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comp.lang.lisp

Re: Python syntax in Lisp and Scheme

William James

7/29/2015 9:17:00 AM

Pascal Costanza wrote:

> What about dealing with an arbitrary number of filters?
>
> (defmacro predicate-collect (list &body predicates)
> (let ((collectors (mapcar (lambda (predicate)
> (declare (ignore predicate))
> (gensym "COLLECT"))
> predicates))
> (collect-t (gensym "COLLECT")))
> `(with-collectors (,@collectors ,collect-t)
> (dolist (l ,list)
> (cond ,@(mapcar (lambda (predicate collector)
> `((funcall ,predicate l) (,collector l)))
> predicates collectors)
> (t (,collect-t l)))))))
>
> An example:
>
> > (predicate-collect '(-5 -4 -3 -2 -1 0 1 2 3 4 5)
> (function evenp)
> (lambda (n) (< n 0))
> (lambda (n) (> n 3)))
> (-4 -2 0 2 4)
> (-5 -3 -1)
> (5)
> (1 3)
>
>
> I use the list collector macros by Tim Bradshaw here - see
> http://www.tfeb.org/lisp/hax.html#...

Gauche Scheme:

(define (predicate-collect items . predicates)
(let go ((xs items) (predicates predicates))
(if (null? predicates)
(list xs)
(receive (picked rejected) (partition (car predicates) xs)
(cons picked (go rejected (cdr predicates)))))))

(predicate-collect '(-5 -4 -3 -2 -1 0 1 2 3 4 5)
even?
negative?
(cut > <> 3))

((-4 -2 0 2 4)
(-5 -3 -1)
(5)
(1 3))

--
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--- Dr. Kevin MacDonald (http://lnrlive.com/tpc/tpc201...)
1 Answer

William James

3/30/2016 12:06:00 AM

0

WJ wrote:

> Pascal Costanza wrote:
>
> > What about dealing with an arbitrary number of filters?
> >
> > (defmacro predicate-collect (list &body predicates)
> > (let ((collectors (mapcar (lambda (predicate)
> > (declare (ignore predicate))
> > (gensym "COLLECT"))
> > predicates))
> > (collect-t (gensym "COLLECT")))
> > `(with-collectors (,@collectors ,collect-t)
> > (dolist (l ,list)
> > (cond ,@(mapcar (lambda (predicate collector)
> > `((funcall ,predicate l) (,collector l)))
> > predicates collectors)
> > (t (,collect-t l)))))))
> >
> > An example:
> >
> > > (predicate-collect '(-5 -4 -3 -2 -1 0 1 2 3 4 5)
> > (function evenp)
> > (lambda (n) (< n 0))
> > (lambda (n) (> n 3)))
> > (-4 -2 0 2 4)
> > (-5 -3 -1)
> > (5)
> > (1 3)
> >
> >
> > I use the list collector macros by Tim Bradshaw here - see
> > http://www.tfeb.org/lisp/hax.html#...

OCaml:

let multi_partition items predicates =
let rec loop items = function
[] -> [items]
| p::preds -> let (yes,no) = List.partition p items
in yes :: loop no preds
in loop items predicates ;;

multi_partition [-5;-4;-3;-2;-1;0;1;2;3;4;5]
[(fun n -> 0=n land 1);
((>) 0);
((<) 3)] ;;

===>
[[-4; -2; 0; 2; 4]; [-5; -3; -1]; [5]; [1; 3]]

--
"To destroy a nation, convince its members that nationalism is evil.
To destroy a race, convince its members that racialism is evil."