From: Jim <jim.pryor@nyu.edu>
Date: Wed, 29 Apr 2015 22:46:28 +0000 (-0400)
Subject: update refunct zippers code
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=27eb9f9e70d526895797ac831defb79ac3769b2e;ds=sidebyside

update refunct zippers code
---

diff --git a/code/refunctionalizing_zippers.rkt b/code/refunctionalizing_zippers.rkt
index 3b51ce99..cac0916b 100644
--- a/code/refunctionalizing_zippers.rkt
+++ b/code/refunctionalizing_zippers.rkt
@@ -17,19 +17,26 @@
     [else (tz1 (pair (cons (nextchar z) (saved z)) (rest z)))]))
 
 ; using explicit continuations
-(define (tc1 l k)
+(define (tc0 l k)
   (cond
     [(null? l) (reverse (k '()))]
+    [(eqv? #\S (car l)) (tc0 (cdr l) (compose k k))]
+    [else (tc0 (cdr l) (lambda (tail) (cons (car l) (k tail))))]))
+
+; improvement: if we flip the order of cons and k in the last line, we can avoid the need to reverse
+(define (tc1 l k)
+  (cond
+    [(null? l) (k '())]
     [(eqv? #\S (car l)) (tc1 (cdr l) (compose k k))]
-    [else (tc1 (cdr l) (lambda (tail) (cons (car l) (k tail))))]))
+    [else (tc1 (cdr l) (lambda (tail) (k (cons (car l) tail))))]))
 
 ; using implicit continuations (reset/shift)
 (define (tr1 l)
   (shift k
     (cond
-      [(null? l) (reverse (k '()))]
+      [(null? l) (k '())]
       [(eqv? #\S (car l)) ((compose k k) (tr1 (cdr l)))]
-      [else ((lambda (tail) (cons (car l) (k tail))) (tr1 (cdr l)))])))
+      [else ((lambda (tail) (k (cons (car l) tail))) (tr1 (cdr l)))])))
 
 ; wrapper functions, there's a (test) function at the end
 
@@ -59,23 +66,21 @@
     [else (tz3 (pair (cons (nextchar z) (saved z)) (rest z)))]))
 
 ; using explicit continuations
-; there are several working solutions
-; but it's a bit tricky to get the reverses in the right place, and the order of appending right
 (define (tc3 l k)
   (cond
-    [(null? l) (reverse (k '()))]
-    [(eqv? #\# (car l)) (append (reverse (k '())) (tc3 (cdr l) identity))]
+    [(null? l) (k '())]
+    [(eqv? #\# (car l)) (append (k '()) (tc3 (cdr l) identity))]
     [(eqv? #\S (car l)) (tc3 (cdr l) (compose k k))]
-    [else (tc3 (cdr l) (lambda (tail) (cons (car l) (k tail))))]))
+    [else (tc3 (cdr l) (lambda (tail) (k (cons (car l) tail))))]))
 
 ; using implicit continuations (reset/shift)
 (define (tr3 l)
   (shift k
     (cond
-      [(null? l) (reverse (k '()))]
-      [(eqv? #\# (car l)) (append (reverse (k '())) (reset (tr3 (cdr l))))]
+      [(null? l) (identity (k '()))]
+      [(eqv? #\# (car l)) (append (k '()) (reset (tr3 (cdr l))))]
       [(eqv? #\S (car l)) ((compose k k) (tr3 (cdr l)))]
-      [else ((lambda (tail) (cons (car l) (k tail))) (tr3 (cdr l)))])))
+      [else ((lambda (tail) (k (cons (car l) tail))) (tr3 (cdr l)))])))
 
 (define (tz4 s)
   (list->string (tz3 (cons '() (string->list s)))))
@@ -93,9 +98,11 @@
     (equal? (t1 inp) (t2 inp)))
   (and
    (equal? (tz2 "abSd") "ababd")
+   (cmp (lambda (s) (list->string (tc0 (string->list s) identity))) tz2 "abSd")
    (cmp tc2 tz2 "abSd")
    (cmp tr2 tz2 "abSd")
    (equal? (tz2 "aSbS") "aabaab")
+   (cmp (lambda (s) (list->string (tc0 (string->list s) identity))) tz2 "aSbS")
    (cmp tc2 tz2 "aSbS")
    (cmp tr2 tz2 "aSbS")
    (equal? (tz4 "ab#ceSfSd") "abcecefcecefd")