add evaluators for let* forms

1 files changed, 20 insertions, 3 deletions

diff --git a/main.scm b/main.scm
index 7476efb..dd60662 100644
--- a/main.scm
+++ b/main.scm
@@ -91,14 +91,26 @@
 
 ;;; let
 (define (let-form? expr) (tagged? expr 'let))
+(define (let*-form? expr) (tagged? expr 'let*))
 (define (let-bindings expr) (cadr expr))
+(define (last-binding? bindings) (null? (cdr bindings)))
 (define (let-body expr) (cddr expr))
 (define (let-binding-variables bindings) (map car bindings))
 (define (let-binding-values bindings) (map cadr bindings))
+(define (make-let bindings body) (list 'let bindings body))
+(define (make-let* bindings body) (list 'let* bindings body))
 (define (let->lambda expr)
   (let ([bindings (let-bindings expr)])
     (make-lambda (let-binding-variables bindings)
                  (let-body expr))))
+(define (let*->let expr)
+  (let ([bindings (let-bindings expr)]
+        [body (sequence->expr (let-body expr))])
+    (cond
+      [(last-binding? bindings) (make-let bindings body)]
+      [else (make-let (list (car bindings))
+                      (let*->let (make-let* (cdr bindings) body)))])))
+
 
 (define (eval-expr expr env)
   (cond
@@ -109,6 +121,7 @@
     [(lambda-form? expr) (make-lambda (lambda-params expr)
                                       (lambda-body expr))]
     [(let-form? expr) (eval-let expr env)]
+    [(let*-form? expr) (eval-let* expr env)]
     [(assignment? expr) (eval-assignment expr env)]
     [(definition? expr) (eval-definition expr env)]
     [(cond-form? expr) (eval-cond expr env)]
@@ -131,9 +144,13 @@
             (eval-sequence (cdr exprs) env))]))
 
 (define (eval-let expr env)
-  (eval-function-application (cons (let->lambda expr)
-                                   (let-binding-values (let-bindings expr)))
-                             env))
+  (eval-function-application 
+    (cons (let->lambda expr)
+          (let-binding-values (let-bindings expr)))
+    env))
+
+(define (eval-let* expr env)
+  (eval-expr (let*->let expr) env))
 
 (define (eval-assignment expr env)
   (set-variable-value!