refactors interpreter pattern

Author: HamidMosalla

Interpreter/Context/TokenReader.cs

@@ -14,15 +14,14 @@ public IExpression ReadToken(List<string> tokenList)
         private IExpression ReadNextToken(List<string> tokenList)
         {
             int i;
+
             if (int.TryParse(tokenList.First(), out i))  //if the token is integer (terminal)
             {
                 tokenList.RemoveAt(0);   //process terminal expression
                 return new NumberExpression(i);
             }
-            else
-            {
-                return ReadNonTerminal(tokenList);  //process nonTerminal expression
-            }
+
+            return ReadNonTerminal(tokenList);  //process nonTerminal expression
         }
 
         private IExpression ReadNonTerminal(List<string> tokenList)

Interpreter/Expression/AddExpression.cs

@@ -2,8 +2,8 @@
 {
     public class AddExpression : IExpression
     {
-        IExpression leftExpression;
-        IExpression rightExpression;
+        readonly IExpression leftExpression;
+        readonly IExpression rightExpression;
 
         public AddExpression(IExpression left, IExpression right)
         {

Interpreter/Expression/NumberExpression.cs

@@ -2,15 +2,16 @@
 {
     public class NumberExpression : IExpression
     {
-        int number;
+        readonly int _number;
+
         public NumberExpression(int i)
         {
-            number = i;
+            _number = i;
         }
 
         int IExpression.Interpret()
         {
-            return number;
+            return _number;
         }
     }
 }

Interpreter/Expression/SubtractExpression.cs

@@ -2,18 +2,18 @@
 {
     public class SubtractExpression : IExpression
     {
-        IExpression leftExpression;
-        IExpression rightExpression;
+        readonly IExpression _leftExpression;
+        readonly IExpression _rightExpression;
 
         public SubtractExpression(IExpression left, IExpression right)
         {
-            leftExpression = left;
-            rightExpression = right;
+            _leftExpression = left;
+            _rightExpression = right;
         }
 
         int IExpression.Interpret()
         {
-            return leftExpression.Interpret() - rightExpression.Interpret();
+            return _leftExpression.Interpret() - _rightExpression.Interpret();
         }
     }
 }