## cons

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### cons

Hi every . i have question about cons in lisp . how can i make this list
(1 (4 9 (12) ))

i used this rule (cons 1 (cons (cons 4 (cons 9 (cons (cons 12()))) ()) ())) but interpreter return error
Ehsan

Posts: 8
Joined: Thu Jul 12, 2012 7:39 am

### Re: cons

I think the main issue is that the empty list must be written as '(). If you write (), then it will try to evaluate the function with no name.

In Common Lisp, '() can also be written as nil.

nuntius

Posts: 532
Joined: Sat Aug 09, 2008 10:44 am
Location: Newton, MA

### Re: cons

tnx . i found how to build a nested list
Ehsan

Posts: 8
Joined: Thu Jul 12, 2012 7:39 am

### Re: cons

Even if unquoted empty lists were recognized, the following part of your expression is not a pair:

Code: Select all
`(cons (cons 12 ()) )`
saulgoode

Posts: 45
Joined: Tue Dec 14, 2010 1:39 am

### Re: cons

Actually, in Common Lisp, () is equivalent to (reads as) NIL, which is self-evaluating, so it's a completely legitimate way of writing the empty list. (Which does not mean it isn't stylistically questionable.)

Kompottkin

Posts: 94
Joined: Mon Jul 21, 2008 7:26 am
Location: München, Germany

### Re: cons

Ehsan wrote:Hi every . i have question about cons in lisp . how can i make this list
(1 (4 9 (12) ))

i used this rule (cons 1 (cons (cons 4 (cons 9 (cons (cons 12()))) ()) ())) but interpreter return error

Maybe it's easier to compose it with list first eg.

Code: Select all
`(list 1 (list 4 9 (list 1 2)))`

Then (list a b c) is an abbrivation for (cons a (cons b (cons c NIL)))
So translating the first list would get you:
Code: Select all
`(cons 1(cons (list 4 9 (list 1 2)) NIL))`

Then do the same with the rest. Note that I'm using NIL here to keep away the the extra parenthesis. If you're using Scheme you need to use '()
It's quite difficult to get it right so before list and quasiquoting it must have been a source of many errors

nuntius wrote:I think the main issue is that the empty list must be written as '(). If you write (), then it will try to evaluate the function with no name.

This is true for Scheme. The empty list needs to be quoted since it cannot be evaluated.

nuntius wrote:In Common Lisp, '() can also be written as nil.

For Common Lisp quoting is optional. (), NIL, '() and 'NIL evaluates to NIL.
I'm the author of two useless languages that uses BF as target machine.
Currently I'm planning a Scheme compiler :p
sylwester

Posts: 130
Joined: Mon Jul 11, 2011 2:53 pm

### Re: cons

tnx everyone . I created this instruction , but this is very confusing

(cons 1 (cons (cons 2 (cons 6 (cons 7 (cons 8 ()))))(cons 3 ( cons (cons 4 (cons 9 (cons (cons 12())())))(cons(cons 5(cons 10 (cons 11()))) ())))))

(1 (2 6 7 8) 3 (4 9 (12)) (5 10 11))
Ehsan

Posts: 8
Joined: Thu Jul 12, 2012 7:39 am