a programming language designed by Carl Hewitt at MIT, and first published in 1969. First, subsets such as Micro-Planner and Pico-Planner were implemented, and then essentially the whole language was implemented in Popler. Derivations such as QA4, Conniver, QLISP and Ether (see Scientific Community Metaphor) were important tools in Artificial Intelligence research in the 1970s, which influenced commercial developments such as KEE and ART.
Procedural approach versus logical approach
The two major paradigms for constructing semantic software systems were procedural and logical. The procedural paradigm was epitomized by Lisp [McCarthy et al. 1962] which featured recursive procedures that operated on list structures.
The logical paradigm was epitomized by uniform proof procedure resolution theorem provers [Robinson 1965]. According to the logical paradigm it was “cheating” to incorporate procedural knowledge [Green 1969].
Procedural embedding of knowledge
Planner was invented for the purposes of the procedural embedding of knowledge [Hewitt 1971] and was a rejection of the resolution uniform proof procedure paradigm [Robinson 1965], which
- Converted everything to clausal form. Converting all information to clausal form is problematic because it hides the underlying structure of the information.
- Then used resolution to attempt to obtain a proof by contradiction by adding the clausal form of the negation of the theorem to be proved. Using only resolution as the rule of inference is problematical because it hides the underlying structure of proofs. Also, using proof by contradiction is problematical because the axiomatizations of all practical domains of knowledge are inconsistent in practice.
Planner was a kind of hybrid between the procedural and logical paradigms because it combined programmability with logical reasoning. Planner featured a procedural interpretation of logical sentences where an implication of the form (P implies Q) can be procedurally interpreted in the following ways using pattern-directed invocation:
- Forward chaining (antecedently):
- If assert P, assert Q
- If assert not Q, assert not P
- Backward chaining (consequently)
- If goal Q, goal P
- If goal not P, goal not Q
A subset called Micro-Planner was implemented by Gerry Sussman, Eugene Charniak and Terry Winograd [Sussman, Charniak, and Winograd 1971] and was used in Winograd's natural-language understanding program SHRDLU, Eugene Charniak's story understanding work, Thorne McCarty's work on legal reasoning, and some other projects. This generated a great deal of excitement in the field of AI. It also generated controversy because it proposed an alternative to the logic approach that had been one of the mainstay paradigms for AI.
At SRI International, Jeff Rulifson, Jan Derksen, and Richard Waldinger developed QA4 which built on the constructs in Planner and introduced a context mechanism to provide modularity for expressions in the database. Earl Sacerdoti and Rene Reboh developed QLISP, an extension of QA4 embedded in INTERLISP, providing Planner-like reasoning embedded in a procedural language and developed in its rich programming environment. QLISP was used by Richard Waldinger and Karl Levitt for program verification, by Earl Sacerdoti for planning and execution monitoring, by Jean-Claude Latombe for computer-aided design, by Richard Fikes for deductive retrieval, and by Steven Coles for an early expert system that guided use of an econometric model.
Computers were expensive. They had only a single slow processor and their memories were very small by comparison with today. So Planner adopted some efficiency expedients including the following:
- Backtracking [Golomb and Baumert 1965] was adopted to economize on the use of time and storage by working on and storing only one possibility at a time in exploring alternatives.
- A unique name assumption was adopted to save space and time by assuming that different names referred to different objects. For example names like Peking and Beijing were assumed to refer to different objects.
- A closed world assumption could be implemented by conditionally testing whether an attempt to prove a goal exhaustively failed. Later this capability was given the misleading name "negation as failure" because for a goal G it was possible to say: "if attempting to achieve G exhaustively fails then assert (Not G)."
Control structure controversy
As related in Hewitt , computer memories were very small by current standards because they were expensive, being made of iron ferrite cores at that time. So Planner adopted the then common expedient of using backtracking control structures to economize on the use of computer memory. In this way, the computer only had to store one possibility at a time in exploring alternatives.
One implementation decision in Micro Planner had unfortunate consequences. Lisp had adopted the programming pun of identifying NIL, the empty list with logical false (at memory location 0) because testing for 0 was faster than anything else. Because of the pun, testing for NIL was extremely common in Lisp programs. Micro Planner extended this pun also to use NIL as a signal to begin backtracking. In Micro Planner, it was common to write programs to perform some operation on every element of a list by using a loop to process the first element of a list, take the rest of the list, and then jump back to the top of the loop to test if the list was empty. If the list tested empty, then the program would go on to do other things. Such a program never made it to testing the empty list after processing all the elements because when the last element was processed and the rest of the list was taken, NIL was returned as a value. The Micro Planner interpreter took this as the signal to begin backtracking and began undoing all the work of processing the elements of the list! People were dumbfounded [Fahlman 1973].
In this and several other ways, backtracking proved unwieldy, helping to fuel the great control structure debate. Hewitt investigated some alternatives in his thesis.
Hairy control structure
According to Hewitt , Peter Landin had introduced an even more powerful control structure using his "J" (for Jump) operator that could perform a non-local goto into the middle of a procedure invocation. In fact the "J" operator could jump back into the middle of a procedure invocation even after it had already returned. Drew McDermott and Gerald Sussman called Landin's concept the "Hairy Control Structure" and used it in the form of a nonlocal goto for the Conniver programming language. Scott Fahlman used Conniver in his planning system for robot construction tasks. This is related to what are now called re-invocable continuations.
Difficulties in communication were a root cause of the control structure difficulties.
Control structures are patterns of passing messages
Hewitt reported: "... we have found that we can do without the paraphernalia of "hairy control structure" (such as possibility lists, non-local gotos, and assignments of values to the internal variables of other procedures in CONNIVER.)... The conventions of ordinary message-passing seem to provide a better structured, more intuitive foundation for constructing the communication systems needed for expert problem-solving modules to cooperate effectively."
