--- /dev/null
+#lang racket
+
+#|
+(define (f x)
+ (letrec ((even?
+ (lambda (n)
+ (if (= n 0)
+ #t
+ (odd? (- n 1)))))
+ (odd?
+ (lambda (n)
+ (if (= n 1)
+ #t
+ (even? (- n 1))))))
+ <body>))
+
+
+
+(letrec ...) is transformed into:
+
+(let ((even? '*unassigned*)
+ (odd? '*unassigned*))
+ (set! even? (lambda (n)
+ (if (= n 0)
+ ...
+ ...)))
+ (set! odd? (lambda (n)
+ ...))
+ <body>)
+
+|#
+
+#|
+
+letrec is of the form:
+
+(letrec ((<var1> <exp1>) ... (<varn> <expn>))
+ <body>)
+
+|#
+(define (tagged-list? expr tag)
+ (or (pair? expr) (eq? (car expr) tag)))
+
+(define (letrec? expr)
+ (tagged-list? expr 'letrec))
+
+(define (letrec-variables expr)
+ (let ((p (car (cdr expr))))
+ (map car p)))
+
+(define (letrec-values expr)
+ (let ((p (car (cdr expr))))
+ (map cadr p)))
+
+(define (letrec->let expr)
+ (if (not (letrec? expr))
+ (error "not a letrec expression -- LETREC")
+ (let ((vars (letrec-variables expr))
+ (vals (letrec-values expr))
+ (body (cdr (cdr expr))))
+ (cons 'let
+ (cons (map (lambda (var)
+ (list var ''*unassigned*))
+ vars)
+ (append (map (lambda (var val)
+ (list 'set! var val))
+ vars
+ vals)
+ body))))))
+
+#|
+
+b. In the case where we use lecrec, this gets transformed into let. So, when we call (f 5)
+a new frame is formed with even? and odd? assigned to '*unassigned. Then these are set to
+the lambda expressions belonging to even? and odd?.
+
+When we use a let in the place of letrec, a frame gets created with even? and odd? assigned
+to the lambda expressions. Then the body of the let expression (i.e. <rest of body of f>)
+is wrapped in a lambda which takes as parameters, even? and odd? is called with the lambda
+expressions corresponding to even? and odd?.
+
+i.e. ((lambda (e? o?)
+ <rest of body of f>)
+ (lambda (n) (...reference to o?..))
+ (lambda (n) (.reference to e?... )))
+
+So, a frame gets created which assigns e? and o? to the corresponding lambdas. The enclosing
+environment for it, does not have e? or o? defined. So, these definitions
+When we draw the environment, the two lambdas passed as parameters do not see each other as their
+environment is the enclosing env of t. so they are undefined. When the body is evaluated, then
+these will generate error for undefined references.
+
+|#
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