From 4e987026148fe65c323afbc93cd560c07bf06b3f Mon Sep 17 00:00:00 2001
From: Yale AI Dept <ai@nebula.cs.yale.edu>
Date: Wed, 14 Jul 1993 13:08:00 -0500
Subject: Import to github.

---
 progs/demo/queens.hs | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100755 progs/demo/queens.hs

(limited to 'progs/demo/queens.hs')

diff --git a/progs/demo/queens.hs b/progs/demo/queens.hs
new file mode 100755
index 0000000..0f8de59
--- /dev/null
+++ b/progs/demo/queens.hs
@@ -0,0 +1,40 @@
+{- This is the n Queens problem. -}
+
+module Main where
+
+queens :: Int -> [[Int]]
+queens size  = queens' size size
+
+queens' :: Int -> Int -> [[Int]]
+queens' 0     _    = [[]]
+queens' (n+1) size = [q:qs | qs <- queens' n size, q <- [1..size],
+			     not (threatens q qs)]
+
+threatens :: Int -> [Int] -> Bool
+threatens q qs = q `elem` qs || q `elem` (diagonals 1 qs)
+
+diagonals :: Int -> [Int] -> [Int]
+diagonals _  []    = []
+diagonals n (q:qs) = (q+n) : (q-n) : diagonals (n+1) qs
+
+main = appendChan stdout "Enter board size: " abort $
+       readChan stdin abort $ \input -> 
+       let line1 : ~(line2 : _) = lines input
+	   size = read line1
+           solns = read line2
+       in if size == 0 then done else  -- This causes the size to actually read
+         appendChan stdout "Number of solutions: " abort $
+         appendChan stdout (concat (map (\x -> showBoard size x)
+                                        (take solns (queens size))))
+  	 abort done
+
+showBoard :: Int -> [Int] -> String
+
+showBoard size pos =
+  concat (map showRow pos) ++ "\n"
+    where
+      showRow n = concat [if i == n then "Q " else ". " | i <- [1..size]]
+                  ++ "\n"
+                
+
+
-- 
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