-- Solve the n-queens puzzle using default rules
-- S. Antoy and M. Hanus, Default Rules for Curry, TPLP, vol. 17(2), 2017, pp. 121-147,

{-# OPTIONS_CYMAKE -F --pgmF=currypp --optF=defaultrules #-}
{-# OPTIONS_CYMAKE -Wnone #-} 

import Control.Search.SetFunctions

main = queens (perm [1..4])

perm []            = []
perm (xs++[x]++ys) = x : perm (xs++ys)

queens (_++[x]++y++[z]++_) | abs (x-z) == length y + 1 = failed
queens'default p = p