From 7c77223255864950aadf1838871d4b091e601aa0 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Fri, 24 Apr 2015 16:16:19 -0400
Subject: [PATCH] Forward-chaining

---
 prospect/examples/forward-chaining.rkt | 100 +++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 prospect/examples/forward-chaining.rkt

diff --git a/prospect/examples/forward-chaining.rkt b/prospect/examples/forward-chaining.rkt
new file mode 100644
index 0000000..15d6211
--- /dev/null
+++ b/prospect/examples/forward-chaining.rkt
@@ -0,0 +1,100 @@
+#lang prospect
+
+(require racket/set)
+(require "../route.rkt")
+(require "../demand-matcher.rkt")
+(require "../drivers/timer.rkt")
+
+(spawn-timer-driver)
+
+(define (insert-record record . monitors)
+  (printf "Record ~v inserted, depending on ~v\n" record monitors)
+  (spawn (lambda (e s)
+           (match e
+             [(? patch/removed?)
+              (printf "Retracting ~v because dependencies ~v vanished\n"
+                      record
+                      (set->list (matcher-project/set (patch-removed e) (compile-projection (?!)))))
+              (quit)]
+             [(message `(retract ,(== record)))
+              (printf "Retracting ~v because we were told to explicitly\n" record)
+              (quit)]
+             [_ #f]))
+         (void)
+         (assert record)
+         (sub `(retract ,record))
+         (patch-seq* (map sub monitors))))
+
+(insert-record `(parent john douglas))
+(insert-record `(parent bob john))
+(insert-record `(parent ebbon bob))
+
+(spawn (lambda (e s)
+         (match e
+           [(? patch?)
+            (transition s
+                        (for/list [(AB (matcher-project/set
+                                        (patch-added e)
+                                        (compile-projection `(parent ,(?!) ,(?!)))))]
+                          (match-define (list A B) AB)
+                          (insert-record `(ancestor ,A ,B)
+                                         `(parent ,A ,B))))]
+           [_ #f]))
+       (void)
+       (sub `(parent ,? ,?)))
+
+(spawn (lambda (e s)
+         (match e
+           [(? patch?)
+            (transition s
+                        (for/list [(AC (matcher-project/set
+                                        (patch-added e)
+                                        (compile-projection `(parent ,(?!) ,(?!)))))]
+                          (match-define (list A C) AC)
+                          (printf "Inductive step for ~v asserted\n" `(parent ,A ,C))
+                          (spawn (lambda (e s)
+                                   (define removed-parents
+                                     (and (patch? e)
+                                          (matcher-project (patch-removed e)
+                                                           (compile-projection
+                                                            `(parent ,(?!) ,(?!))))))
+                                   (if (matcher-non-empty? removed-parents)
+                                       (begin
+                                         (printf
+                                          "Inductive step for ~v retracted because of removal ~v\n"
+                                          `(parent ,A ,C)
+                                          (matcher-key-set removed-parents))
+                                         (quit))
+                                       (and (patch? e)
+                                            (transition s
+                                                        (for/list [(CB (matcher-project/set
+                                                                        (patch-added e)
+                                                                        (compile-projection
+                                                                         `(ancestor ,(?!) ,(?!)))))]
+                                                          (match-define (list _ B) CB)
+                                                          (insert-record `(ancestor ,A ,B)
+                                                                         `(parent ,A ,C)
+                                                                         `(ancestor ,C ,B)))))))
+                                 (void)
+                                 (sub `(parent ,A ,C))
+                                 (sub `(ancestor ,C ,?)))))]
+           [_ #f]))
+       (void)
+       (sub `(parent ,? ,?)))
+
+(define (after msec thunk)
+  (define id (gensym 'after))
+  (list
+   (spawn (lambda (e s) (and (message? e) (quit (thunk))))
+          (void)
+          (sub (timer-expired id ?)))
+   (message (set-timer id msec 'relative))))
+
+(after 1000 (lambda ()
+              (printf "----- Retracting\n")
+              (message `(retract (parent bob john)))))
+(after 2000 (lambda ()
+              (printf "----- Asserting\n")
+              (list (insert-record `(parent bob mary))
+                    (insert-record `(parent mary sue))
+                    (insert-record `(parent sue john)))))