Added Flatten method to unwrap the optional

Author: sakno

src/DotNext.Tests/OptionalTests.cs

@@ -352,4 +352,11 @@ public static void ConcatTwoValues()
         optional = Optional<int>.None.Concat<long>(42L);
         False(optional.HasValue);
     }
+
+    [Fact]
+    public static async Task FlattenTask()
+    {
+        await ThrowsAsync<InvalidOperationException>(static () => Task.FromResult(Optional.None<int>()).Flatten());
+        Equal(42, await Task.FromResult(Optional.Some(42)).Flatten());
+    }
 }

src/DotNext/Optional.cs

@@ -36,6 +36,16 @@ public static class Optional
     public static async Task<T?> Or<T>(this Task<Optional<T>> task, T? defaultValue)
         => (await task.ConfigureAwait(false)).Or(defaultValue);
 
+    /// <summary>
+    /// Returns a task that contains unwrapped value; or exception if <see cref="Optional{T}"/> has no value.
+    /// </summary>
+    /// <param name="task">The task representing optional value.</param>
+    /// <typeparam name="T">The type of the value.</typeparam>
+    /// <returns>The task containing a value of type <typeparamref name="T"/>; or the exception if <see cref="Optional{T}"/> has no value.</returns>
+    /// <exception cref="InvalidOperationException">No value is present.</exception>
+    public static async Task<T> Flatten<T>(this Task<Optional<T>> task)
+        => (await task.ConfigureAwait(false)).Value;
+
     /// <summary>
     /// If a value is present, apply the provided mapping function to it, and if the result is
     /// non-null, return an Optional describing the result. Otherwise, returns <see cref="Optional{T}.None"/>.