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

Program.cs

@@ -8,60 +8,76 @@ 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");
+      {
-      else
+        string input = readREPL();
-        try
+        while (input != null)
         {
-          string[] expr = args[0].Split();
+          writeREPL(evaluate(input));
-          Stack<double> st = new Stack<double>();
+          input = readREPL();
+        }
+      }
+      else
+      {
+        writeREPL(evaluate(args[0]));
+      }
+    }
 
-          for (int i = 0; i < expr.Length; i++)
+    static private double evaluate(string s)
+    {
+      try
+      {
+        string[] expr = s.Split();
+        Stack<double> st = new Stack<double>();
+
+        for (int i = 0; i < expr.Length; i++)
+        {
+          object val = convertOne(expr[i]);
+
+          if (val is double number)
+            st.Push(number);
+          else if (val is char oper)
           {
-            object val = convertOne(expr[i]);
+            try
-
-            if (val is double number)
-              st.Push(number);
-            else if (val is char oper)
             {
-              try
+              double b = st.Pop();
+              double a = st.Pop();
+              switch (oper)
               {
-                double b = st.Pop();
+                case '+':
-                double a = st.Pop();
+                  st.Push(a + b);
-                switch (oper)
+                  break;
-                {
+                case '-':
-                  case '+':
+                  st.Push(a - b);
-                    st.Push(a + b);
+                  break;
-                    break;
+                case '*':
-                  case '-':
+                  st.Push(a * b);
-                    st.Push(a - b);
+                  break;
-                    break;
+                case '/':
-                  case '*':
+                  st.Push(a / b);
-                    st.Push(a * b);
+                  break;
-                    break;
+                case '^':
-                  case '/':
+                  st.Push(Math.Pow(a, b));
-                    st.Push(a / b);
+                  break;
-                    break;
+                default:
-                  case '^':
+                  throw new Exception("Got unknown operator");
-                    st.Push(Math.Pow(a, b));
-                    break;
-                  default:
-                    throw new Exception("Got unknown operator");
-                }
-              }
-              catch (InvalidOperationException)
-              {
-                throw new Exception(oper + " operation requires two operands");
               }
             }
-            else throw new Exception("Stack corrupted");
+            catch (InvalidOperationException)
+            {
+              throw new Exception(oper + " operation requires two operands");
+            }
           }
+          else throw new Exception("Stack corrupted");
+        }
 
-          Console.WriteLine(st.Pop());
+        return st.Pop();
-        }
+      }
-        catch (Exception e)
+      catch (Exception e)
-        {
+      {
-          Console.WriteLine(e.Message);
+        Console.WriteLine(e.Message);
-        }
+        Environment.Exit(1);
+        return -1;
+      }
     }
 
     static private object convertOne(string s)
@@ -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);
+    }
   }
 }