Common Lisp - Compiler and Interpreter

Compiler and Interpreter

Several implementations of earlier Lisp dialects provided both an interpreter and a compiler. Unfortunately often the semantics were different. These earlier Lisps implemented lexical scoping in the compiler and dynamic scoping in the interpreter. Common Lisp requires that both the interpreter and compiler use lexical scoping by default. The Common Lisp standard describes both the semantics of the interpreter and a compiler. The compiler can be called using the function compile for individual functions and using the function compile-file for files. Common Lisp allows type declarations and provides ways to influence the compiler code generation policy. For the latter various optimization qualities can be given values between 0 (not important) and 3 (most important): speed, space, safety, debug and compilation-speed.

There is also a function to evaluate Lisp code: eval. eval takes code as pre-parsed s-expressions and not, like in some other languages, as text strings. This way code can be constructed with the usual Lisp functions for constructing lists and symbols and then this code can be evaluated with eval. Several Common Lisp implementations (like Clozure CL and SBCL) are implementing eval using their compiler. This way code is compiled, even though it is evaluated using the function eval.

The file compiler is invoked using the function compile-file. The generated file with compiled code is called a fasl (from fast load) file. These fasl files and also source code files can be loaded with the function load into a running Common Lisp system. Depending on the implementation, the file compiler generates byte-code (for example for the Java Virtual Machine), C language code (which then is compiled with a C compiler) or, directly, native code.

Common Lisp implementations can be used interactively, even though the code gets fully compiled. The idea of an Interpreted language thus does not apply for interactive Common Lisp.

The language makes distinction between read-time, compile-time, load-time and run-time, and allows user code to also make this distinction to perform the wanted type of processing at the wanted step.

Some special operators are provided to especially suit interactive development; for instance, DEFVAR will only assign a value to its provided variable if it wasn't already bound, while DEFPARAMETER will always perform the assignment. This distinction is useful when interactively evaluating, compiling and loading code in a live image.

Some features are also provided to help writing compilers and interpreters. Symbols consist of first-level objects and are directly manipulable by user code. The PROGV special operator allows to create lexical bindings programmatically, while packages are also manipulable. The Lisp compiler itself is available at runtime to compile files or individual functions. These make it easy to use Lisp as an intermediate compiler or interpreter for another language.