Re: Chained Object Type Predicates

[pjb@informatimago.com (Pascal J. Bourguignon)]

Nordlöw <per.nordlow@gmail.com> writes:

> How should i implement chained type predicate functions, for example
>
> (defun list-of-numbers-p (object) (and (listp object) ...))
>
> should return t if object is of type list (listp) and its elements in
> turn all are of type string (stringp) otherwise nil.

Then why don't you call it list-of-strings-p ?


There are several ways to do so.

One way is to notice that a list is either nil (meaning the empty list)
or made of a first item followed by a rest sublist.
So you could write: 

(defun list-of-numbers-p (object)
    (or (null object)
        (and (consp object)
             (numberp (first object))
             (list-of-numbersp (rest object)))))

Another would be to use reduce:

(require 'cl)
(defun list-of-numbers-p (object)
  (and (listp object)
       (reduce (lambda (&optional (r rp) e)
                   (if rp (and r (numberp e)) t)) object)))



Finally, you may notice that in a list of numbers, every element is a number:

(require 'cl)
(defun list-of-numbers-p (object)
   (and (listp object) (every (function numberp) object)))




You may write a macro to build these functions automatically:

(defun proper-list-p (object)
  (labels ((step (slow fast)
             (cond ((eq slow fast)           nil) ; circular list
                    ((null (cdr fast))       t) 
                    ((atom (cdr fast))       nil) ; dotted list
                    ((null (cdr (cdr fast))) t) 
                    ((atom (cdr (cdr fast))) nil) ; dotted list 
                    (t (step (cdr slow) (cdr (cdr fast)))))))
   (cond            
      ((null object)       t)   ; empty list
      ((atom object)       nil) ; not a list
      ((null (cdr object)) t)   ; one-element list
      (t (step object (cdr object))))))

(defmacro proper-list-of (predicate)
   `(lambda (object) 
       (and (proper-list-p object)
            (every (function ,predicate) object))))


(funcall (proper-list-of integerp) '(1 2 3 4))             --> t
(funcall (proper-list-of integerp) '(1 2 "3" 4))           --> nil
(funcall (proper-list-of integerp) '(1 2 3 . 4))           --> nil
(funcall (proper-list-of integerp) '(1 2 #1=(3 4 . #1#)))  --> nil
         
-- 
__Pascal Bourguignon__