Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z
faqs.org - Internet FAQ Archives

FAQ: Lisp Frequently Asked Questions 3/7 [Monthly posting]
Section - [3-2] Why can't I apply #'AND and #'OR?

( Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Single Page )
[ Usenet FAQs | Web FAQs | Documents | RFC Index | Neighborhoods ]


Top Document: FAQ: Lisp Frequently Asked Questions 3/7 [Monthly posting]
Previous Document: [3-1] Why can't it deduce from (READ-FROM-STRING "foobar" :START 3) that the intent is to specify the START keyword parameter rather than
Next Document: [3-3] I used a destructive function (e.g. DELETE, SORT), but it didn't seem to work. Why?
See reader questions & answers on this topic! - Help others by sharing your knowledge

Here's the simple, but not necessarily satisfying, answer: AND and OR are
macros, not functions; APPLY and FUNCALL can only be used to invoke
functions, not macros and special operators.

OK, so what's the *real* reason?  The reason that AND and OR are macros
rather than functions is because they implement control structure in
addition to computing a boolean value.  They evaluate their subforms
sequentially from left/top to right/bottom, and stop evaluating subforms as
soon as the result can be determined (in the case of AND, as soon as a
subform returns NIL; in the case of OR, as soon as one returns non-NIL);
this is referred to as "short circuiting" in computer language parlance.
APPLY and FUNCALL, however, are ordinary functions; therefore, their
arguments are evaluated automatically, before they are called.  Thus, were
APPLY able to be used with #'AND, the short-circuiting would be defeated.

Perhaps you don't really care about the short-circuiting, and simply want
the functional, boolean interpretation.  While this may be a reasonable
interpretation of trying to apply AND or OR, it doesn't generalize to other
macros well, so there's no obvious way to have the Lisp system "do the
right thing" when trying to apply macros.  The only function associated
with a macro is its expander function; this function accepts and returns
and form, so it cannot be used to compute the value.

The Common Lisp functions EVERY and SOME can be used to get the
functionality you intend when trying to apply #'AND and #'OR.  For
instance, the erroneous form:

   (apply #'and *list*)

can be translated to the correct form:

   (every #'identity *list*)

User Contributions:

Comment about this article, ask questions, or add new information about this topic:

CAPTCHA