ackerman and halting problem

ackerman.bak

@@ -0,0 +1,8 @@
+#lang racket
+(define A
+  (lambda ( n m)
+    (cond
+      ((zero? n) ( add1 m))
+      ((zero? m) (A (sub1 n) 1))
+      (else (A (sub1 n)
+               (A n (sub1 m))))))) 

ackerman.rkt

@@ -0,0 +1,13 @@
+#lang racket
+(define A
+  (lambda (n m)
+    (cond
+      ((zero? n) (add1 m))
+      ((zero? m) (A (sub1 n) 1))
+      (else (A (sub1 n)
+               (A n (sub1 m)))))))
+
+(A 2 1) ;5
+(A 3 2) ;29
+
+;(A 4 3) ;Keeps going; presumabley stops

exactfilter.rkt

@@ -0,0 +1,9 @@
+#lang racket
+(define exact-filters '(('type1 (cons '1 '2))
+                        ('type2 (cons '100 111)
+                        ('type1 (cons '3 '4))
+                        )))
+
+(define result (car (group-by (λ (type) (eq? type type)) exact-filters)))
+
+result

little-schemer-halting-problem.bak

@@ -0,0 +1,22 @@
+#lang racket
+
+(define pick
+  (lambda (pos lat)
+    (cond
+      ((eq? pos 1)(car lat))
+      (else (pick (sub1 pos)(cdr lat))))))
+
+(pick 1 '(a b c))
+(pick 2 '(a b c))
+
+(define looking
+  (lambda (a choice lat)
+    (keep-looking a (pick choice lat) lat)))
+
+(define keep-looking
+  (lambda (a pos lat)
+    (cond
+      ((eq? (pick pos lat) a) #t)
+      (else (looking a pos lat)))))
+
+(looking  'caviar 1 '(6 2 3 caviar 5 7 3))

little-schemer-halting-problem.rkt

@@ -19,4 +19,5 @@
       ((eq? (pick pos lat) a) #t)
       (else (looking a pos lat)))))
 
-(looking  'caviar 1 '(6 2 4 caviar 5 7 3))
+(looking  'caviar 1 '(6 2 4 caviar 5 7 3))
+;(looking  'caviar 1 '(7 1 2 caviar 5 6 3))