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Lisp help - is this function right?

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Join Date: May 2005

Posts: 18

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Mr.

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Lisp help - is this function right?

Nov 23rd, 2005

Hello,

the listp function doesn't apply the full "list" rules.

Example

(cons 1 ( cons 2 3 ))

Isn't a list as the definition goes:

* either an empty list (nil)
* or a cons cell that has a list for a cdr

altough listp yelds true to that "list"

So I'm writing a function that will do the full thing

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(defun isList (list)
   (if (null list)
        t
        (if (not (consp list ))
             nil
             (and t (isList (cdr list))))))

This is correct for the following examples

(cons 1 ( cons 2 ( cons 3 ( cons 4 5 ))))
returns nil
(cons 1 (cons 2 3))
returns nul
(cons 1 ( cons 2 ( cons 3 ( cons 4 nil ))))
returns t
(1 2)
returns t

It seams ok. Is it?

Thamls.

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Join Date: Jun 2005

Posts: 2,039

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Rashakil Fol Rashakil Fol is offline

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Re: Lisp help - is this function right?

Nov 23rd, 2005

It looks like a fine implementation of the behavior of the Scheme function, LIST?. I don't know why you'd use (and t (isList (cdr list))) when that means the same thing as (isList (cdr list)).

However, Common Lisp (I'll assume that's the variant you're using, since you didn't specify) has a different meaning for listp.

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Originally Posted by Common Lisp the Language, 2nd Edition

listp object

listp is true if its argument is a cons or the empty list (), and otherwise is false. It does not check for whether the list is a ``true list'' (one terminated by nil) or a ``dotted list'' (one terminated by a non-null atom).

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