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Tony Sidaway (talk | contribs) →Lisp: We're getting ahead of ourselves. Micro-Planner and SHRDLU are covered under the Hewitt/Actor model section |
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ALGOL introduced the use of block structure and lexical scope. It was also notorious for its difficult [[call by name]] default parameter passing mechanism, which was defined to as to require textual substitution of the expression representing the actual parameter in place of the formal parameter during execution of a procedure or function, causing it to be re-evaluated each time it is referenced during execution. ALGOL implementors developed a mechanism they called a [[thunk]], which captured the context of the actual parameter enabling it to be evaluated during execution of the procedure or function.
In 1973 Carl Hewitt of MIT, along with his collaborators Peter Bishop and Richard Steiger, published their work on the Actor model, which they were developing as a means of programming concurrent systems. <ref name="hewitt1973">{{cite paper|author=Carl Hewitt|coauthors=Peter Bishop and Richard Steiger|title=A Universal Modular Actor Formalism for Artificial Intelligence|publisher=IJCAI|year=1973}}</ref> In 1970, Sussman and Hewitt had worked together along with others on Muddle (later MDL), an extended Lisp which formed a component of Hewitt's ambitious Planner project, and in 1971 Sussman, Drew McDermott, and Eugene Charniak had developed a system called Micro-Planner which was a partial and somewhat unsatisfactory implementation of Planner. Micro-Planner would later form a basis for the famous [[SHRDLU]] system. Drew McDermott, and Sussman in 1972 developed the Lisp-based language Conniver, which revised the use of automatic backtracking in Planner which they thought was unproductive.
Meanwhile Hewitt worked on Planner-73, later called PLASMA, which was also written in Lisp and embodied his newly developed ideas which formed the [[Actor model]]. Steele, then a graduate student at MIT, had been following these developments, and he and Sussman decided to implement a version of the Actor model in their own "tiny Lisp" developed on top of [[MacLisp]], in order to understand the model better. Using this basis they then began to develop mechanisms for creating actors and sending messages. <ref name="revisited">{{cite journal
| author = [[Gerald Jay Sussman]] and [[Guy L. Steele, Jr.]]
| month = December
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| accessdate = 2006-06-19
}}</ref>
Sussman had studied ALGOL, and some aspects of PLASMA's modeling suggested to him that the use of lexical scope would greatly simplify the creation of actors and closures. Their modeling system went well and they called it Schemer, eventually changing it to Scheme to fit the six-character limit on the ITS file system on their DEC PDP-10.
==Influence==
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