Re: to big nest level of recursion

[Pascal Bourguignon <usenet@informatimago.com>]

"Anton V. Belyaev" <anton.belyaev@gmail.com> writes:

> Hi! I wrote a simple function which fails on lists long enougth with
> reason "(error "Lisp nesting exceeds `max-lisp-eval-depth'")". But the
> function contains right recursion. Does Emacs LISP interpreter unwind
> right recursion calls?
>
> (defun contains (lst el)
>   (if (null lst)
>       nil
>     (if (eq (car lst) el)
> 	t
>       (contains (cdr lst) el)
>       )
>     )
>   )

Let's make it readable:

1- indentation and parenthesis placement:

(defun contains (lst el)
   (if (null lst)
       nil
       (if (eq (car lst) el)
          t
          (contains (cdr lst) el))))

2- use of cond in place of if sequences:

 (defun contains (lst el)
   (cond ((null lst)         nil)
         ((eq (car lst) el)  t))
         (t                  (contains (cdr lst) el)))

3- since all the branches return a boolean anyway, we can write it as
   a boolean expression directly:

 (defun contains (lst el)
   (and (not (null lst))  (or (eq (car lst) el)  (contains (cdr lst) el))))



Otherwise, emacs doesn't do TCO.


> PS: Does Emacs built-in functions have an equivalent of my contains?

member


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