import Miranda 2.066 from upstream

1 files changed, 22 insertions, 0 deletions

diff --git a/miralib/ex/queens.m b/miralib/ex/queens.m
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+||this generates all solutions to the 8 queens problem -- say
+||      solns
+||and it will print the solutions one per line - all 92 of  them.   This
+||is  a  good  program  for testing the garbage collector.  Say `/gc' to
+||switch on garbage collector diagnostics.
+
+solns = layn(map show (queens 8))
+queens 0 = [[]]
+queens (n+1) = [q:b|b<-queens n;q<-[1..8];safe q b]
+safe q b = and[~checks q b i|i<-index b]
+checks q b i = q=b!i \/ abs(q-b!i)=i+1
+
+||Note that the function `queens n' returns a list of all  solutions  to
+||the n queens problem (placing queens in the first n columns of a chess
+||board, so that no queen gives check  to  another).   A  board  with  n
+||queens  is represented as a list of n numbers, namely the positions of
+||the queens in each column
+
+||This  example  exhibits  a  basic   technique   of   lazy   functional
+||programming,  which is to eliminate backtracking from a search problem
+||by working at the level of a list of  all  solutions,  rather  than  a
+||single solution.