Re: lists, plain conses and length

[Lars Ingebrigtsen]

"T.V Raman" <raman@google.com> writes:

> (length a)=> Debugger entered--Lisp error: (wrong-type-argument listp 2)
>
> Q: how does one distinguish a cons from a list.
> The error  message for length may need fixing since it says the
> argument passed to it is not a list (it's correct) but then listp is wrong?

`length' (and many other functions) require what's called a "proper
list", which simply means that the cdr of the last cons cell is nil.

(length '(1 2 . 3))
-> (wrong-type-argument listp 3)

The predicate to use here is `proper-list-p'.

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