Merge pull request #131 from AndrewSav/where-message

Added optional error message to the Where combinator

Author: nblumhardt

src/Superpower/Combinators.cs

@@ -1034,11 +1034,13 @@ public static TextParser<U> Value<T, U>(this TextParser<T> parser, U value)
         /// <typeparam name="T">The type of value being parsed.</typeparam>
         /// <param name="parser">The parser.</param>
         /// <param name="predicate">The predicate to apply.</param>
+        /// <param name="message">An optional error message when parsing fails.</param>
         /// <returns>The resulting parser.</returns>
-        public static TokenListParser<TKind, T> Where<TKind, T>(this TokenListParser<TKind, T> parser, Func<T, bool> predicate)
+        public static TokenListParser<TKind, T> Where<TKind, T>(this TokenListParser<TKind, T> parser, Func<T, bool> predicate, string message = "unsatisfied condition")
         {
             if (parser == null) throw new ArgumentNullException(nameof(parser));
             if (predicate == null) throw new ArgumentNullException(nameof(predicate));
+            if (message == null) throw new ArgumentNullException(nameof(message));
 
             return input =>
             {
@@ -1049,7 +1051,7 @@ public static TokenListParser<TKind, T> Where<TKind, T>(this TokenListParser<TKi
                 if (predicate(rt.Value))
                     return rt;
 
-                return TokenListParserResult.Empty<TKind, T>(input, "unsatisfied condition");
+                return TokenListParserResult.Empty<TKind, T>(input, message);
             };
         }
 
@@ -1060,11 +1062,13 @@ public static TokenListParser<TKind, T> Where<TKind, T>(this TokenListParser<TKi
         /// <typeparam name="T">The type of value being parsed.</typeparam>
         /// <param name="parser">The parser.</param>
         /// <param name="predicate">The predicate to apply.</param>
+        /// <param name="message">An optional error message when parsing fails.</param>
         /// <returns>The resulting parser.</returns>
-        public static TextParser<T> Where<T>(this TextParser<T> parser, Func<T, bool> predicate)
+        public static TextParser<T> Where<T>(this TextParser<T> parser, Func<T, bool> predicate, string message = "unsatisfied condition")
         {
             if (parser == null) throw new ArgumentNullException(nameof(parser));
             if (predicate == null) throw new ArgumentNullException(nameof(predicate));
+            if (message == null) throw new ArgumentNullException(nameof(message));
 
             return input =>
             {
@@ -1075,7 +1079,7 @@ public static TextParser<T> Where<T>(this TextParser<T> parser, Func<T, bool> pr
                 if (predicate(rt.Value))
                     return rt;
 
-                return Result.Empty<T>(input, "unsatisfied condition");
+                return Result.Empty<T>(input, message);
             };
         } 
 

test/Superpower.Tests/Combinators/WhereCombinatorTests.cs

@@ -0,0 +1,30 @@
+using Superpower.Parsers;
+using Superpower.Tests.Support;
+using Xunit;
+
+namespace Superpower.Tests.Combinators
+{
+    public class WhereCombinatorTests
+    {
+        [Fact]
+        public void WhereFailsIfPrecedingParserFails()
+        {
+            AssertParser.Fails(Character.EqualTo('a').Where(_ => true), "b");
+        }
+
+        [Fact]
+        public void WhereSucceedsWhenPredicateMatches()
+        {
+            AssertParser.SucceedsWith(Character.EqualTo('a').Where(a => a == 'a'), "a", 'a');
+        }
+
+        [Fact]
+        public void WhereFailsWhenPredicateDoesNotMatch()
+        {
+            AssertParser.FailsWithMessage(
+                Character.EqualTo('a').Where(a => a != 'a', "character should be an A"),
+                "a",
+                "Syntax error (line 1, column 1): character should be an A.");
+        }
+    }
+}