lists, plain conses and length

lists, plain conses and length

[T.V Raman]

Ran into this while trying to distinguish a cons from a list:

Try this:

(setq a (cons 1 2)) => (1 . 2)
(consp a) => t
(listp a) => t
(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?
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Re: lists, plain conses and length

[Stefan Monnier]

> Q: how does one distinguish a cons from a list.

A: check `null`

(tongue in cheek)


        Stefan


PS: Hint: `(listp X)` is the same as `(or (consp X) (null X))`

Re: lists, plain conses and length

[T.V Raman]

In the spirit of the US election campaign, if it isn't shown to be
true, it must be false -- if it's shown to be false, it  must be fake ...Stefan Monnier writes:
 > > Q: how does one distinguish a cons from a list.
 > 
 > A: check `null`
 > 
 > (tongue in cheek)
 > 
 > 
 >         Stefan
 > 
 > 
 > PS: Hint: `(listp X)` is the same as `(or (consp X) (null X))`

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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'.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Re: lists, plain conses and length

[tomas]

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On Fri, Sep 04, 2020 at 04:29:47AM +0200, Lars Ingebrigtsen wrote:
> "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'.

Note that `proper-list-p' has to run down the whole list to make
sure. For short lists, that's fine.

Cheers
 - t

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