#lang scheme/base
;;; PLT Scheme Inference Collection
;;; ancestors-vector.ss
;;;
;;; Ancestors is a trivial knowledge-based system that determines
;;; a given person's ancestors based on asserted parent relationships.

(require (planet williams/inference/inference))

(define-ruleset ancestors-vector-ruleset)

(define-rule (initialize ancestors-vector-ruleset)
    (?start <- (start))
  ==>
    (retract ?start)
    (printf "Please enter the first name of a~n")
    (printf "person whose ancestors you would~n")
    (printf "like to find:~n")
    (assert `#(request ,(read))))

(define-rule (print-ancestors ancestors-vector-ruleset)
    (?request <- #(request ?name))
    #(parents ?name ?mother ?father)
  ==>
    (retract ?request)
    (when ?mother
      (printf "~a is an ancestor via ~a~n" ?mother ?name)
      (assert `#(request ,?mother)))
    (when ?father
      (printf "~a is an ancestor via ~a~n" ?father ?name)
      (assert `#(request ,?father))))

(define-rule (remove-request ancestors-vector-ruleset #:priority -100)
    (?request <- #(request ?))
  ==>
    (retract ?request))

(define (find-ancestors)
  (with-new-inference-environment
   (activate ancestors-vector-ruleset)
   (assert '#(parents penelope jessica jeremy))
   (assert '#(parents jessica mary-elizabeth homer))
   (assert '#(parents jeremy jenny steven))
   (assert '#(parents steven loree john))
   (assert '#(parents loree #f jason))
   (assert '#(parents homer stephanie #f))
   (start-inference)))

(find-ancestors)