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curry functional logic set_functions

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Created by mh on April 9, 2014 (last modified on November 2, 2023)

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Description: An implementation of the n-qeeens problem in Curry using set functions

Source code (Latest version):

-------------------------------------------------------------------------
-- Example for the use of set functions as described in the paper

-- Sergio Antoy, Michael Hanus:
-- Set Functions for Functional Logic Programming
-- Proc. 11th International ACM SIGPLAN Conference on Principles and Practice
-- of Declarative Programming (PPDP'09), pp. 73-82, 2009

import Control.Search.SetFunctions

-------------------------------------------------------------------------
-- Solving the n-queens problem in Curry with set functions

-- Permutations of a list defined as a non-deterministic operation
perm [] = []
perm (x:xs) = ndinsert (perm xs)
 where
  ndinsert ys     = x : ys
  ndinsert (y:ys) = y : ndinsert ys

-- A placement is a solution if it is not unsafe.
-- Set functions are used to negate unsafe, and unsafe is defined
-- with a functional pattern.
queens n | isEmpty ((set1 unsafe) p) = p
 where
   p = perm [1..n]

unsafe (_++[x]++y++[z]++_) = abs (x-z) =:= length y + 1

goal1 = queens 4  --> [3,1,4,2],[2,4,1,3]

goal2 = queens 6  --> [5,3,1,6,4,2],[4,1,5,2,6,3],[3,6,2,5,1,4],[2,4,6,1,3,5]

main = goal1

-------------------------------------------------------------------------

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Queens with set functions

implemented in Curry