simplify eval.

Eval.hs

@@ -12,22 +12,18 @@ evaluate exp0 = eval exp0 empty
 
 eval :: Exp -> Stack Exp -> Exp
 eval (exp1 :. exp2) stack = eval exp1 $ push exp2 stack
-eval exp1           stack = exec stack exp1
-
-exec :: Stack Exp -> Exp -> Exp
-exec stack0 exp1
-  | isEmpty stack0 = exp1
-exec stack0 exp1@(Var _) = foldl (:.) exp1 $ map evaluate stack0
-exec stack0 S = eval (x :. z :. (y :. z)) stack3
+eval exp1 stack
+  | isEmpty stack = exp1
+eval exp1@(Var _) stack = foldl (:.) exp1 $ map evaluate stack
+eval S stack = eval (x :. z :. (y :. z)) stack3
   where
-    (x,stack1) = pop stack0
+    (x,stack1) = pop stack
     (y,stack2) = pop stack1
     (z,stack3) = pop stack2
-exec stack0 K = eval x stack2
+eval K stack = eval x stack2
   where
-    (x,stack1) = pop stack0
+    (x,stack1) = pop stack
     (_,stack2) = pop stack1
-exec stack0 exp0@(_ :. _) = eval exp0 stack0
 
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