<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=iso-8859-1"> <META content="MSHTML 6.00.6000.16788" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY> <DIV><FONT face="Courier New" size=2>#lang scheme</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>; permute : list-of-a (a a -> boolean) -> (list-of list-of-a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>; take all permutations of a list, but do not include more than one</FONT></DIV> <DIV><FONT face="Courier New" size=2>; copy of all permutations A and B for which (andmap eq A B)</FONT></DIV><FONT face="Courier New" size=2> <DIV><BR>(define (permute lst eq)<BR> (if (null? lst) '(())<BR>  (apply append (map (curry process-rotation eq) (rotations lst eq)))))</DIV> <DIV> </DIV> <DIV>; rotations : list-of-a (a a -> boolean) -> (list-of list-of-a)</DIV> <DIV>; take all rotations of a list, but only those with different cars.</DIV> <DIV></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(define (rotations lst eq)<BR> (let loop ((head lst) (tail '()))<BR>  (cond<BR>   ((null? head) '())<BR>   ((ormap (curry eq (car head)) tail)<BR>    (loop (cdr head) (cons (car head) tail)))<BR>   (else<BR>    (cons<BR>     (append head (reverse tail)) ; (*)<BR>     (loop (cdr head) (cons (car head) tail)))))))</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>; process-rotation : (a a -> boolean) (list-of a) -> (listof list-of a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>; cons the car of a rotation to all permutations of the cdr of that rotation.</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(define (process-rotation eq rot)<BR> (map (curry cons (car rot)) (permute (cdr rot) eq)))</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>; curry : procedure any ... -> procedure</FONT></DIV> <DIV><FONT face="Courier New" size=2>; curry from left</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(define (curry fun . args)<BR> (lambda other-args<BR>  (apply fun (append args other-args))))</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>;test</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(equal? (permute '(a a b c) eq?)</FONT></DIV> <DIV><FONT face="Courier New" size=2>'((a a b c)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (a a c b)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (a b c a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (a b a c)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (a c a b)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (a c b a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (b c a a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (b a a c)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (b a c a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (c a a b)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (c a b a)</FONT></DIV> <DIV><FONT face="Courier New" size=2>  (c b a a))) ; --> #t</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>(*) the reversal is not necessary, but without it the word 'rotation' would be misleading </FONT><FONT face="Courier New" size=2>and the permutations may be listed in another order.</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>Jos</FONT></DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2></FONT> </DIV> <DIV><FONT face="Courier New" size=2>----- Original Message ----- </FONT> <DIV><FONT face="Courier New" size=2>From: "Ben Simon" <</FONT><A href="mailto:benjisimon@gmail.com"><FONT face="Courier New" size=2>benjisimon@gmail.com</FONT></A><FONT face="Courier New" size=2>></FONT></DIV> <DIV><FONT face="Courier New" size=2>To: <</FONT><A href="mailto:plt-scheme@list.cs.brown.edu"><FONT face="Courier New" size=2>plt-scheme@list.cs.brown.edu</FONT></A><FONT face="Courier New" size=2>></FONT></DIV> <DIV><FONT face="Courier New" size=2>Sent: Sunday, January 04, 2009 8:48 PM</FONT></DIV> <DIV><FONT face="Courier New" size=2>Subject: [plt-scheme] Re: Why do layman programmers care about Currying?</FONT></DIV></DIV> <DIV><FONT face="Courier New"><BR><FONT size=2></FONT></FONT></DIV><FONT face="Courier New" size=2>On Jan 2, 9:57 am, "Grant Rettke" <</FONT><A href="mailto:gret...@acm.org"><FONT face="Courier New" size=2>gret...@acm.org</FONT></A><FONT face="Courier New" size=2>> wrote:<BR>> On Tue, Dec 30, 2008 at 11:44 PM, Richard Cleis <</FONT><A href="mailto:rcl...