name—a symbol; not evaluated.
initial-value—a form; evaluated.
documentation—a string; not evaluated.
defconstant
causes the global variable named by name to be
given a value that is the result of evaluating initial-value.
A constant defined by defconstant can be redefined
with defconstant.
However, the consequences are undefined if an attempt is made to assign
a value to the symbol using another operator, or to
assign it to a different
value using a subsequent
defconstant.
If documentation is supplied, it is attached to name as a documentation string of kind variable.
defconstant
normally appears as a top level form, but it is meaningful
for it to appear as a non-top-level form.
However, the compile-time side
effects described below
only take place when defconstant appears as a
top level form.
The consequences are undefined if there are any
bindings
of the variable named by name at the time defconstant
is executed or if the value is not eql to the value of
initial-value.
The consequences are undefined when constant symbols are rebound
as either lexical or dynamic variables. In other words, a reference to a
symbol declared with defconstant always refers to its global value.
The side effects of the execution of defconstant must
be equivalent to at least the side effects of the execution of the following
code:
(setf (symbol-value 'name) initial-value) (setf (documentation 'name 'variable) 'documentation)
If a defconstant form appears as a top level form,
the compiler must recognize that name names
a constant variable. An implementation may choose to
evaluate the value-form at compile time, load time, or both.
Therefore, users must ensure that the initial-value
can be evaluated at compile time
(regardless of whether or not references to name
appear in the file) and that it always evaluates
to the same value.
(defconstant this-is-a-constant 'never-changing "for a test") → THIS-IS-A-CONSTANT this-is-a-constant → NEVER-CHANGING (documentation 'this-is-a-constant 'variable) → "for a test" (constantp 'this-is-a-constant) → true
declaim, defparameter, defvar, documentation, proclaim, Section 3.1.2.1.1.3 (Constant Variables), Section 3.2 (Compilation)