From acbf2d03baf8c63ad14c440d12def0a45c3b2cd6 Mon Sep 17 00:00:00 2001
From: Julian T <julian@jtle.dk>
Date: Tue, 28 Sep 2021 22:24:02 +0200
Subject: Add exercises for pp lecture 2 and 3

---
 sem7/pp/lec3.scm | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 sem7/pp/lec3.scm

(limited to 'sem7/pp/lec3.scm')

diff --git a/sem7/pp/lec3.scm b/sem7/pp/lec3.scm
new file mode 100644
index 0000000..7babbb8
--- /dev/null
+++ b/sem7/pp/lec3.scm
@@ -0,0 +1,125 @@
+(define (to-string thing)
+  (let ([format-pair-tail
+	 (lambda (tail)
+	   (apply string-append (cond [(null? tail) (list "")]
+				      [(pair? tail) (list " " (to-string tail))]
+				      [else (list " . " (to-string tail))])))
+	 ])
+    (cond [(number? thing) (number->string thing)]
+	  [(symbol? thing) (symbol->string thing)]
+	  [(null? thing) "()"]
+	  [(procedure? thing) "#function#"]
+	  [(pair? thing) (string-append
+			  (to-string (car thing))
+			  (format-pair-tail (cdr thing)))]
+	  [(string? thing) thing]
+	  [else "?"]
+	  )))
+
+(define (create-dispatch methods super-func)
+  (lambda (msg)
+    (let ([method (assoc msg methods)])
+      (if method (cdr method) ((super-func) msg))
+      )))
+
+(define (send obj msg . para)
+  (let ((method (obj msg)))
+    (apply method para)))
+
+(define (new-instance class . para)
+  (apply class (cons '() para)))
+
+;; TODO implement str 
+(define (object self-into)
+  (letrec ([whoami 'object]
+	   [dispatch (create-dispatch (list
+				       [cons 'pretty-str (lambda ()
+							   (to-string (send (self) 'pretty)))]
+				       [cons 'pretty (lambda () (list whoami))]
+				       [cons 'type-of (lambda () whoami)]
+				       [cons 'class-of (lambda () object)]
+				       ) '())]
+	   [self (lambda () (if (null? self-into) dispatch self-into))]
+	   ) dispatch))
+
+(define (point self-into x y)
+  (letrec ([whoami 'point]
+	   [super (lambda () (object (self)))]
+	   [self (lambda () (if (null? self-into) (dispatch) self-into))]
+	   [pretty (lambda () (list whoami x y))]
+	   [move (lambda (dx dy)
+		   (send (self) 'add (new-instance point dx dy)))]
+	   [add (lambda (other)
+		  (new-instance (send (self) 'class-of) (+ x (send other 'getx))
+		    (+ y (send other 'gety))
+		    ))]
+	   [with-color (lambda (color) (new-instance color-point
+						(send (self) 'getx)
+						(send (self) 'gety)
+						color))]
+	   [dispatch (lambda () (create-dispatch (list
+						  [cons 'getx (lambda () x)]
+						  [cons 'gety (lambda () y)]
+						  [cons 'add add]
+						  [cons 'pretty pretty]
+						  [cons 'move move]
+						  [cons 'type-of (lambda () whoami)]
+						  [cons 'class-of (lambda () point)]
+						  [cons 'with-color with-color]
+						  ) super))]
+	   ) (dispatch)))
+
+(define p1 (new-instance point 10 20))
+(define p2 (new-instance point 1 1))
+
+(define (color-point self-into x y . color)
+  (let ((color (if (null? color) 'green (car color))))
+    (letrec ([whoami 'color-point]
+	     [super (lambda () (point (self) x y))]
+	     [self (lambda () (if (null? self-into) (dispatch) self-into))]
+	     [pretty (lambda () (list whoami
+				      (send (self) 'getx)
+				      (send (self) 'gety)
+				      color))]
+	     [dispatch (lambda () (create-dispatch (list
+						    [cons 'get-color (lambda () color)]
+						    [cons 'type-of (lambda () whoami)]
+						    [cons 'pretty pretty]
+						    [cons 'class-of (lambda () color-point)]
+						    ) super))]
+	     ) (dispatch))))
+
+;;# Exercise 3.7
+;;? What is the value of the expression
+(let ((x 1)
+      (y 2)
+      (z 3)
+      (v 5))
+  (cons x 
+	(call/cc (lambda (e) 
+		   (cons y  
+			 (cons z 
+			       (if (even? v) v (e (+ v 1)))))))))
+;; Okay lets start by looking at it:
+;; Okay, we define x y z and v
+;; Then we create a cons with x and the continuations, so the result must be
+;; something like (1 . ???)
+;; If v is even, it will do some cons stuff with the other guys, but it's not.
+;; Therefore the result of (call/cc ...) is (e (+ v 1)) which is 6.
+;; Therefore the result must be (1 . 6)
+
+;; Okay when running it is (1 . 6).
+
+;;# Exercise 3.4
+;;? Program (lambda (a b c) (- (* b b) (* 4 a c))) in CPS
+(define (sub a b k)
+  (k (- a b)))
+(define (mult a b k)
+  (k (* a b)))
+(define (dist a b c k)
+  (mult a c (lambda (v)
+	      (mult 4 v (lambda (v1)
+			  (mult b b (lambda (v2)
+				      (sub v2 v1 k))))))))
+
+(define dist-old (lambda (a b c) (- (* b b) (* 4 a c))))
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