Define efficient global variables in Common Lisp.

;; Similar to defparameter with regard to redefinitions
(define-global-parameter -x- 3)

;; Similar to defvar with regard to redefinitions
(define-global-var -y- 4)

;; ...

(setf -x- 5)
(setf -y- 6)

In Common Lisp, a special variable that is never dynamically bound typically serves as a stand-in for a global variable. The global-vars library provides true global variables that are implemented by some compilers. An attempt to rebind a global variable properly results in a compiler error. That is, a global variable cannot be dynamically bound.

Global variables therefore allow us to communicate an intended usage that differs from special variables. Global variables are also more efficient than special variables, especially in the presence of threads.

• [macro] define-global-var name value &optional documentation

  Define a global variable with a compile-time value.

  Subsequent redefinitions will not change the value (like defvar).

  The value argument is evaluated at compile-time. On SBCL, this permits optimizations based upon the invariant that name is always bound.

• [macro] define-global-var* name value &optional documentation

  Same as define-global-var except value is evaluated at load time, not compile time.

• [macro] define-global-parameter name value &optional documentation

  Same as define-global-var except subsequent redefinitions will update the value (like defparameter).

• [macro] define-global-parameter* name value &optional documentation

  Same as define-global-parameter except value is evaluated at load time, not compile time.

global-vars wraps the following implementation-specific features:

• SBCL: sb-ext:defglobal and sb-ext:define-load-time-global.

• CCL: ccl:defstatic and ccl:defstaticvar.

• LispWorks: hcl:special-global, in particular defglobal-parameter and defglobal-variable.

For these implementations, rebinding a global variable is a compilation error.

On other implementations, a global variable is implemented as a symbol macro which expands to a symbol-value form. Rebinding a global variable will (unfortunately) not signal an error.

It is recommended to use a naming convention for global variables, such as -foo-. This makes it clear that (let ((-foo- 9)) ...) is a mistake even if the compiler doesn't catch it.

James M. Lawrence [email protected]