evaluate cond forms, by forming if else chain

1 files changed, 26 insertions, 0 deletions

diff --git a/main.scm b/main.scm
index 59dae1d..8db3289 100644
--- a/main.scm
+++ b/main.scm
@@ -20,6 +20,8 @@
 (define variable? symbol?)
 
 ;;; if-else
+(define (make-if predicate consequent alternative)
+  (list 'if predicate consequent alternative))
 (define (if-else? expr) (tagged? expr 'if))
 (define (if-predicate expr) (cadr expr))
 (define (if-consequent expr) (caddr expr))
@@ -28,6 +30,26 @@
       (cadddr expr)
       'false))
 
+;;; cond forms
+(define (cond-form? expr) (tagged? expr 'cond))
+(define (cond-clauses expr) (cdr expr))
+(define (cond-predicate clause) (car clause))
+(define (cond-consequent clause) (cadr clause))
+(define (cond-else? clause) (tagged? clause 'else))
+(define (last-clause? clauses) (null? (cdr clauses)))
+(define (make-cond clauses) (cons 'cond clauses))
+(define (cond->if expr)
+  (let ([clauses (cond-clauses expr)])
+    (cond
+      [(cond-else? (car clauses)) (cond-consequent (car clauses))]
+      [(last-clause? clauses) (make-if (cond-predicate (car clauses))
+                                       (cond-consequent (car clauses))
+                                       '())]
+      [else (make-if (cond-predicate (car clauses))
+                     (cond-consequent (car clauses))
+                     (cond->if (make-cond (cdr clauses))))])))
+
+
 ;;; function application
 (define (function-application? expr) (pair? expr))
 (define (operator expr) (car expr))
@@ -47,6 +69,7 @@
   (cond
     [(self-evaluating? expr) expr]
     [(if-else? expr) (eval-if expr env)]
+    [(cond-form? expr) (eval-cond expr env)]
     [(quote-form? expr) (quotation-text expr)]
     [(function-application? expr)
      (apply-expr (operator expr)
@@ -57,6 +80,9 @@
       (eval-expr (if-consequent expr) env)
       (eval-expr (if-alternative expr) env)))
 
+(define (eval-cond expr env)
+  (eval-expr (cond->if expr) env))
+
 (define (primitive? proc)
   (is-elem-assoc-list proc primitives))