(define (range n)(define (recur n)(if (= n -1)'()(cons n (recur (- n 1)))))(recur (- n 1)))(define (flatten a)(if (null? a)'()
        (append (car a) (flatten (cdr a)))))(define (safe? x y sln)(if (null? sln)#t(let ((px (car (car sln))) (py (cadr (car sln))))(if (or (= y py) (= (- py y) (- px x)) (= (- py y) (- x px)))#f (safe? x y (cdr sln))))))(define (nqueen n)(define (recur x)(if (= x -1)(list '())(flatten (map (lambda (y) (map (lambda (sln) (cons (list x y) sln)) (filter (lambda (sln) (safe? x y sln)) (recur (- x 1))))) (range n)))))(recur (- n 1)))(define (pl a)(if (null? a)'()(begin (display (car a)) (display "\n") (pl (cdr a)))))(pl (nqueen 4))

#lang racket
(require racket/stream)(define-syntax-rule (scons a b) (stream-cons a b))
(define-syntax-rule (scar a) (stream-first a))
(define-syntax-rule (scdr a) (stream-rest a))
(define-syntax-rule (smap f s) (stream-map f s))
(define-syntax-rule (sfilter f s) (stream-filter f s))
(define-syntax-rule (sreduce f i s) (stream-fold f i s))
(define-syntax-rule (snull? s) (stream-empty? s))
(define-syntax-rule (sappend s ...) (stream-append s ...))
(define-syntax-rule (sfor f s) (stream-for-each f s))(define (sflatten a)(if (snull? a)empty-stream(sappend (scar a) (sflatten (scdr a)))))(define (srange n)(let recur ((x 0))(if (= x n)empty-stream(scons x (recur (+ x 1))))))(define (dis x)(begin (display x)(newline)))(define (sd s)(sfor dis s))(define (safe? x y sln)(if (snull? sln)#t(let ((px (scar (scar sln))) (py (scar (scdr (scar sln)))))(if (or (= y py) (= (- py y) (- px x)) (= (- py y) (- x px)))#f (safe? x y (scdr sln))))))(define (nqueen n)(define (recur x)(if (= x -1)(stream empty-stream)(sflatten (smap (lambda (y) (smap (lambda (sln) (scons (stream x y) sln)) (sfilter (lambda (sln) (safe? x y sln)) (recur (- x 1))))) (srange n)))))(recur (- n 1)))(define ass (sflatten (stream (srange 4) (srange 5))))
;(define ass  (stream (srange 4) (srange 5)))
(sfor (lambda (x) (sfor (lambda (y) (sfor display y)(display ",")) x)(newline)) (nqueen 8))