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+module squeen
+
+/*
+The Queens Problem.
+
+Or: How to put n queens on a n*n chessboard in such a way that they
+cannot attack each other.
+	
+The result of this program is the number of possible solutions for
+the queens problem for a certain boardsize together with one solution.
+When BoardSize is 8 the result will be: (92,[4,2,7,3,6,8,5,1]),
+which means the queens are on a4, b2, c7, d3, e6, f8, g5 and h1.
+
+Strictness annotations are used at certain points, because that makes
+this program more than twice as fast (the strictness analyzer is not
+able to deduce this strictness information). However, other Clean programs
+for the Queens problem exist without strictness annotations added by the 
+programmer that are only 40% slower than this solution (lqueen.icl).
+
+*/
+
+import StdEnv
+
+BoardSize :== 8 // The size of the chessboard.
+
+//	Finding all solutions for the queens problem.
+	
+Queens::Int [Int] [[Int]] -> [[Int]]
+Queens row board boards
+	| row>BoardSize	=  [board : boards]
+	| otherwise		=  TryCols BoardSize row board boards
+
+TryCols::Int Int [Int] [[Int]] -> [[Int]]
+TryCols 0 row board boards =  boards
+TryCols col row board boards
+	| Save col 1 board	=	TryCols (col-1) row board queens
+	| otherwise			= 	TryCols (col-1) row board boards
+where	queens	= Queens (row+1) [col : board] boards
+
+/*	The strictness analyzer can't derive strictness for the first and second
+	argument of Save, because they are not used in the first alternative
+	of that function. However, Save is strict in these arguments (in the
+	context of this program) and adding the strictness annotations speeds
+	up this program considerably. */
+
+Save::!Int !Int [Int] -> Bool
+Save  c1 rdiff [] =  True
+Save  c1 rdiff [c2:cols]
+	| cdiff==0 || cdiff==rdiff || cdiff==0-rdiff	=	False
+	| otherwise										=	Save c1 (rdiff+1) cols
+where	cdiff	= c1 - c2
+
+/*	The Start Rule: Calculate the list of solutions, show the first
+	solution and the length of that list. */
+	
+Start::(Int,[Int])
+Start 	= 	(length solutions, hd solutions)
+where	solutions	= Queens 1 [] []