Added extension JustWhen to Maybe<T> (#7)

bridgefield · May 2, 2021 · 3472ef1 · 3472ef1
1 parent 7a9479f
commit 3472ef1
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Showing 2 changed files with 29 additions and 2 deletions.
diff --git a/MonadicBits/MaybeExtensions.cs b/MonadicBits/MaybeExtensions.cs
@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 
 namespace MonadicBits
@@ -12,5 +13,12 @@ public static Maybe<T> JustNotNull<T>(this T value) =>
 
         public static Maybe<T> FirstOrNothing<T>(this IEnumerable<T> source) =>
             source.Select(value => value.Just()).DefaultIfEmpty(Maybe<T>.Nothing()).First();
+
+        public static Maybe<T> JustWhen<T>(this T value, Func<T, bool> predicate)
+        {
+            if (predicate == null) throw new ArgumentNullException(nameof(predicate));
+
+            return predicate(value) ? Maybe<T>.Just(value) : Maybe<T>.Nothing();
+        }
     }
 }
diff --git a/MonadicBitsTests/MaybeExtensionsTests.cs b/MonadicBitsTests/MaybeExtensionsTests.cs
@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using MonadicBits;
 using NUnit.Framework;
 
@@ -34,5 +35,23 @@ public static void FirstOrNothing_with_list_of_values_returns_maybe_of_first_val
         [Test]
         public static void FirstOrNothing_with_empty_list_returns_nothing() =>
             new List<int>().FirstOrNothing().Match(_ => Assert.Fail(), Assert.Pass);
+
+        [Test]
+        public static void JustWhen_with_null_predicate_throws_exception() =>
+            Assert.Throws<ArgumentNullException>(() => "Test".JustWhen(null));
+
+        [Test]
+        public static void JustWhen_with_false_predicate_returns_nothing()
+        {
+            const string input = "Test";
+            input.JustWhen(s => s.Length == input.Length + 1).Match(_ => Assert.Fail(), Assert.Pass);
+        }
+
+        [Test]
+        public static void JustWhen_with_true_predicate_returns_just()
+        {
+            const string input = "Test";
+            input.JustWhen(s => s.Length == input.Length).Match(_ => Assert.Pass(), Assert.Fail);
+        }
     }
 }