Comments on: Lisp labels and anonymous functions

By: Mek

Mek — Sat, 11 Sep 2010 23:34:51 +0000

Or (endp lst), or (not (listp lst))

By: foo

foo — Tue, 16 Dec 2008 11:19:05 +0000

CAR and CDR return NIL for NIL. But that’s not what you want.

See (car ‘(nil)) and (car ‘()) . Both return nil.

The usual check for list recursion is (null list), then take things apart.

By: Christopher

Christopher — Tue, 16 Dec 2008 10:27:43 +0000

oh yeah true, it’s a special operator.
i read about it in “Practical Common Lisp” by Peter Seibel.
About checking the list first: good remark, but as it seems, it doesnt fail with an empty list, since car and cdr don’t either. maybe this is only in sbcl? does it fail in other implementations?
of course, it still would be better to check that first, since it is better and more robust code.

By: foo

foo — Tue, 16 Dec 2008 09:52:48 +0000

a few remarks:

LABELS is not a macro, but a built-in special operator.

Common Lisp has also FIRST, REST and IF.

You should test the list for elements with NULL before taking FIRST and REST (CAR and CDR in your case).

So, the function should look a bit different.