diff --git a/psalm-baseline.xml b/psalm-baseline.xml
index 917586c..c24c750 100644
--- a/psalm-baseline.xml
+++ b/psalm-baseline.xml
@@ -13,7 +13,7 @@
       <code>$route</code>
     </MissingClosureParamType>
     <MissingClosureReturnType occurrences="1">
-      <code>function ($matched, $route) use ($path, $method) {</code>
+      <code>static function ($matched, $route) use ($path, $method) {</code>
     </MissingClosureReturnType>
     <MixedArgument occurrences="11">
       <code>$allowedMethods</code>
@@ -114,10 +114,7 @@
   </file>
   <file src="test/FastRouteRouterFactoryTest.php">
     <MissingClosureReturnType occurrences="4">
-      <code>function () {</code>
-      <code>function () {</code>
-      <code>function () {</code>
-      <code>function () {</code>
+      <code>fn()</code>
     </MissingClosureReturnType>
     <MissingPropertyType occurrences="1">
       <code>$container</code>
@@ -150,9 +147,7 @@
   </file>
   <file src="test/FastRouteRouterTest.php">
     <MissingClosureReturnType occurrences="3">
-      <code>function () {</code>
-      <code>function () {</code>
-      <code>function () {</code>
+      <code>fn()</code>
     </MissingClosureReturnType>
     <MissingParamType occurrences="2">
       <code>$method</code>
@@ -279,8 +274,8 @@
     </UndefinedThisPropertyFetch>
   </file>
   <file src="test/UriGeneratorTest.php">
-    <MissingClosureReturnType occurrences="1">
-      <code>function () {</code>
+    <MissingClosureReturnType occurrences="2">
+      <code>fn()</code>
     </MissingClosureReturnType>
     <MissingParamType occurrences="4">
       <code>$expected</code>
diff --git a/src/FastRouteRouter.php b/src/FastRouteRouter.php
index 04e0c27..714ad65 100644
--- a/src/FastRouteRouter.php
+++ b/src/FastRouteRouter.php
@@ -18,6 +18,7 @@
 use function array_reduce;
 use function array_reverse;
 use function array_unique;
+use function assert;
 use function dirname;
 use function file_exists;
 use function file_put_contents;
@@ -74,56 +75,46 @@ class FastRouteRouter implements RouterInterface
 
     /**
      * Cache generated route data?
-     *
-     * @var bool
      */
-    private $cacheEnabled = false;
+    private bool $cacheEnabled = false;
 
     /**
      * Cache file path relative to the project directory.
-     *
-     * @var string
      */
-    private $cacheFile = 'data/cache/fastroute.php.cache';
+    private string $cacheFile = 'data/cache/fastroute.php.cache';
 
     /** @var callable A factory callback that can return a dispatcher. */
     private $dispatcherCallback;
 
     /**
      * Cached data used by the dispatcher.
-     *
-     * @var array
      */
-    private $dispatchData = [];
+    private array $dispatchData = [];
 
     /**
      * True if cache is enabled and valid dispatch data has been loaded from
      * cache.
-     *
-     * @var bool
      */
-    private $hasCache = false;
+    private bool $hasCache = false;
 
     /**
      * FastRoute router
-     *
-     * @var RouteCollector
      */
-    private $router;
+    private ?RouteCollector $router = null;
 
     /**
      * All attached routes as Route instances
      *
      * @var Route[]
      */
-    private $routes = [];
+    private array $routes = [];
 
     /**
      * Routes to inject into the underlying RouteCollector.
      *
      * @var Route[]
      */
-    private $routesToInject = [];
+    private array $routesToInject = [];
 
     /**
      * Constructor
@@ -361,9 +352,7 @@ private function getDispatcher($data): Dispatcher
      */
     private function createDispatcherCallback(): callable
     {
-        return function ($data) {
-            return new GroupCountBased($data);
-        };
+        return static fn($data): GroupCountBased => new GroupCountBased($data);
     }
 
     /**
@@ -387,7 +376,7 @@ private function marshalFailedRoute(array $result): RouteResult
     private function marshalMatchedRoute(array $result, string $method): RouteResult
     {
         $path  = $result[1];
-        $route = array_reduce($this->routes, function ($matched, $route) use ($path, $method) {
+        $route = array_reduce($this->routes, static function ($matched, $route) use ($path, $method) {
             if ($matched) {
                 return $matched;
             }
@@ -447,6 +436,8 @@ private function injectRoute(Route $route): void
             $methods = self::HTTP_METHODS_STANDARD;
         }
 
