diff --git a/.swift-version b/.swift-version
index 9f55b2c..5186d07 100644
--- a/.swift-version
+++ b/.swift-version
@@ -1 +1 @@
-3.0
+4.0
diff --git a/.travis.yml b/.travis.yml
index cfa256b..d532a53 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode8
+osx_image: xcode9
 script:
   - set -o pipefail
   - xcodebuild test -scheme OptionalExtensions-Mac | xcpretty -c
diff --git a/OptionalExtensions.xcodeproj/project.pbxproj b/OptionalExtensions.xcodeproj/project.pbxproj
index bfea69c..3f7d8d2 100644
--- a/OptionalExtensions.xcodeproj/project.pbxproj
+++ b/OptionalExtensions.xcodeproj/project.pbxproj
@@ -822,7 +822,7 @@
 				PRODUCT_NAME = OptionalExtensions;
 				SKIP_INSTALL = YES;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};
@@ -841,7 +841,7 @@
 				PRODUCT_NAME = OptionalExtensions;
 				SKIP_INSTALL = YES;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Release;
 		};
diff --git a/OptionalExtensions/Source/OptionalExtensions.swift b/OptionalExtensions/Source/OptionalExtensions.swift
index c1ded18..4dc469d 100644
--- a/OptionalExtensions/Source/OptionalExtensions.swift
+++ b/OptionalExtensions/Source/OptionalExtensions.swift
@@ -12,11 +12,11 @@ public extension Optional {
         return map(predicate) == .some(true) ? self : .none
     }
 
-    func mapNil(_ predicate: (Void) -> Wrapped) -> Optional {
+    func mapNil(_ predicate: () -> Wrapped) -> Optional {
         return self ?? .some(predicate())
     }
 
-    func flatMapNil(_ predicate: (Void) -> Optional) -> Optional {
+    func flatMapNil(_ predicate: () -> Optional) -> Optional {
         return self ?? predicate()
     }
 
@@ -33,7 +33,7 @@ public extension Optional {
         return self
     }
 
-    func onNone(_ f: (Void) -> Void) -> Optional {
+    func onNone(_ f: () -> Void) -> Optional {
         if isNone { f() }
         return self
     }