@me.com"><FONT face="Courier New" size=2>rcl...@me.com</FONT></A><FONT face="Courier New" size=2>> wrote:<BR>> > On Dec 30, 2008, at 9:12 PM, Grant Rettke wrote:<BR>> >> Why do layman (working programmers) care about Currying? Or how are<BR>> >> they applied in daily use to help one out?<BR>><BR>[...]<BR>> I don't understand the practical application of currying in Scheme, so<BR>> I'm not asking about one or the other; the wikipedia link was just my<BR>> going in position on trying to understand what is currying.<BR><BR>For me, the practical side of currying has to do with providing some<BR>arguments to a function, while leaving others unset. The result is a<BR>new function that takes in just the unset arguments.  This may not<BR>sound very useful, but as the example below attempts to show, it can<BR>actually be quite handy.<BR><BR>My suggestion is to read up on SRFI 26 and make it one of the standard<BR>tools you use.<BR><BR>;;; ----------------------------------------------------------<BR>#lang scheme<BR><BR>;; Scheme doesn't offer automatic currying, but SRFI-26 provides<BR>;; the (cut ...) operator which gets us close enough.<BR>(require srfi/26)<BR><BR>;; A function to send e-mail. This is a bogus implementation,<BR>;; could easily be a "real" implementation<BR>(define (send-mail smtp-host smtp-port from to subject message)<BR>  (printf "Would have sent a message: \n")<BR>  (printf "  SMTP server: ~a:~a\n" smtp-host smtp-port)<BR>  (printf "  From: ~a\n" from)<BR>  (printf "  To: ~a\n" to)<BR>  (printf "  Subject: ~a\n" subject)<BR>  (printf "  Message: ~a\n" message))<BR><BR>;; A place holder function to return a list of e-mail addresses. Used<BR>;; for testing below.<BR>(define (list-of-emails)<BR>  '("</FONT><A href="mailto:foo@nowhere.com"><FONT face="Courier New" size=2>foo@nowhere.com</FONT></A><FONT face="Courier New" size=2>" "</FONT><A href="mailto:bar@nowhere.com"><FONT face="Courier New" size=2>bar@nowhere.com</FONT></A><FONT face="Courier New" size=2>" "</FONT><A href="mailto:baz@nowhere.com"><FONT face="Courier New" size=2>baz@nowhere.com</FONT></A><FONT face="Courier New" size=2>"))<BR><BR>;; Let's start currying...<BR><BR>;; EXAMPLE 1<BR>;; Here we provide two arguments to our send-mail function, and leave<BR>;; 3 unset (<> serves as a place holder). The result of this is a<BR>those<BR>;; function that takes in just  3 remaining arguments.<BR>;;<BR>;; Now we can use `mailer' throughout our application and not have to<BR>worry about providing<BR>;; the SMTP host and port.<BR>(define mailer (cut send-mail "mail.myhost.com" 25<BR>"</FONT><A href="mailto:noreply@myhost.com"><FONT face="Courier New" size=2>noreply@myhost.com</FONT></A><FONT face="Courier New" size=2>" <> <> <>))<BR><BR>;; Use our mailer<BR>(mailer "</FONT><A href="mailto:example@nowhere.com"><FONT face="Courier New" size=2>example@nowhere.com</FONT></A><FONT face="Courier New" size=2>" "Hello World" "This is just a test...")<BR><BR>;; EXAMPLE 2<BR>;; Let's say we want to send the same message over and over again. We<BR>can use (cut ...) to<BR>;; provide every argument but the to address.  Notice how we can<BR>operate on our already curried<BR>;; function.<BR>(define broadcaster (cut mailer <> "A Special offer, Just For You"<BR>"Here's a special offer, sent only to you. We promise..."))<BR><BR>;; Use our broadcaster<BR>(for-each broadcaster<BR>          (list-of-emails))<BR><BR>;; EXAMPLE 3<BR>;; We can provide all the arguments except for a message, and turn our<BR>mail<BR>;; function into a logger.<BR>(define logger (cut mailer "</FONT><A href="mailto:logging@nowhere.com"><FONT face="Courier New" size=2>logging@nowhere.com</FONT></A><FONT face="Courier New" size=2>" "Log Message" <>))<BR><BR>;; Pretend this is some complicated function<BR>(define (something-complicated logger)<BR>  (logger "Starting to do something complicated...")<BR>  (void)<BR>  (logger "Whew, and we're done."))<BR>(something-complicated logger)<BR>_________________________________________________<BR>  For list-related administrative tasks:<BR>  </FONT><A href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme"><FONT face="Courier New" size=2>http://list.cs.brown.edu/mailman/listinfo/plt-scheme</FONT></A></BODY></HTML>