Fix #5: validate lambda expression upon construction

CHANGELOG.txt

@@ -1,3 +1,9 @@
+------------------------------------------------------------------------------
+  qSharp 2.0.7 [TBA]
+------------------------------------------------------------------------------
+
+  - Fix: validate lambda expression upon construction
+
 ------------------------------------------------------------------------------
   qSharp 2.0.6 [2014.09.11]
 ------------------------------------------------------------------------------

qSharp/src/QLambda.cs

@@ -28,13 +28,26 @@ public sealed class QLambda
 
         /// <summary>
         ///     Creates new QLambda instance with given body and parameters.
+        ///     Note that expression is trimmed and required to be enclosed in { and } brackets.
         /// </summary>
         public QLambda(string expression, Array parameters = null)
         {
             if (expression == null)
             {
                 throw new ArgumentException("Lambda expression cannot be null");
             }
+            expression = expression.Trim();
+
+            if (expression.Length == 0)
+            {
+                throw new ArgumentException("Lambda expression cannot be empty");
+            }
+
+            if (expression[0] != '{' || expression[expression.Length - 1] != '}')
+            {
+                throw new ArgumentException("Lambda expression is expected to be enclosed in {} brackets");
+            }
+
             this.expression = expression;
             this.parameters = parameters;
         }