From 9477dbe667f250ecd23f8fc0d56b942191526421 Mon Sep 17 00:00:00 2001
From: Franciszek Malinka <franciszek.malinka@gmail.com>
Date: Thu, 25 Feb 2021 14:42:55 +0100
Subject: Stare semestry, niepoukladane

---
 Semestr 2/racket/l9/zad4.rkt | 202 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 202 insertions(+)
 create mode 100644 Semestr 2/racket/l9/zad4.rkt

(limited to 'Semestr 2/racket/l9/zad4.rkt')

diff --git a/Semestr 2/racket/l9/zad4.rkt b/Semestr 2/racket/l9/zad4.rkt
new file mode 100644
index 0000000..7b5e0bc
--- /dev/null
+++ b/Semestr 2/racket/l9/zad4.rkt	
@@ -0,0 +1,202 @@
+#lang racket
+
+; Do fun.rkt dodajemy rekurencyjne let-y
+;
+; Miejsca, ktore sie zmienily oznaczone sa przez !!!
+
+; --------- ;
+; Wyrazenia ;
+; --------- ;
+
+(struct const       (val)      #:transparent)
+(struct binop       (op l r)   #:transparent)
+(struct var-expr    (id)       #:transparent)
+(struct let-expr    (id e1 e2) #:transparent)
+(struct letrec-expr (id e1 e2) #:transparent) ; <----------------- !!!
+(struct if-expr     (eb et ef) #:transparent)
+(struct cons-expr   (e1 e2)    #:transparent)
+(struct car-expr    (e)        #:transparent)
+(struct cdr-expr    (e)        #:transparent)
+(struct null-expr   ()         #:transparent)
+(struct null?-expr  (e)        #:transparent)
+(struct app         (f e)      #:transparent)
+(struct lam         (id e)     #:transparent)
+(struct citation    (q)        #:transparent)
+
+(define (expr? e)
+  (match e
+    [(const n) (or (number? n) (boolean? n))]
+    [(binop op l r) (and (symbol? op) (expr? l) (expr? r))]
+    [(var-expr x) (symbol? x)]
+    [(let-expr x e1 e2)
+     (and (symbol? x) (expr? e1) (expr? e2))]
+    [(letrec-expr x e1 e2) ; <------------------------------------ !!!
+     (and (symbol? x) (expr? e1) (expr? e2))]
+    [(if-expr eb et ef)
+     (and (expr? eb) (expr? et) (expr? ef))]
+    [(cons-expr e1 e2) (and (expr? e1) (expr? e2))]
+    [(car-expr e) (expr? e)]
+    [(cdr-expr e) (expr? e)]
+    [(null-expr) true]
+    [(null?-expr e) (expr? e)]
+    [(app f e) (and (expr? f) (expr? e))]
+    [(lam id e) (and (symbol? id) (expr? e))]
+    [(citation q) true]
+    [_ false]))
+
+(define (parse q)
+  (cond
+    [(number? q) (const q)]
+    [(eq? q 'true)  (const true)]
+    [(eq? q 'false) (const false)]
+    [(eq? q 'null)  (null-expr)]
+    [(symbol? q) (var-expr q)]
+    [(and (list? q) (eq? 'quote (first q))) (citation (second q))]
+    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'null?))
+     (null?-expr (parse (second q)))]
+    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'cons))
+     (cons-expr (parse (second q))
+                (parse (third q)))]
+    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'car))
+     (car-expr (parse (second q)))]
+    [(and (list? q) (eq? (length q) 2) (eq? (first q) 'cdr))
+     (cdr-expr (parse (second q)))]
+    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'let))
+     (let-expr (first (second q))
+               (parse (second (second q)))
+               (parse (third q)))]
+    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'letrec)) ; <!!!
+     (letrec-expr (first (second q))
+                  (parse (second (second q)))
+                  (parse (third q)))]
+    [(and (list? q) (eq? (length q) 4) (eq? (first q) 'if))
+     (if-expr (parse (second q))
+              (parse (third q))
+              (parse (fourth q)))]
+    [(and (list? q) (eq? (length q) 3) (eq? (first q) 'lambda))
+     (parse-lam (second q) (third q))]
+    [(and (list? q) (pair? q) (not (op->proc (car q))))
+     (parse-app q)]
+    [(and (list? q) (eq? (length q) 3) (symbol? (first q)))
+     (binop (first q)
+            (parse (second q))
+            (parse (third q)))]))
+
+(define (parse-app q)
+  (define (parse-app-accum q acc)
+    (cond [(= 1 (length q)) (app acc (parse (car q)))]
