Add XDelimitedBy for better error message on failure.

Author: xoposhiy

src/Sprache.Tests/ParseTests.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using NUnit.Framework;
 
@@ -357,6 +358,17 @@ public void CanParseSequence()
             Assert.IsTrue(r.Remainder.AtEnd);
         }
 
+        [Test]
+        public void FailGracefullyOnSequence()
+        {
+            var sequence = Parse.Char('a').XDelimitedBy(Parse.Char(','));
+            AssertParser.FailsWith(sequence, "a,a,b", result =>
+            {
+                StringAssert.Contains("unexpected 'b'", result.Message);
+                CollectionAssert.Contains(result.Expectations, "a");
+            });
+        }
+
         [Test]
         public void CanParseContained()
         {

src/Sprache/Parse.Sequence.cs

@@ -28,6 +28,28 @@ from item in parser
                    select head.Concat(tail);
         }
 
+        /// <summary>
+        /// Fails on the first itemParser failure, if it reads at least one character.
+        /// </summary>
+        /// <param name="itemParser"></param>
+        /// <param name="delimiter"></param>
+        /// <typeparam name="T"></typeparam>
+        /// <typeparam name="U"></typeparam>
+        /// <returns></returns>
+        /// <exception cref="ArgumentNullException"></exception>
+        public static Parser<IEnumerable<T>> XDelimitedBy<T, U>(this Parser<T> itemParser, Parser<U> delimiter)
+        {
+            if (itemParser == null) throw new ArgumentNullException("itemParser");
+            if (delimiter == null) throw new ArgumentNullException("delimiter");
+
+            return from head in itemParser.Once()
+                   from tail in
+                       (from separator in delimiter
+                        from item in itemParser
+                        select item).XMany()
+                   select head.Concat(tail);
+        }
+
         /// <summary>
         /// 
         /// </summary>