The Actor model provided the foundation for solving the Artificial Intelligence control structure problem. It took considerable time to develop programming methodologies for the Actor model. Indeed, the implementation of the scientific community metaphor requires sophisticated message passing that is still the subject of research.
The genesis of Prolog
Gerry Sussman, Winograd and (Charniak 1971), Seymour Papert (Minsky and Papert 1971), and Terry Winograd (Winograd 1971) visited Edinburgh spreading the news about Micro-Planner and SHRDLU casting doubt on the resolution uniform proof procedure approach that had been the mainstay of the Edinburgh Logicists. At the University of Edinburgh, Bruce Anderson implemented a subset of Micro-Planner called PICO-PLANNER (Anderson 1972) and Julian Davies (1973) implemented essentially all of Planner.
According to Donald MacKenzie, Pat Hayes recalled the impact of a visit from Papert to Edinburgh, which had become the "heart of artificial intelligence's Logicland," according to Papert's MIT colleague, Carl Hewitt. Papert eloquently voiced his critique of the resolution approach dominant at Edinburgh "...and at least one person upped sticks and left because of Papert." [MacKenzie 2001 pg 82.]
The above developments generated tension among the Logicists at Edinburgh. These tensions were exacerbated when the UK Science Research Council commissioned Sir James Lighthill to write a report on the AI research situation in the UK. The resulting report [Lighthill 1973; McCarthy 1973] was highly critical although SHRDLU was favorably mentioned.
Pat Hayes visited Stanford where he learned about Planner. When he returned to Edinburgh, he tried to influence his friend Bob Kowalski to take Planner into account in their joint work on automated theorem proving. "Resolution theorem-proving was demoted from a hot topic to a relic of the misguided past. Bob Kowalski doggedly stuck to his faith in the potential of resolution theorem proving. He carefully studied Planner.” according to Bruynooghe, Pereira, Sickmann, and van Emden . Kowalski  states "I can recall trying to convince Hewitt that Planner was similar to SL-resolution." But Planner was invented for the purposes of the procedural embedding of knowledge and was a rejection of the resolution uniform proof procedure paradigm. Colmerauer and Roussel recalled their reaction to learning about Planner in the following way:
"While attending an IJCAI convention in September ‘71 with Jean Trudel, we met Robert Kowalski again and heard a lecture by Terry Winograd on natural language processing. The fact that he did not use a unified formalism left us puzzled. It was at this time that we learned of the existence of Carl Hewitt’s programming language, Planner [Hewitt, 1969]. The lack of formalization of this language, our ignorance of Lisp and, above all, the fact that we were absolutely devoted to logic meant that this work had little influence on our later research." [Colmerauer and Roussel 1996]
In the fall of 1972, Roussel implemented a language called Prolog (an abbreviation for PROgrammation en LOGique - French for "programming in logic"). Prolog programs are generically of the following form (which is a special case of the backward-chaining in Planner):
- When goal Q, goal P1 and ... and goal Pn
Prolog duplicated the following aspects of Micro-Planner:
- Pattern directed invocation of procedures from goals (i.e. backward chaining)
- An indexed data base of pattern-directed procedures and ground sentences.
- Giving up on the completeness paradigm that had characterized previous work on theorem proving and replacing it with the programming language procedural embedding of knowledge paradigm.
Prolog also duplicated the following capabilities of Micro-Planner which were pragmatically useful for the computers of the era because they saved space and time:
- Backtracking control structure
- Unique Name Assumption by which different names are assumed to refer to distinct entities, e.g., Peking and Beijing are assumed to be different.
- Reification of Failure. The way that Planner established that something was provable was to successfully attempt it as a goal and the way that it establish that something was unprovable was to attempt it as a goal and explicitly fail. Of course the other possibility is that the attempt to prove the goal runs forever and never returns any value. Planner also had a (not expression) construct which succeeded if expression failed, which gave rise to the “Negation as Failure” terminology in Planner.
Use of the Unique Name Assumption and Negation as Failure became more questionable when attention turned to Open Systems [Hewitt and de Jong 1983, Hewitt 1985, Hewitt and Inman 1991].
The following capabilities of Micro-Planner were omitted from Prolog:
- Pattern-directed invocation of procedural plans from assertions (i.e., forward chaining)
- Logical negation, e.g., (not (human Socrates)).
Prolog did not include negation in part because it raises implementation issues. Consider for example if negation were included in the following Prolog program:
- not Q.
- Q :- P.
The above program would be unable to prove not P even though it follows by the rules of mathematical logic. This is an illustration of the fact that Prolog (like Planner) is intended to be a programming language and so does not (by itself) prove many of the logical consequences that follow from a declarative reading of its programs.
The work on Prolog was valuable in that it was much simpler than Planner. However, as the need arose for greater expressive power in the language, Prolog began to include many of the capabilities of Planner that were left out of the original version of Prolog.
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- Earl Sacerdoti, et al., QLISP A Language for the Interactive Development of Complex Systems AFIPS. 1976
- William Kornfeld and Carl Hewitt. The Scientific Community Metaphor MIT AI Memo 641. January 1981.
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- Carl Hewitt and Jeff Inman. DAI Betwixt and Between: From ‘Intelligent Agents’ to Open Systems Science IEEE Transactions on Systems, Man, and Cybernetics. Nov. /Dec. 1991.
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- Carl Hewitt. The repeated demise of logic programming and why it will be reincarnated What Went Wrong and Why: Lessons from AI Research and Applications. Technical Report SS-06-08. AAAI Press. March 2006.
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