1 files changed, 102 insertions, 0 deletions
diff --git a/ps06_rule_systems/ghelper.scm b/ps06_rule_systems/ghelper.scm
new file mode 100644
index 0000000..7b8613d
--- /dev/null
+++ b/ps06_rule_systems/ghelper.scm
@@ -0,0 +1,102 @@
+;;;;           Most General Generic-Operator Dispatch
+
+(declare (usual-integrations))
+
+;;; Generic-operator dispatch is implemented here by a discrimination
+;;; list, where the arguments passed to the operator are examined by
+;;; predicates that are supplied at the point of attachment of a
+;;; handler (by ASSIGN-OPERATION).
+
+;;; To be the correct branch all arguments must be accepted by
+;;; the branch predicates, so this makes it necessary to
+;;; backtrack to find another branch where the first argument
+;;; is accepted if the second argument is rejected.  Here
+;;; backtracking is implemented by OR.
+
+(define (make-generic-operator arity default-operation)
+  (let ((record (make-operator-record arity)))
+
+    (define (operator . arguments)
+      (if (not (= (length arguments) arity))
+          (error:wrong-number-of-arguments operator arity arguments))
+      (let ((succeed
+	     (lambda (handler)
+	       (apply handler arguments))))
+	(let per-arg
+	    ((tree (operator-record-tree record))
+	     (args arguments)
+	     (fail
+	      (lambda ()
+		(error:no-applicable-methods operator arguments))))
+	  (let per-pred ((tree tree) (fail fail))
+	    (cond ((pair? tree)
+		   (if ((caar tree) (car args))
+		       (if (pair? (cdr args))
+			   (per-arg (cdar tree)
+				    (cdr args)
+				    (lambda ()
+				      (per-pred (cdr tree) fail)))
+			   (succeed (cdar tree)))
+		       (per-pred (cdr tree) fail)))
+		  ((null? tree)
+		   (fail))
+		  (else
+		   (succeed tree)))))))
+
+    (hash-table/put! *generic-operator-table* operator record)
+    (if default-operation
+	(assign-operation operator default-operation))
+    operator))
+
+(define *generic-operator-table*
+  (make-eq-hash-table))
+
+(define (make-operator-record arity) (cons arity '()))
+(define (operator-record-arity record) (car record))
+(define (operator-record-tree record) (cdr record))
+(define (set-operator-record-tree! record tree) (set-cdr! record tree))
+
+(define (assign-operation operator handler . argument-predicates)
+  (let ((record
+         (let ((record (hash-table/get *generic-operator-table* operator #f))
+               (arity (length argument-predicates)))
+           (if record
+               (begin
+                 (if (not (<= arity (operator-record-arity record)))
+                     (error "Incorrect operator arity:" operator))
+                 record)
+               (let ((record (make-operator-record arity)))
+                 (hash-table/put! *generic-operator-table* operator record)
+                 record)))))
+    (set-operator-record-tree! record
+                               (bind-in-tree argument-predicates
+                                             handler
+                                             (operator-record-tree record))))
+  operator)
+
+(define defhandler assign-operation)
+
+(define (bind-in-tree keys handler tree)
+  (let loop ((keys keys) (tree tree))
+    (if (pair? keys)
+	(let find-key ((tree* tree))
+	  (if (pair? tree*)
+	      (if (eq? (caar tree*) (car keys))
+		  (begin
+		    (set-cdr! (car tree*)
+			      (loop (cdr keys) (cdar tree*)))
+		    tree)
+		  (find-key (cdr tree*)))
+	      (cons (cons (car keys)
+			  (loop (cdr keys) '()))
+		    tree)))
+	(if (pair? tree)
+	    (let ((p (last-pair tree)))
+	      (if (not (null? (cdr p)))
+		  (warn "Replacing a handler:" (cdr p) handler))
+	      (set-cdr! p handler)
+	      tree)
+	    (begin
+	      (if (not (null? tree))
+		  (warn "Replacing top-level handler:" tree handler))
+	      handler)))))
+\ No newline at end of file