+        assert($this->router instanceof RouteCollector);
+
         $this->router->addRoute($methods, $route->getPath(), $route->getPath());
     }
 
@@ -462,6 +453,8 @@ private function getDispatchData(): array
             return $this->dispatchData;
         }
 
+        assert($this->router instanceof RouteCollector);
+
         $dispatchData = (array) $this->router->getData();
 
         if ($this->cacheEnabled) {
@@ -479,7 +472,7 @@ private function getDispatchData(): array
      */
     private function loadDispatchData(): void
     {
-        set_error_handler(function () {
+        set_error_handler(static function (): void {
         }, E_WARNING); // suppress php warnings
         $dispatchData = include $this->cacheFile;
         restore_error_handler();
@@ -545,7 +538,7 @@ private function cacheDispatchData(array $dispatchData)
     private function marshalMethodNotAllowedResult(array $result): RouteResult
     {
         $path           = $result[1];
-        $allowedMethods = array_reduce($this->routes, function ($allowedMethods, $route) use ($path) {
+        $allowedMethods = array_reduce($this->routes, static function ($allowedMethods, $route) use ($path) {
             if ($path !== $route->getPath()) {
                 return $allowedMethods;
             }
diff --git a/test/FastRouteRouter/ConfigProviderTest.php b/test/FastRouteRouter/ConfigProviderTest.php
index 0daa327..be7fc2e 100644
--- a/test/FastRouteRouter/ConfigProviderTest.php
+++ b/test/FastRouteRouter/ConfigProviderTest.php
@@ -11,8 +11,7 @@
 
 class ConfigProviderTest extends TestCase
 {
-    /** @var ConfigProvider */
-    private $provider;
+    private ConfigProvider $provider;
 
     protected function setUp(): void
     {
diff --git a/test/FastRouteRouterFactoryTest.php b/test/FastRouteRouterFactoryTest.php
index c5b193a..4d630fd 100644
--- a/test/FastRouteRouterFactoryTest.php
+++ b/test/FastRouteRouterFactoryTest.php
@@ -16,11 +16,10 @@ class FastRouteRouterFactoryTest extends TestCase
 {
     use ProphecyTrait;
 
-    /** @var FastRouteRouterFactory */
-    private $factory;
+    private FastRouteRouterFactory $factory;
 
     /** @var ObjectProphecy<ContainerInterface> */
-    private $container;
+    private ObjectProphecy $container;
 
     protected function setUp(): void
     {
@@ -35,14 +34,10 @@ public function testCreatesRouterWithEmptyConfig()
         $router = ($this->factory)($this->container->reveal());
 
         $this->assertInstanceOf(FastRouteRouter::class, $router);
-        $cacheEnabled = Closure::bind(function () {
-            return $this->cacheEnabled;
-        }, $router, FastRouteRouter::class)();
+        $cacheEnabled = Closure::bind(fn() => $this->cacheEnabled, $router, FastRouteRouter::class)();
         $this->assertFalse($cacheEnabled);
 
-        $cacheFile = Closure::bind(function () {
-            return $this->cacheFile;
-        }, $router, FastRouteRouter::class)();
+        $cacheFile = Closure::bind(fn() => $this->cacheFile, $router, FastRouteRouter::class)();
         $this->assertSame('data/cache/fastroute.php.cache', $cacheFile);
     }
 
@@ -62,14 +57,10 @@ public function testCreatesRouterWithConfig()
 
         $this->assertInstanceOf(FastRouteRouter::class, $router);
 
-        $cacheEnabled = Closure::bind(function () {
-            return $this->cacheEnabled;
-        }, $router, FastRouteRouter::class)();
+        $cacheEnabled = Closure::bind(fn() => $this->cacheEnabled, $router, FastRouteRouter::class)();
         $this->assertTrue($cacheEnabled);
 
