From 23a8766b451975cfa7d1310e8c03c8fea0b1093d Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Mon, 18 Jan 2021 08:19:32 -0500
Subject: [PATCH] Implement 4.8

---
 ex-4_01-xx.scm | 61 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 49 insertions(+), 12 deletions(-)

diff --git a/ex-4_01-xx.scm b/ex-4_01-xx.scm
index b6b00c4..890c6db 100644
--- a/ex-4_01-xx.scm
+++ b/ex-4_01-xx.scm
@@ -51,7 +51,7 @@
                          env)))
   (put 'begin (lambda (exp env) (eval-sequence (begin-actions exp) env)))
   (put 'cond (lambda (exp env) (eval (cond->if exp) env)))
-  (put 'let (lambda (exp env) (eval (let->lambda exp) env)))
+  (put 'let (lambda (exp env) (eval (let->combination exp) env)))
   (put 'let* (lambda (exp env) (eval (let*->nested-lets exp) env)))
   (put 'and (lambda (exp env) (eval-and (clauses exp) env)))
   (put 'or (lambda (exp env) (eval-or (clauses exp) env)))
@@ -122,7 +122,7 @@
                     clauses)))
           ((cond-test-recipient-clause? first)
            (let ((test-evaluated (eval (cond-test exp) env)))
-             (if test-evaluated
+             (if (true? test-evaluated)
                (eval (list (cond-recipient exp) test-evaluated) env)
                (expand-clauses rest))))
           (else
@@ -135,20 +135,20 @@
 (display "\nex-4.6 - let\n")
 
 (define (let? exp) (tagged-list exp 'let))
-(define (let-init exp) (cadr exp))
+(define (let-bindings exp) (cadr exp))
 (define (let-body exp) (cddr exp))
-(define (let-init-var init) (car init))
-(define (let-init-exp init) (cadr init))
-(define (let-init-vars exp) (map let-init-var (let-init exp)))
-(define (let-init-exps exp) (map let-init-exp (let-init exp)))
+(define (let-binding-var binding) (car binding))
+(define (let-binding-exp binding) (cadr binding))
+(define (let-vars exp) (map let-binding-var (let-bindings exp)))
+(define (let-exps exp) (map let-binding-exp (let-bindings exp)))
 
-(define (let->lambda exp)
-  (let ((let-variables (let-init-vars exp))
-        (let-expressions (let-init-exps exp)))
+(define (let->combination exp)
+  (let ((let-variables (let-vars exp))
+        (let-expressions (let-exps exp)))
     (cons (make-lambda let-variables (let-body exp))
           let-expressions)))
 
-(assert (let->lambda '(let ((a 3) (b 4)) (* 3 a b)))
+(assert (let->combination '(let ((a 3) (b 4)) (* 3 a b)))
         '((lambda (a b) (* 3 a b)) 3 4))
 
 (display "\nex-4.7 - let*\n")
@@ -173,8 +173,45 @@
 (assert (let*->nested-lets '(let* ((x 1) (y 2)) (* x y) (+ x y)))
         '(let ((x 1)) (let ((y 2)) (begin (* x y) (+ x y)))))
 
+
 (display "\nex-4.8 - named-let\n")
 
+(define (simple-let? exp) (pair? (let-bindings exp)))
+(define (named-let? exp) (symbol? (let-bindings exp)))
+(define (named-let-var exp) (cadr exp))
+(define (named-let-bindings exp) (caddr exp))
 
-(display "\nex-4.9\n")
+(define (named-let-var exp) (cadr exp))
+(define (named-let-bindings exp) (caddr exp))
+(define (named-let-body exp) (cdddr exp))
+(define (named-let-vars exp) (map let-binding-var (named-let-bindings exp)))
+(define (named-let-exps exp) (map let-binding-exp (named-let-bindings exp)))
+
+(define (let->combination exp)
+  (cond
+    ((simple-let? exp)
+     (let ((let-variables (let-vars exp))
+           (let-expressions (let-exps exp)))
+       (cons (make-lambda let-variables (let-body exp))
+             let-expressions)))
+    ((named-let? exp)
+     (let ((let-name (named-let-var exp))
+           (let-variables (named-let-vars exp))
+           (let-expressions (named-let-exps exp)))
+       (cons
+         (list 'define
+               (cons let-name let-variables)
+               (cons 'begin (named-let-body exp)))
+         let-expressions)))
+    (else (error "Unsupported let expression -- LET->COMBINATION"))))
+
+(assert (let->combination '(let ((a 3) (b 4)) (* 3 a b)))
+        '((lambda (a b) (* 3 a b)) 3 4))
+
+(assert (let->combination '(let fib-iter ((a 1)) (fib-iter (+ a 1))))
+        '((define (fib-iter a) (begin (fib-iter (+ a 1)))) 1))
+
+(display "\nex-4.9 - iteration-constructs\n")
+
+(display "\nex-4.10\n")