Merge pull request #45 from messerli-informatik-ag/option-where

Add Option.Where

Author: FreeApophis

Funcky.Test/OptionTest.cs

@@ -199,6 +199,32 @@ public static IEnumerable<object[]> TestValues()
             yield return new object[] { "None", Option<MyEnum>.None() };
         }
 
+        [Theory]
+        [MemberData(nameof(WhereTestValues))]
+        public void WhereFiltersOptionCorrectly(Option<string> expectedResult, Option<string> input, Func<string, bool> predicate)
+        {
+            Assert.Equal(expectedResult, input.Where(predicate));
+        }
+
+        [Theory]
+        [MemberData(nameof(WhereTestValues))]
+        public void OptionSupportsLinqWhereSyntax(Option<string> expectedResult, Option<string> input, Func<string, bool> predicate)
+        {
+            var result = from x in input
+                where predicate(x)
+                select x;
+            Assert.Equal(expectedResult, result);
+        }
+
+        public static TheoryData<Option<string>, Option<string>, Func<string, bool>> WhereTestValues()
+            => new TheoryData<Option<string>, Option<string>, Func<string, bool>>
+            {
+                { Option.Some("foo"), Option.Some("foo"), True },
+                { Option<string>.None(), Option.Some("foo"), False },
+                { Option<string>.None(), Option<string>.None(), True },
+                { Option<string>.None(), Option<string>.None(), False },
+            };
+
         [Theory]
         [MemberData(nameof(TestValues))]
 

Funcky/Monads/Option.cs

@@ -50,6 +50,9 @@ public Option<TResult> SelectMany<TMaybe, TResult>(Func<TItem, Option<TMaybe>> m
             return Option<TResult>.None();
         }
 
+        public Option<TItem> Where(Func<TItem, bool> predicate)
+            => AndThen(item => predicate(item) ? Option.Some(item) : None());
+
         public TResult Match<TResult>(TResult none, Func<TItem, TResult> some)
             => _hasItem
                 ? some(_item)

changelog.md

@@ -15,3 +15,4 @@
 
 ## Unreleased
 * Added overload for AndThen which flattens the Option
+* Add `Where` method to `Option<T>`, which allows filtering the `Option` by a predicate.