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;;;======================================================
;;; Farmer's Dilemma Problem
;;;
;;; Another classic AI problem (cannibals and the
;;; missionary) in agricultural terms. The point is
;;; to get the farmer, the fox the cabbage and the
;;; goat across a stream.
;;; But the boat only holds 2 items. If left
;;; alone with the goat, the fox will eat it. If
;;; left alone with the cabbage, the goat will eat
;;; it.
;;; This example uses rules and fact pattern
;;; matching to solve the problem.
;;;
;;; CLIPS Version 6.0 Example
;;;
;;; To execute, merely load, reset and run.
;;;======================================================
(defmodule MAIN
(export deftemplate status))
;;;*************
;;;* TEMPLATES *
;;;*************
;;; The status facts hold the state
;;; information of the search tree.
(deftemplate MAIN::status
(slot search-depth (type INTEGER) (range 1 ?VARIABLE))
(slot parent (type FACT-ADDRESS SYMBOL) (allowed-symbols no-parent))
(slot farmer-location
(type SYMBOL) (allowed-symbols shore-1 shore-2))
(slot fox-location
(type SYMBOL) (allowed-symbols shore-1 shore-2))
(slot goat-location
(type SYMBOL) (allowed-symbols shore-1 shore-2))
(slot cabbage-location
(type SYMBOL) (allowed-symbols shore-1 shore-2))
(slot last-move
(type SYMBOL) (allowed-symbols no-move alone fox goat cabbage)))
;;;*****************
;;;* INITIAL STATE *
;;;*****************
(deffacts MAIN::initial-positions
(status (search-depth 1)
(parent no-parent)
(farmer-location shore-1)
(fox-location shore-1)
(goat-location shore-1)
(cabbage-location shore-1)
(last-move no-move)))
(deffacts MAIN::opposites
(opposite-of shore-1 shore-2)
(opposite-of shore-2 shore-1))
;;;***********************
;;;* GENERATE PATH RULES *
;;;***********************
(defrule MAIN::move-alone
?node <- (status (search-depth ?num)
(farmer-location ?fs))
(opposite-of ?fs ?ns)
=>
(duplicate ?node (search-depth =(+ 1 ?num))
(parent ?node)
(farmer-location ?ns)
(last-move alone)))
(defrule MAIN::move-with-fox
?node <- (status (search-depth ?num)
(farmer-location ?fs)
(fox-location ?fs))
(opposite-of ?fs ?ns)
=>
(duplicate ?node (search-depth =(+ 1 ?num))
(parent ?node)
(farmer-location ?ns)
(fox-location ?ns)
(last-move fox)))
(defrule MAIN::move-with-goat
?node <- (status (search-depth ?num)
(farmer-location ?fs)
(goat-location ?fs))
(opposite-of ?fs ?ns)
=>
(duplicate ?node (search-depth =(+ 1 ?num))
(parent ?node)
(farmer-location ?ns)
(goat-location ?ns)
(last-move goat)))
(defrule MAIN::move-with-cabbage
?node <- (status (search-depth ?num)
(farmer-location ?fs)
(cabbage-location ?fs))
(opposite-of ?fs ?ns)
=>
(duplicate ?node (search-depth =(+ 1 ?num))
(parent ?node)
(farmer-location ?ns)
(cabbage-location ?ns)
(last-move cabbage)))
;;;******************************
;;;* CONSTRAINT VIOLATION RULES *
;;;******************************
(defmodule CONSTRAINTS
(import MAIN deftemplate status))
(defrule CONSTRAINTS::fox-eats-goat
(declare (auto-focus TRUE))
?node <- (status (farmer-location ?s1)
(fox-location ?s2&~?s1)
(goat-location ?s2))
=>
(retract ?node))
(defrule CONSTRAINTS::goat-eats-cabbage
(declare (auto-focus TRUE))
?node <- (status (farmer-location ?s1)
(goat-location ?s2&~?s1)
(cabbage-location ?s2))
=>
(retract ?node))
(defrule CONSTRAINTS::circular-path
(declare (auto-focus TRUE))
(status (search-depth ?sd1)
(farmer-location ?fs)
(fox-location ?xs)
(goat-location ?gs)
(cabbage-location ?cs))
?node <- (status (search-depth ?sd2&:(< ?sd1 ?sd2))
(farmer-location ?fs)
(fox-location ?xs)
(goat-location ?gs)
(cabbage-location ?cs))
=>
(retract ?node))
;;;*********************************
;;;* FIND AND PRINT SOLUTION RULES *
;;;*********************************
(defmodule SOLUTION
(import MAIN deftemplate status))
(deftemplate SOLUTION::moves
(slot id (type FACT-ADDRESS SYMBOL) (allowed-symbols no-parent))
(multislot moves-list
(type SYMBOL) (allowed-symbols no-move alone fox goat cabbage)))
(defrule SOLUTION::recognize-solution
(declare (auto-focus TRUE))
?node <- (status (parent ?parent)
(farmer-location shore-2)
(fox-location shore-2)
(goat-location shore-2)
(cabbage-location shore-2)
(last-move ?move))
=>
(retract ?node)
(assert (moves (id ?parent) (moves-list ?move))))
(defrule SOLUTION::further-solution
?node <- (status (parent ?parent)
(last-move ?move))
?mv <- (moves (id ?node) (moves-list $?rest))
=>
(modify ?mv (id ?parent) (moves-list ?move ?rest)))
(defrule SOLUTION::print-solution
?mv <- (moves (id no-parent) (moves-list no-move $?m))
=>
(retract ?mv)
(printout t t "Solution found: " t t)
(bind ?length (length ?m))
(bind ?i 1)
(bind ?shore shore-2)
(while (<= ?i ?length)
(bind ?thing (nth ?i ?m))
(if (eq ?thing alone)
then (printout t "Farmer moves alone to " ?shore "." t)
else (printout t "Farmer moves with " ?thing " to " ?shore "." t))
(if (eq ?shore shore-1)
then (bind ?shore shore-2)
else (bind ?shore shore-1))
(bind ?i (+ 1 ?i))))
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