diff --git a/src/Pool.php b/src/Pool.php
index a27d2d6..4bd650f 100644
--- a/src/Pool.php
+++ b/src/Pool.php
@@ -234,24 +234,24 @@ public function markAsFailed(Runnable $process)
         $this->failed[$process->getPid()] = $process;
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         // TODO
 
         return false;
     }
 
-    public function offsetGet($offset)
+    public function offsetGet($offset): Runnable
     {
         // TODO
     }
 
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         $this->add($value);
     }
 
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         // TODO
     }