Julian Davies at Edinburgh. Planner also influenced the later development of other AI research languages such as Muddle and Conniver, as well as the Smalltalk object-oriented programming language. Hewitt's own work on Planner continued with Muddle (later called MDL), which was developed in the early 1970s by Sussman, Hewitt, Chris Reeve, and David Cressey as a stepping-stone towards a full implementation of Planner. Muddle was implemented as an extended version of Lisp, and introduced several features that were later adopted by Conniver, Lisp Machine Lisp, and Common Lisp. However, in late 1972 Hewitt abruptly halted his development of the Planner design in his thesis, when he and his graduate students invented the actor model of computation. Actor model Hewitt's work on the actor model of computation has spanned over 30 years, beginning with the introduction of the model in a 1973 paper authored by Hewitt, Peter Bishop, and Richard Steiger, and including new results on actor model semantics published as recently as 2006. Much of this work was carried out in collaboration with students in Hewitt's Message Passing Semantics Group at MIT's Artificial Intelligence Lab. Sussman and Steele developed the Scheme programming language in an effort to gain a better understanding of the actor model. However, their Scheme interpreter was not capable of fully implementing the actor model because actor customers cannot be implemented as lambda calculus continuations and actors can change their local state in a way that is impossible in the lambda calculus A number of programming languages were developed to specifically implement the actor model, such as ACT-1, SALSA, Caltrop, E and ActorScript. The actor model also influenced the development of the π-calculus. (See actor model and process calculi history.) Selected works Carl Hewitt (1969). PLANNER: A Language for Proving Theorems in Robots IJCAI'69. Carl Hewitt, Peter Bishop and Richard Steiger (1973). A Universal Modular Actor Formalism for Artificial Intelligence IJCAI'73. Carl Hewitt and Henry Baker (1977a). Laws for Communicating Parallel Processes IFIP'77. Carl Hewitt and Henry Baker (1977b). Actors and Continuous Functionals Proceeding of IFIP Working Conference on Formal Description of Programming Concepts. August 1–5, 1977. William Kornfeld and Carl Hewitt (1981). The Scientific Community Metaphor IEEE Transactions on Systems, Man, and Cybernetics. January 1981. Henry Lieberman and Carl E. Hewitt (1983). A Real-Time Garbage Collector Based on the Lifetimes of Objects Communications of the ACM, 26(6). Carl Hewitt (1985). The Challenge of Open Systems Byte