Re: 'length' function for lists and cons cells?

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Thorsten Jolitz <tjolitz@gmail.com> writes:

> So if both, true lists and cons cells, are processed in a 'dolist' or

proper-lists, not true lists.  

They all are true lists, ie. cons cells or nil.

> 'mapc', how do you get that second string element without writing
> something like 
>
> ,----------------------------------
> | (defun tj/act-conditional (lst)
> |    (if (cdr (last lst))
> |       (cdr lst)
> |      (cadr lst)))
> | 
> | (tj/act-conditional '("a" . "1"))
> | "1"
> | 
> | (tj/act-conditional '("a" "1"))
> | "1"
> `----------------------------------

This is like asking how do you get the second digit of 123 and "123"
without writing something like:

(defun second-digit (thing)
   (etypecase thing
     (integer (mod (truncate thing (expt 10 (truncate (1- (log thing 10))))) 10))
     (string  (- (aref thing 1) ?0))))

(second-digit 12345)  --> 2
(second-digit "1234") --> 2


The point is that lisp is a typed programming language.

If a function expects data of type proper-list, then you don't pass it
circular lists or dotted lists or strings or numbers, or whatever else.

If you want to write a function that takes a or type argument, then you
have to do the type casing yourself, or use other functions that take
the same or type.


(defun second-element (thing)
  (check-type thing (or null cons))
  (typecase thing
    (null nil)
    (cons (typecase (cdr thing)
            (cons (cadr thing))
            (t    (cdr thing))))))

(second-element '())        --> nil
(second-element '(1))       --> nil
(second-element '(1 2))     --> 2
(second-element '(1 2 3))   --> 2
(second-element '(1 2 . 3)) --> 2 
(second-element '(1 . 2))   --> 2
(second-element "123")  error: (wrong-type-argument (or null cons) "123" thing)

It is up to you to define what type of data you want to work with.




-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.