Scheme?

[sans-hubris]

Scheme sounds cool because it's all recursion (recursion is your friend). Does anyone have any sample code?

[Strike]

Here's something from my functional languages programming class:


;________________________________________
;Revised version of the "queens" problem

;written by Danny DiPaolo, 11-1-99

;Most of the functions were taken from the specific
;8 x 8 "queens" problem addressed in Springer and
;Friedman's text, but I have modified it to fit the
;specifications of Laboratory 14. It will now
;print all the solutions to a given boardsize if
;the function "queens" is applied to that number.
;
;Examples:
;
;> (queens 4)
;((2 4 1 3) (3 1 4 2))
;
;> (queens 3)
;()
;
;> (length (queens 10))
;724
;
;I couldn't, however, think of an easier, more efficient
;way to test the validity of the solution than by testing
;the length of it.


;_________________________________________________ ____
;This function tests to see if the next attempted move (try)
;is legal, given the list that has been constructed thus far
;(if any) - legal-pl (LEGAL PLacement list)

;N.B. - this function is an EXACT copy of the one from
;Springer and Friedman

(define legal?
(lambda (try legal-pl)
(letrec
((good?
(lambda (new-pl up down)
(cond
((null? new-pl) #t)
(else (let ((next-pos (car new-pl)))
(and
(not (= next-pos try))
(not (= next-pos up))
(not (= next-pos down))
(good? (cdr new-pl)
(add1 up)
(sub1 down)))))))))
(good? legal-pl (add1 try) (sub1 try)))))

;_________________________________________________ ____
;This function tests the length of the solution to
;see if we need to continue "cons"ing on more terms
;or not given to the specified board size.
;
;I modified this function so that it could test the
;validity of any solution for a given boardsize.

(define solution?
(lambda (legal-pl boardsize)
(= (length legal-pl) boardsize)))


;_________________________________________________ ____
;I had to modify this function so that it was passed
;the boardsize in its call, but other than that (and
;simply replacing "fresh-start" with boardsize), just
;about no changes were made. This function simply
;generates a solution.

(define build-solution
(lambda (legal-pl boardsize)
(cond
((solution? legal-pl boardsize) legal-pl)
(else (forward boardsize legal-pl boardsize)))))

;_________________________________________________ ____
;This function dictates how the next solution will be
;chosen, as it is only called when the last solution
;was proven to be legal, and we are ready to try a new
;placement.
;
;I had to modify this function to include the boardsize
;as well, since it invokes "build-solution".

(define forward
(lambda (try legal-pl boardsize)
(cond
((zero? try) (backtrack legal-pl boardsize))
((legal? try legal-pl) (build-solution (cons try legal-pl) boardsize))
(else (forward (sub1 try) legal-pl boardsize)))))

;_________________________________________________ ____
;This function is used when the last move is found to
;be unhelpful (although valid) - instead it tries another
;one until it finds a new solution.
;
;Again, I had to modify this function to include boardsize
;since it calls "forward", which has boardsize as a
;parameter due to the "build-solution" call within it

(define backtrack
(lambda (legal-pl boardsize)
(cond
((null? legal-pl) '())
(else (forward (sub1 (car legal-pl)) (cdr legal-pl) boardsize)))))

;_________________________________________________ ____
;This is pretty much the same function as the one in the book
;with just my minor "boardsize" tweaks, since build-solution
;is called.

(define build-all-solutions
(lambda (boardsize)
(letrec
((loop (lambda (sol)
(cond
((null? sol) '())
(else (cons sol (loop (backtrack sol boardsize))))))))
(loop (build-solution '() boardsize)))))

;_________________________________________________ ____
;This function I made up entirely myself, and I only
;made it really to satisfy the syntactical limitations
;of the laboratory instructions. This makes it so that
;the input of "(queens 4)" will return a list of the
;two possible configurations that are valid solutions,
;even though my modifiend functions would return the same
;value by simply inputting "(build-all-solutions 4)".

(define queens
(lambda (n)
(build-all-solutions n)))

[klamath]

I'm learning Common LISP right now (which is similar to scheme). It's pretty damn cool...