Just for the information of any Allegro CL\PC users out there, I have
discovered a bug in the lisp interpreter. I reported it today to
Franz, Inc., and they have confirmed it as a bona fide bug. I am
hoping for a fix in the near future.
In the meantime, here's a description of the bug, and a workaround:
THE BUG
The code (not (null (member 'foo '(foo bar baz))))
returns (foo bar baz)
when it should return T
The weirdness is in the interaction between NOT and NULL when (I
think) the argument to NULL is the result of a regular function call:
(not (null '(a b c))) => T ; as expected
(not (null (list 'a 'b 'c))) => (a b c) ; bitten by the bug
Note:
(null (list 'a 'b 'c)) => NIL ; expected result
WHEN DOES THIS BUG BITE?
You will have a problem any time you try to compare the results of two
different calls to (not (null..., e.g.,
(if (eq (not (null (list 'a 'b 'c)))
(not (null (list 'd 'e 'f))))
'not-bitten
'bitten)
The above form, when evaluated, SHOULD return NOT-BITTEN.
Unfortunately, in Allegro CL\PC 1.0, it returns BITTEN.
Admittedly, the reasons for wanting to make such a comparison are
obscure. Thus you are unlikely to be BITTEN. Nevertheless, it would
be a wise idea to check your code carefully for such calls, just to be
safe.
A simple and obvious workaround is to replace all calls of the form:
(not (null ...))
with
(if (not (null ...)) t)
Happy hacking!
Brant
--
Brant A. Cheikes
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