Fix rvar subtraction, convert tests to rackunit

rvar.rkt

@@ -23,6 +23,7 @@
            [(Var name) (eval-symbol env name)]
            [(Prim 'read '()) (read-fixnum)]
            [(Prim '- (list e)) (fx- 0 ((interp-exp env) e))]
+           [(Prim '- `(,e1 ,e2)) (fx- ((interp-exp env) e1) ((interp-exp env) e2))]
            [(Prim '+ `(,e1 ,e2)) (fx+ ((interp-exp env) e1) ((interp-exp env) e2))]
            [(Let var rexp body)
             (let ([value ((interp-exp env) rexp)])

test-rvar.rkt

@@ -1,39 +1,50 @@
 #lang racket
 
-(require "test-util.rkt")
+(provide test-rvar-tests)
+
+(require rackunit)
 (require "rvar.rkt")
 
 (define (interp-exp env e)
   ((send (new interp-RVar-class) interp-exp env) e))
 
-(test-eq
+(define test-rvar-tests
+    (test-suite 
+     "RVar interpretation testsuite"
+
+     (check-equal?
        (let ([env `((a . 1) (b . 2))])
          (interp-exp env (Var 'a)))
        1)
 
-(test-eq
+     (check-equal?
        (let ([env `((a . 1) (b . 2))])
          (interp-exp env (Prim '+ (list (Var 'a) (Int 3)))))
        4)
 
-(test-eq
+     (check-equal?
        (let ([env `((a . 1) (b . 2))])
          (interp-exp env (Prim '- `(,(Int 3)))))
        -3)
 
-(test-eq
+     (check-equal?
+       (let ([env `((a . 1))])
+         (interp-exp env (Prim '- (list (Int 3) (Var 'a)))))
+       2)
+
+     (check-equal?
        (let ([env `((a . 1) (b . 2))])
          (interp-exp env (Prim '+ (list (Var 'a) (Prim `- (list (Var 'b)))))))
        -1)
 
-(test-eq
+     (check-equal?
        (interp-exp `() (Let 'a
                             (Prim '+ (list (Int 1) (Int 2)))
                             (Prim '+ (list (Var 'a) (Int 3)))))
        6)
 
-(test-eq
+     (check-equal?
        (interp-RVar (Program `() (Let 'a
                                       (Prim '+ (list (Int 1) (Int 2)))
                                       (Prim '+ (list (Var 'a) (Int 3))))))
-  6)
+       6)))