Separated expression string processing to its own method and created basic REPL

Program.cs

@@ -8,11 +8,25 @@ namespace calculator
     static void Main(string[] args)
     {
       if (args.Length == 0)
-        Console.WriteLine("REPL is not ready yet. Please, provide mathematic expression as an argument");
+      {
+        string input = readREPL();
+        while (input != null)
+        {
+          writeREPL(evaluate(input));
+          input = readREPL();
+        }
+      }
       else
+      {
+        writeREPL(evaluate(args[0]));
+      }
+    }
+
+    static private double evaluate(string s)
+    {
       try
       {
-          string[] expr = args[0].Split();
+        string[] expr = s.Split();
         Stack<double> st = new Stack<double>();
 
         for (int i = 0; i < expr.Length; i++)
@@ -56,11 +70,13 @@ namespace calculator
           else throw new Exception("Stack corrupted");
         }
 
-          Console.WriteLine(st.Pop());
+        return st.Pop();
       }
       catch (Exception e)
       {
         Console.WriteLine(e.Message);
+        Environment.Exit(1);
+        return -1;
       }
     }
 
@@ -71,5 +87,29 @@ namespace calculator
       else if (s.Length == 1) return s[0];
       else throw new Exception("Got multiple characters operator");
     }
+
+    static private string readREPL()
+    {
+      Console.Write(">> ");
+      string input = Console.ReadLine();
+
+      switch (input)
+      {
+        case "exit":
+        case "exit()":
+        case "quit":
+        case "quit()":
+        case "q":
+        case "":
+          return null;
+        default:
+          return input;
+      }
+    }
+
+    static private void writeREPL<T>(T o)
+    {
+      Console.WriteLine(o);
+    }
   }
 }