Common Lisp the Language, 2nd Edition

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20. The Evaluator

The mechanism that executes Lisp programs is called the evaluator. More precisely, the evaluator accepts a form and performs the computation specified by the form. This mechanism is made available to the user through the function eval.

The evaluator is typically implemented as an interpreter that traverses the given form recursively, performing each step of the computation as it goes. An interpretive implementation is not required, however. A permissible alternative approach is for the evaluator first to completely compile the form into machine-executable code and then invoke the resulting code. This technique virtually eliminates incompatibilities between interpreted and compiled code but also renders the evalhook mechanism relatively useless. Various mixed strategies are also possible. All of these approaches should produce the same results when executing a correct program but may produce different results for incorrect programs. For example, the approaches may differ as to when macro calls are expanded; macro definitions should not depend on the time at which they are expanded. Implementors should document the evaluation strategy for each implementation.