+          [else (parse-app-accum (cdr q) (app acc (parse (car q))))]))
+  (parse-app-accum (cdr q) (parse (car q))))
+
+(define (parse-lam pat e)
+  (cond [(= 1 (length pat))
+         (lam (car pat) (parse e))]
+        [else
+         (lam (car pat) (parse-lam (cdr pat) e))]))
+
+; ---------- ;
+; Srodowiska ;
+; ---------- ;
+
+(struct blackhole () #:transparent) ; <------------------------- !!!
+(struct environ (xs) #:transparent)
+
+(define env-empty (environ null))
+(define (env-add x v env)
+  (environ (cons (mcons x v) (environ-xs env)))) ; <-------------- !!!
+(define (env-lookup x env)
+  (define (assoc-lookup xs)
+    (cond [(null? xs) (error "Unknown identifier" x)]
+          [(eq? x (mcar (car xs))) ; <---------------------------- !!!
+             (match (mcdr (car xs))
+               [(blackhole) (error "Stuck forever in a black hole!")]
+               [x x])]
+          [else (assoc-lookup (cdr xs))]))
+  (assoc-lookup (environ-xs env)))
+(define (env-update! x v xs) ; <---------------------------------- !!!
+  (define (assoc-update! xs)
+    (cond [(null? xs) (error "Unknown identifier" x)]
+          [(eq? x (mcar (car xs))) (set-mcdr! (car xs) v)]
+          [else (env-update! x v (cdr xs))]))
+  (assoc-update! (environ-xs xs)))
+
+; --------- ;
+; Ewaluacja ;
+; --------- ;
+
+(struct clo (id e env) #:transparent)
+
+(define (value? v)
+  (or (number? v)
+      (boolean? v)
+      (and (pair? v) (value? (car v)) (value? (cdr v)))
+      (null? v)
+      (clo? v)
+      (blackhole? v))) ; <---------------------------------------- !!!
+
+(define (op->proc op)
+  (match op ['+ +] ['- -] ['* *] ['/ /] ['% modulo]
+            ['= =] ['> >] ['>= >=] ['< <] ['<= <=]
+            ['and (lambda (x y) (and x y))]
+            ['or  (lambda (x y) (or  x y))]
+            [_ false]))
+
+(define (eval-env e env)
+  (match e
+    [(const n) n]
+    [(citation q) q]
+    [(binop op l r) ((op->proc op) (eval-env l env)
+                                   (eval-env r env))]
+    [(let-expr x e1 e2)
+     (eval-env e2 (env-add x (eval-env e1 env) env))]
+    [(letrec-expr x e1 e2) ; <------------------------------------ !!!
+     (let* ([new-env (env-add x (blackhole) env)]
+            [v (eval-env e1 new-env)])
+       (begin
+          (env-update! x v new-env)
+          (eval-env e2 new-env)))]
+    [(var-expr x) (env-lookup x env)]
+    [(if-expr eb et ef) (if (eval-env eb env)
+                            (eval-env et env)
+                            (eval-env ef env))]
+    [(cons-expr e1 e2) (cons (eval-env e1 env)
+                             (eval-env e2 env))]
+    [(car-expr e) (car (eval-env e env))]
+    [(cdr-expr e) (cdr (eval-env e env))]
+    [(null-expr) null]
+    [(null?-expr e) (null? (eval-env e env))]
+    [(lam x e) (clo x e env)]
+    [(app f e)
+     (let ([vf (eval-env f env)]
+           [ve (eval-env e env)])
+       (match vf [(clo x body fun-env)
+                  (eval-env body (env-add x ve fun-env))]))]))
+
+(define (eval e) (eval-env e env-empty))
+
+(define program
+  '(letrec
+     [fact (lambda (n) (if (= n 0) 1 (* n (fact (- n 1)))))]
+   (letrec
+     [even-odd
+       (cons
+         (lambda (x)
+           (if (= x 0) true  ((cdr even-odd) (- x 1))))
+         (lambda (x)
+           (if (= x 0) false ((car even-odd) (- x 1)))))]
+   (let [even (car even-odd)]
+   (let [odd  (cdr even-odd)]
+   (even (fact 6)))))))
+
+(define PROGRAM
+  '(letrec [from-to (lambda (n k)
+                      (if (> n k)
+                          null
+                          (cons n (from-to (+ n 1) k))))]
+   (letrec [sum (lambda (xs)
+                  (if (null? xs)
+                      0
+                      (+ (car xs) (sum (cdr xs)))))]
+   (sum (from-to 1 36)))))
+
+(define (test-eval) (eval (parse PROGRAM)))
\ No newline at end of file
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