Our evaluator for Lisp will be implemented as a Lisp program. It may seem circular to think about evaluating Lisp programs using an evaluator that is itself implemented in Lisp. However, evaluation is a process, so it is appropriate to describe the evaluation process using Lisp, which, after all, is our tool for describing processes. An evaluator that is written in the same language that it evaluates is said to be metacircular.
The metacircular evaluator is essentially a Scheme formulation of the environment model of evaluation described in section . Recall that the model has two basic parts:
These two rules describe the essence of the evaluation process, a basic cycle in which expressions to be evaluated in environments are reduced to procedures to be applied to arguments, which in turn are reduced to new expressions to be evaluated in new environments, and so on, until we get down to symbols, whose values are looked up in the environment, and to primitive procedures, which are applied directly (see figure ). This evaluation cycle will be embodied by the interplay between the two critical procedures in the evaluator, eval and apply, which are described in section (see figure ).
The implementation of the evaluator will depend upon procedures that define the syntax of the expressions to be evaluated. We will use data abstraction to make the evaluator independent of the representation of the language. For example, rather than committing to a choice that an assignment is to be represented by a list beginning with the symbol set! we use an abstract predicate assignment? to test for an assignment, and we use abstract selectors assignment-variable and assignment-value to access the parts of an assignment. Implementation of expressions will be described in detail in section . There are also operations, described in section , that specify the representation of procedures and environments. For example, make-procedure constructs compound procedures, lookup-variable-value accesses the values of variables, and apply-primitive-procedure applies a primitive procedure to a given list of arguments.