zl:1 - FTP

~7 oddp
oddp integer Function
Returns t if integer is odd, otherwise nil. If integer is not an integer, oddp
signals an error.
For a table of related items: See the section "Numeric Property-checking
Predicates" in Symbolics Common Lisp: Language Concepts.
once-only variable-list &body body
A once-only form looks like:
(once-onl y var-list
forml
form2
... )
Macro
var-list is a list of variables. once-only is usually used in macros where
these variables are Lisp forms. The forms are a Lisp program that
presumably uses the values of those variables. When the form resulting
from the expansion of the once-only is evaluated, it fIrst inspects the
values of each of the variables in var-list; these values are assumed to be
Lisp forms. It binds each variable either to its current value, if the current
value is a trivial form, or to a generated symbol. Next, once-only
e,·aluates the forms, in this new binding environment, and when they have
been evaluated it undoes the bindings. The result of the evaluation of the
last form is presumed to be a Lisp form, typically the expansion of a
macro. If all of the variables had been bound to trivial forms, then
once-only just returns that result. Otherwise, once-only returns the result
wrapped in a lambda-combination that binds the generated symbols to the
result of evaluating the respective nontrivial forms.
The effect is that the program produced by evaluating the once-only form
is coded in such a way that it only evaluates each form once, unless evaluation
of the form has no side effects, for each of the forms that were the
values of variables in var-list. At the same time, no unnecessary lambdabinding
appears in this program, but the body of the once-only is not cluttered
up with extraneous code to decide whether or not to introduce
lambda-binding in the program it constructs.
Note well: while once-only attempts to prevent multiple evaluation, it does
not necessarily preserve the order of evaluation of the forms! Since it
generates the new bindings, the evaluation of complex forms (for which a
new variable needs to be created) may be moved ahead of the evaluation of
simple forms (such as variable references). once-only does not solve all of
the problems mentioned in this section.
I