From: Guillermo J. Rozas <edu/mit/csail/zurich/gjr>
Date: Mon, 17 Feb 1992 21:20:57 +0000 (+0000)
Subject: pattern-lookup was not taking care of repeated pattern variables
X-Git-Tag: 20090517-FFI~9722
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=37065df6243a4208f44b8f54d89269423744f40d;p=mit-scheme.git

pattern-lookup was not taking care of repeated pattern variables
correctly.  It was looking for the pattern in the list of values!
Patched for now to assume that sharing is rare, and thus extra
work is done then.

To do handle this Correctly, pattern-variables should be modified to
replace each occurrence of a pattern variable with a pattern-variable
+ position, where position is false if this is the first occurrence of
the pattern variable or the index for list-ref of the previous value
if not.  This avoids the runtime memq, etc.
---

diff --git a/v7/src/compiler/base/pmlook.scm b/v7/src/compiler/base/pmlook.scm
index f074eb2b5..96ec4b930 100644
--- a/v7/src/compiler/base/pmlook.scm
+++ b/v7/src/compiler/base/pmlook.scm
@@ -1,8 +1,8 @@
 #| -*-Scheme-*-
 
-$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/compiler/base/pmlook.scm,v 1.7 1989/04/15 18:06:14 cph Rel $
+$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/compiler/base/pmlook.scm,v 1.8 1992/02/17 21:20:57 jinx Exp $
 
-Copyright (c) 1988, 1989 Massachusetts Institute of Technology
+Copyright (c) 1988-1992 Massachusetts Institute of Technology
 
 This material was developed by the Scheme project at the Massachusetts
 Institute of Technology, Department of Electrical Engineering and
@@ -33,6 +33,7 @@ promotional, or sales literature without prior written consent from
 MIT in each case. |#
 
 ;;;; Very Simple Pattern Matcher: Lookup
+;;; package: (compiler pattern-matcher/lookup)
 
 (declare (usual-integrations))
 
@@ -45,24 +46,28 @@ MIT in each case. |#
 ;;; as the list of values.
 
 (define (pattern-lookup entries instance)
-  (define (lookup-loop entries values)
+  (define (lookup-loop entries values bindings)
     (define (match pattern instance)
       (if (pair? pattern)
 	  (if (eq? (car pattern) pattern-variable-tag)
-	      (let ((entry (memq (cdr pattern) values)))
-		(if entry
-		    (eqv? (cdr entry) instance)
-		    (begin (set! values (cons instance values))
-			   true)))
+	      (let ((entry (memq (cdr pattern) bindings)))
+		(if (not entry)
+		    (begin (set! bindings (cons (cdr pattern) bindings))
+			   (set! values (cons instance values))
+			   true)
+		    (eqv? instance
+			  (list-ref values (- (length bindings)
+					      (length entry))))))
 	      (and (pair? instance)
 		   (match (car pattern) (car instance))
 		   (match (cdr pattern) (cdr instance))))
 	  (eqv? pattern instance)))
+
     (and (not (null? entries))
 	 (or (and (match (caar entries) instance)
 		  (pattern-lookup/bind (cdar entries) values))
-	     (lookup-loop (cdr entries) '()))))
-  (lookup-loop entries '()))
+	     (lookup-loop (cdr entries) '() '()))))
+  (lookup-loop entries '() '()))
 
 (define-integrable (pattern-lookup/bind binder values)
   (apply binder values))