Execution of code can be accomplished by a variety of means ranging from direct interpretation of a form representing a program to invocation of compiled code produced by a compiler.
Evaluation is the process by which a program is executed in Common Lisp. The mechanism of evaluation is manifested both implicitly through the effect of the Lisp read-eval-print loop, and explicitly through the presence of the functions eval, compile, compile-file, and load. Any of these facilities might share the same execution strategy, or each might use a different one.
The behavior of a conforming program processed by eval and by compile-file might differ; see Section 18.104.22.168 (Semantic Constraints).
Evaluation can be understood in terms of a model in which an interpreter recursively traverses a form performing each step of the computation as it goes. This model, which describes the semantics of Common Lisp programs, is described in Section 3.1.2 (The Evaluation Model).
3.1.1 Introduction to Environments
3.1.2 The Evaluation Model
3.1.3 Lambda Expressions
3.1.4 Closures and Lexical Binding
3.1.7 Return Values