From f753f1a30cf57c78a88e36182440b9b4c0d5763a Mon Sep 17 00:00:00 2001
From: Atif Aziz <code@raboof.com>
Date: Fri, 23 Dec 2016 12:43:45 +0100
Subject: [PATCH] Better integration of Option<T> with string formatting

Closes #20
---
 Optional.Tests/MaybeTests.cs | 34 +++++++++++++++++++++++++++++++++-
 Optional/Option_Maybe.cs     | 11 +++++++++--
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/Optional.Tests/MaybeTests.cs b/Optional.Tests/MaybeTests.cs
index 29d12e3..548606c 100644
--- a/Optional.Tests/MaybeTests.cs
+++ b/Optional.Tests/MaybeTests.cs
@@ -1,6 +1,7 @@
 using System;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using System.IO;
 using System.Runtime.Serialization.Formatters.Binary;
@@ -166,7 +167,38 @@ public void Maybe_StringRepresentation()
         }
 
         [TestMethod]
-        public void Maybe_GetValue()
+        public void Maybe_Format()
+        {
+            Assert.AreEqual("", ((IFormattable) Option.None<int>()).ToString(null, null));
+            Assert.AreEqual("", ((IFormattable) Option.None<int?>()).ToString(null, null));
+            Assert.AreEqual("", ((IFormattable) Option.None<string>()).ToString(null, null));
+
+            Assert.AreEqual("", ((IFormattable) Option.Some<int?>(null)).ToString(null, null));
+            Assert.AreEqual("", ((IFormattable) Option.Some<string>(null)).ToString(null, null));
+
+            // Test number formatting using a culture that uses a different
+            // separator for grouping than the current culture.
+
+            var numberFormatProvider =
+                CultureInfo.GetCultures(CultureTypes.SpecificCultures)
+                           .First(ci => ci.NumberFormat.NumberGroupSeparator != NumberFormatInfo.CurrentInfo.NumberGroupSeparator);
+
+            Assert.AreEqual(int.MaxValue.ToString("N0", numberFormatProvider), ((IFormattable) Option.Some<int>(int.MaxValue)).ToString("N0", numberFormatProvider));
+            Assert.AreEqual(int.MaxValue.ToString("N0", numberFormatProvider), ((IFormattable) Option.Some<int?>(int.MaxValue)).ToString("N0", numberFormatProvider));
+
+            // Test date formatting using a culture that would yield a
+            // different result than the current culture.
+
+            var now = DateTime.Now;
+            var dateFormatProvider =
+                CultureInfo.GetCultures(CultureTypes.SpecificCultures)
+                           .First(ci => now.ToString("D", ci) != now.ToLongDateString());
+
+            Assert.AreEqual(now.ToString("D", dateFormatProvider), ((IFormattable) Option.Some<DateTime>(now)).ToString("D", dateFormatProvider));
+        }
+
+        [TestMethod]
+    public void Maybe_GetValue()
         {
             var noneStruct = Option.None<int>();
             var noneNullable = Option.None<int?>();
diff --git a/Optional/Option_Maybe.cs b/Optional/Option_Maybe.cs
index f1d7417..7a65ce9 100644
--- a/Optional/Option_Maybe.cs
+++ b/Optional/Option_Maybe.cs
@@ -14,7 +14,7 @@ namespace Optional
     [Serializable]
 #endif
     [DebuggerDisplay("{" + nameof(DebugString) + "}")]
-    public struct Option<T> : IEquatable<Option<T>>
+    public struct Option<T> : IEquatable<Option<T>>, IFormattable
     {
         private readonly bool hasValue;
         private readonly T value;
@@ -391,5 +391,12 @@ public Option<T> Filter(Func<T, bool> predicate)
         /// </summary>
         /// <returns>The filtered optional.</returns>
         public Option<T> NotNull() => hasValue && value == null ? Option.None<T>() : this;
-    }
+
+        string IFormattable.ToString(string format, IFormatProvider formatProvider)
+        {
+            return hasValue
+                 ? string.Format(formatProvider, "{0:" + format + "}", value)
+                 : string.Empty;
+        }
+}
 }