diff --git a/composer.json b/composer.json
index 663bbb2..8fe68fb 100644
--- a/composer.json
+++ b/composer.json
@@ -6,7 +6,8 @@
 		"php": ">=7.4"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "9.*"
+		"phpunit/phpunit": "9.*",
+		"phpstan/phpstan": ">=0.12.42"
 	},
 
 	"autoload": {
diff --git a/composer.lock b/composer.lock
index 4afeffb..0d5a2ca 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "c738cc31376e09de8edb72c7039476dc",
+    "content-hash": "d8a937720c006b21dc286e5111bfcf13",
     "packages": [],
     "packages-dev": [
         {
@@ -490,6 +490,62 @@
             ],
             "time": "2020-12-19T10:15:11+00:00"
         },
+        {
+            "name": "phpstan/phpstan",
+            "version": "0.12.71",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/phpstan.git",
+                "reference": "d508fa3b0ecc5fc91ac70c6c7ac2862f968ba2b5"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan/zipball/d508fa3b0ecc5fc91ac70c6c7ac2862f968ba2b5",
+                "reference": "d508fa3b0ecc5fc91ac70c6c7ac2862f968ba2b5",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.1|^8.0"
+            },
+            "conflict": {
+                "phpstan/phpstan-shim": "*"
+            },
+            "bin": [
+                "phpstan",
+                "phpstan.phar"
+            ],
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "0.12-dev"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "PHPStan - PHP Static Analysis Tool",
+            "funding": [
+                {
+                    "url": "https://github.com/ondrejmirtes",
+                    "type": "github"
+                },
+                {
+                    "url": "https://www.patreon.com/phpstan",
+                    "type": "patreon"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/phpstan/phpstan",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2021-02-01T18:24:00+00:00"
+        },
         {
             "name": "phpunit/php-code-coverage",
             "version": "9.2.5",
@@ -790,16 +846,16 @@
         },
         {
             "name": "phpunit/phpunit",
-            "version": "9.5.0",
+            "version": "9.5.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/phpunit.git",
-                "reference": "8e16c225d57c3d6808014df6b1dd7598d0a5bbbe"
+                "reference": "f661659747f2f87f9e72095bb207bceb0f151cb4"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/phpunit/zipball/8e16c225d57c3d6808014df6b1dd7598d0a5bbbe",
-                "reference": "8e16c225d57c3d6808014df6b1dd7598d0a5bbbe",
+                "url": "https://api.github.com/repos/sebastianbergmann/phpunit/zipball/f661659747f2f87f9e72095bb207bceb0f151cb4",
+                "reference": "f661659747f2f87f9e72095bb207bceb0f151cb4",
                 "shasum": ""
             },
             "require": {
@@ -885,7 +941,7 @@
                     "type": "github"
                 }
             ],
-            "time": "2020-12-04T05:05:53+00:00"
+            "time": "2021-02-02T14:45:58+00:00"
         },
         {
             "name": "sebastian/cli-parser",
@@ -1789,16 +1845,16 @@
         },
         {
             "name": "symfony/polyfill-ctype",
-            "version": "v1.20.0",
+            "version": "v1.22.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/polyfill-ctype.git",
-                "reference": "f4ba089a5b6366e453971d3aad5fe8e897b37f41"
+                "reference": "c6c942b1ac76c82448322025e084cadc56048b4e"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/symfony/polyfill-ctype/zipball/f4ba089a5b6366e453971d3aad5fe8e897b37f41",
-                "reference": "f4ba089a5b6366e453971d3aad5fe8e897b37f41",
+                "url": "https://api.github.com/repos/symfony/polyfill-ctype/zipball/c6c942b1ac76c82448322025e084cadc56048b4e",
+                "reference": "c6c942b1ac76c82448322025e084cadc56048b4e",
                 "shasum": ""
             },
             "require": {
@@ -1810,7 +1866,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-main": "1.20-dev"
+                    "dev-main": "1.22-dev"
                 },
                 "thanks": {
                     "name": "symfony/polyfill",
@@ -1861,7 +1917,7 @@
                     "type": "tidelift"
                 }
             ],
-            "time": "2020-10-23T14:02:19+00:00"
+            "time": "2021-01-07T16:49:33+00:00"
         },
         {
             "name": "theseer/tokenizer",
diff --git a/src/Pool.php b/src/Pool.php
index db094e6..7ed9249 100644
--- a/src/Pool.php
+++ b/src/Pool.php
@@ -3,13 +3,13 @@
 
 class Pool {
 	/** @var Process[] Associative array of name=>Process */
-	protected $processList;
+	protected array $processList;
 
 	public function __construct() {
 		$this->processList = [];
 	}
 
-	public function add(string $name, Process $process) {
+	public function add(string $name, Process $process):void {
 		$this->processList[$name] = $process;
 	}
 
diff --git a/src/Process.php b/src/Process.php
index 3bcd510..8871195 100644
--- a/src/Process.php
+++ b/src/Process.php
@@ -6,21 +6,21 @@ class Process {
 	const PIPE_OUT = 1;
 	const PIPE_ERROR = 2;
 
-	/** @var string[] */
-	protected $command;
-	/** @var string */
-	protected $cwd;
-	/** @var array Indexed array of streams: 0=>input, 1=>output, 2=>error. */
-	protected $pipes;
+	/** @var array<int, string> */
+	protected array $command;
+	protected string $cwd;
+	/** @var array<int, resource> Indexed array of streams: 0=>input, 1=>output, 2=>error. */
+	protected array $pipes;
 	/** @var resource The process as returned from proc_open. */
 	protected $process = null;
-	protected $status;
-	/** @var bool */
-	protected $isBlocking = false;
+	/** @var array<string, mixed> */
+	protected array $status;
+	protected bool $isBlocking = false;
 
 	public function __construct(string...$command) {
 		$this->command = $command;
 		$this->cwd = getcwd();
+		$this->pipes = [];
 	}
 
 	public function __destruct() {
@@ -35,7 +35,7 @@ public function setExecCwd(string $cwd):void {
 	 * Runs the command in a concurrent thread.
 	 * Sets the input, output and errors streams.
 	 */
-	public function exec() {
+	public function exec():void {
 		$descriptor = [
 			self::PIPE_IN => ["pipe", "r"],
 			self::PIPE_OUT => ["pipe", "w"],
@@ -45,7 +45,11 @@ public function exec() {
 		$oldCwd = getcwd();
 		chdir($this->cwd);
 
+		// Parameter #1 of proc_open is an array
+		// @see https://www.php.net/manual/en/function.proc-open.php
+
 		$this->process = proc_open(
+		/** @phpstan-param string[] */
 			$this->command,
 			$descriptor,
 			$this->pipes
@@ -89,6 +93,7 @@ public function hasNotEnded():bool {
 		return $this->status["exitcode"] === 0;
 	}
 
+	/** @return array<int, string> */
 	public function getCommand():array {
 		return $this->command;
 	}
@@ -153,8 +158,8 @@ public function setBlocking(bool $blocking = true):void {
 	 * @see https://php.net/manual/function.proc-get-status.php
 	 **/
 	protected function refreshStatus():void {
-		if($this->status["running"] ?? null
-		|| empty($this->status)) {
+		$running = $this->status["running"] ?? null;
+		if($running || empty($this->status)) {
 			if(is_resource($this->process)) {
 				$this->status = proc_get_status($this->process);
 			}