* Chained Object Type Predicates
@ 2009-06-08 12:39 Nordlöw
2009-06-08 12:58 ` Pascal J. Bourguignon
2009-06-08 13:36 ` Lennart Borgman
0 siblings, 2 replies; 3+ messages in thread
From: Nordlöw @ 2009-06-08 12:39 UTC (permalink / raw
To: help-gnu-emacs
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.
Thanks in advance,
Per Nordlöw
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Chained Object Type Predicates
2009-06-08 12:39 Chained Object Type Predicates Nordlöw
@ 2009-06-08 12:58 ` Pascal J. Bourguignon
2009-06-08 13:36 ` Lennart Borgman
1 sibling, 0 replies; 3+ messages in thread
From: Pascal J. Bourguignon @ 2009-06-08 12:58 UTC (permalink / raw
To: help-gnu-emacs
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__
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Chained Object Type Predicates
2009-06-08 12:39 Chained Object Type Predicates Nordlöw
2009-06-08 12:58 ` Pascal J. Bourguignon
@ 2009-06-08 13:36 ` Lennart Borgman
1 sibling, 0 replies; 3+ messages in thread
From: Lennart Borgman @ 2009-06-08 13:36 UTC (permalink / raw
To: Nordlöw; +Cc: help-gnu-emacs
On Mon, Jun 8, 2009 at 2:39 PM, Nordlöw<per.nordlow@gmail.com> wrote:
> 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.
Maybe there is something special implemented for it, but otherwise
just loop over the list and jump out with catch/throw.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2009-06-08 12:58 ` Pascal J. Bourguignon
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