-        $cacheFile = Closure::bind(function () {
-            return $this->cacheFile;
-        }, $router, FastRouteRouter::class)();
+        $cacheFile = Closure::bind(fn() => $this->cacheFile, $router, FastRouteRouter::class)();
         $this->assertSame('/foo/bar/file-cache', $cacheFile);
     }
 }
diff --git a/test/FastRouteRouterTest.php b/test/FastRouteRouterTest.php
index e996da9..2612f94 100644
--- a/test/FastRouteRouterTest.php
+++ b/test/FastRouteRouterTest.php
@@ -45,9 +45,7 @@ protected function setUp(): void
     {
         $this->fastRouter       = $this->prophesize(RouteCollector::class);
         $this->dispatcher       = $this->prophesize(Dispatcher::class);
-        $this->dispatchCallback = function () {
-            return $this->dispatcher->reveal();
-        };
+        $this->dispatchCallback = fn() => $this->dispatcher->reveal();
     }
 
     private function getRouter(): FastRouteRouter
@@ -66,9 +64,7 @@ private function getMiddleware(): MiddlewareInterface
     public function testWillLazyInstantiateAFastRouteCollectorIfNoneIsProvidedToConstructor(): void
     {
         $router         = new FastRouteRouter();
-        $routeCollector = Closure::bind(function () {
-            return $this->router;
-        }, $router, FastRouteRouter::class)();
+        $routeCollector = Closure::bind(fn() => $this->router, $router, FastRouteRouter::class)();
 
         $this->assertInstanceOf(RouteCollector::class, $routeCollector);
     }
@@ -79,9 +75,7 @@ public function testAddingRouteAggregatesRoute(): void
         $router = $this->getRouter();
         $router->addRoute($route);
 
-        $routesToInject = Closure::bind(function () {
-            return $this->routesToInject;
-        }, $router, FastRouteRouter::class)();
+        $routesToInject = Closure::bind(fn() => $this->routesToInject, $router, FastRouteRouter::class)();
         $this->assertContains($route, $routesToInject);
     }
 
@@ -104,9 +98,7 @@ public function testMatchingInjectsRouteIntoFastRoute(): void
         $uri->getPath()->willReturn('/foo');
 
         $request = $this->prophesize(ServerRequestInterface::class);
-        $request->getUri()->will(function () use ($uri) {
-            return $uri->reveal();
-        });
+        $request->getUri()->will(fn(): object => $uri->reveal());
         $request->getMethod()->willReturn(RequestMethod::METHOD_GET);
 
         $router->match($request->reveal());
@@ -636,9 +628,7 @@ public function createServerRequestProphecy(
         $uri->getPath()->willReturn($path);
 
         $request = $this->prophesize(ServerRequestInterface::class);
-        $request->getUri()->will(function () use ($uri) {
-            return $uri->reveal();
-        });
+        $request->getUri()->will(fn(): object => $uri->reveal());
 
         $request->getMethod()->willReturn($method);
 
diff --git a/test/UriGeneratorTest.php b/test/UriGeneratorTest.php
index f0427f5..587cf03 100644
--- a/test/UriGeneratorTest.php
+++ b/test/UriGeneratorTest.php
@@ -28,8 +28,7 @@ class UriGeneratorTest extends TestCase
     /** @var callable */
     private $dispatchCallback;
 
-    /** @var FastRouteRouter */
-    private $router;
+    private FastRouteRouter $router;
 
     /**
      * Test routes taken from https://github.com/nikic/FastRoute/blob/master/test/RouteParser/StdTest.php
@@ -70,7 +69,7 @@ public function provideRouteTests(): array
             ['/test/{param:\d+}', ['param' => 1], '/test/1'],
             //['/test/{param:\d+}', ['param' => 'foo'], 'exception', null],
             ['/test/{ param : \d{1,9} }', ['param' => 1], '/test/1'],
-            ['/test/{ param : \d{1,9} }', ['param' => 123456789], '/test/123456789'],
+            ['/test/{ param : \d{1,9} }', ['param' => 123_456_789], '/test/123456789'],
             ['/test/{ param : \d{1,9} }', ['param' => 0], '/test/0'],
             ['/test[opt]', [], '/testopt'],
             ['/test[/{param}]', [], '/test'],
@@ -106,7 +105,7 @@ public function exceptionalRoutes(): iterable
             ],
             [
                 '/test/{ param : \d{1,9} }',
-                ['param' => 1234567890],
+                ['param' => 1_234_567_890],
                 RuntimeException::class,
                 'Parameter value for [param] did not match the regex `\d{1,9}`',
             ],
@@ -123,9 +122,7 @@ protected function setUp(): void
     {
         $this->fastRouter       = $this->prophesize(RouteCollector::class);
         $this->dispatcher       = $this->prophesize(Dispatcher::class);
-        $this->dispatchCallback = function () {
-            return $this->dispatcher->reveal();
-        };
+        $this->dispatchCallback = fn() => $this->dispatcher->reveal();
 
         $this->router = new FastRouteRouter(
             $this->fastRouter->reveal(),