diff --git a/.gitignore b/.gitignore
index 4e3728b..321655a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@ composer.phar
 composer.lock
 vendor/
 bin/
+phpstan.phar
diff --git a/.travis.yml b/.travis.yml
index d8ec128..299fe07 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,13 @@ php:
     - 7.2
     - nightly
 
-matrix:
+jobs:
+    include:
+        - stage: Code Quality
+          php: 7.2
+          env: STATIC_ANALYSIS
+          install: travis_retry composer install --prefer-dist
+          script: composer static-analysis
     allow_failures:
         - php: nightly
 
@@ -22,10 +28,7 @@ before_install:
     - phpenv config-rm xdebug.ini || true
     - composer selfupdate
 
-install:
-    - composer install --dev --prefer-dist
-    - composer require react/promise:2.*
-    - composer require psr/http-message:1.*
+install: composer install --dev --prefer-dist
 
 script: if [ "$TRAVIS_PHP_VERSION" == "5.6" ]; then bin/phpunit --coverage-clover build/logs/clover.xml --group default,ReactPromise; else bin/phpunit --group default,ReactPromise; fi
 
diff --git a/composer.json b/composer.json
index d48b76a..4178927 100644
--- a/composer.json
+++ b/composer.json
@@ -14,13 +14,11 @@
   },
   "require-dev": {
     "phpunit/phpunit": "^4.8",
-    "psr/http-message": "^1.0"
+    "psr/http-message": "^1.0",
+    "react/promise": "2.*"
   },
   "config": {
     "bin-dir": "bin",
-    "platform": {
-      "php": "5.6.0"
-    },
     "preferred-install": "dist",
     "sort-packages": true
 },
@@ -36,6 +34,15 @@
       "GraphQL\\Examples\\Blog\\": "examples/01-blog/Blog/"
     }
   },
+  "scripts": {
+    "static-analysis": [
+      "rm phpstan.phar || true",
+      "@composer req --ansi --no-interaction --dev phpstan/phpstan-shim",
+      "cp -f vendor/phpstan/phpstan-shim/phpstan.phar .",
+      "@composer rem --ansi --dev phpstan/phpstan-shim",
+      "@php phpstan.phar analyse --ansi -l 1 -c phpstan.neon src"
+    ]
+  },
   "suggest": {
     "react/promise": "To leverage async resolving on React PHP platform",
     "psr/http-message": "To use standard GraphQL server"
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..e69de29
diff --git a/src/Executor/ExecutionContext.php b/src/Executor/ExecutionContext.php
index f31a444..0a73b1b 100644
--- a/src/Executor/ExecutionContext.php
+++ b/src/Executor/ExecutionContext.php
@@ -2,6 +2,7 @@
 namespace GraphQL\Executor;
 
 use GraphQL\Error\Error;
+use GraphQL\Executor\Promise\PromiseAdapter;
 use GraphQL\Language\AST\FragmentDefinitionNode;
 use GraphQL\Language\AST\OperationDefinitionNode;
 use GraphQL\Type\Schema;
@@ -56,6 +57,11 @@ class ExecutionContext
      */
     public $errors;
 
+    /**
+     * @var PromiseAdapter
+     */
+    public $promises;
+
     public function __construct(
         $schema,
         $fragments,
diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 4460e20..e63a42a 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -560,7 +560,7 @@ class Executor
             switch ($selection->kind) {
                 case NodeKind::FIELD:
                     if (!$this->shouldIncludeNode($selection)) {
-                        continue;
+                        continue 2;
                     }
                     $name = self::getFieldEntryKey($selection);
                     if (!isset($fields[$name])) {
@@ -572,7 +572,7 @@ class Executor
                     if (!$this->shouldIncludeNode($selection) ||
                         !$this->doesFragmentConditionMatch($selection, $runtimeType)
                     ) {
-                        continue;
+                        continue 2;
                     }
                     $this->collectFields(
                         $runtimeType,
@@ -584,14 +584,14 @@ class Executor
                 case NodeKind::FRAGMENT_SPREAD:
                     $fragName = $selection->name->value;
                     if (!empty($visitedFragmentNames[$fragName]) || !$this->shouldIncludeNode($selection)) {
-                        continue;
+                        continue 2;
                     }
                     $visitedFragmentNames[$fragName] = true;
 
                     /** @var FragmentDefinitionNode|null $fragment */
                     $fragment = isset($exeContext->fragments[$fragName]) ? $exeContext->fragments[$fragName] : null;
                     if (!$fragment || !$this->doesFragmentConditionMatch($fragment, $runtimeType)) {
-                        continue;
+                        continue 2;
                     }
                     $this->collectFields(
                         $runtimeType,
diff --git a/src/Language/Visitor.php b/src/Language/Visitor.php
index ab20d1e..fda7540 100644
--- a/src/Language/Visitor.php
+++ b/src/Language/Visitor.php
@@ -6,15 +6,6 @@ use GraphQL\Language\AST\NodeKind;
 use GraphQL\Language\AST\NodeList;
 use GraphQL\Utils\TypeInfo;
 
-class VisitorOperation
-{
-    public $doBreak;
-
-    public $doContinue;
-
-    public $removeNode;
-}
-
 /**
  * Utility for efficient AST traversal and modification.
  *
@@ -258,6 +249,7 @@ class Visitor
 
                 if ($visitFn) {
                     $result = call_user_func($visitFn, $node, $key, $parent, $path, $ancestors);
+                    $editValue = null;
 
                     if ($result !== null) {
                         if ($result instanceof VisitorOperation) {
diff --git a/src/Language/VisitorOperation.php b/src/Language/VisitorOperation.php
new file mode 100644
index 0000000..493073d
--- /dev/null
+++ b/src/Language/VisitorOperation.php
@@ -0,0 +1,11 @@
+<?php
+namespace GraphQL\Language;
+
+class VisitorOperation
+{
+    public $doBreak;
+
+    public $doContinue;
+
+    public $removeNode;
+}
diff --git a/src/Server/StandardServer.php b/src/Server/StandardServer.php
index 9684205..2ea4480 100644
--- a/src/Server/StandardServer.php
+++ b/src/Server/StandardServer.php
@@ -5,7 +5,7 @@ use GraphQL\Error\FormattedError;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Executor\ExecutionResult;
 use GraphQL\Executor\Promise\Promise;
-use GraphQL\Utils;
+use GraphQL\Utils\Utils;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamInterface;
diff --git a/src/Type/Definition/FieldDefinition.php b/src/Type/Definition/FieldDefinition.php
index 19199c3..a98dcd5 100644
--- a/src/Type/Definition/FieldDefinition.php
+++ b/src/Type/Definition/FieldDefinition.php
@@ -62,6 +62,11 @@ class FieldDefinition
      */
     public $config;
 
+    /**
+     * @var null|callable
+     */
+    public $complexityFn;
+
     /**
      * @var OutputType
      */
@@ -108,7 +113,7 @@ class FieldDefinition
     }
 
     /**
-     * @param array|Config $field
+     * @param array $field
      * @param string $typeName
      * @return FieldDefinition
      */
diff --git a/src/Type/Definition/InputObjectField.php b/src/Type/Definition/InputObjectField.php
index 1744e51..68f2a32 100644
--- a/src/Type/Definition/InputObjectField.php
+++ b/src/Type/Definition/InputObjectField.php
@@ -24,7 +24,7 @@ class InputObjectField
     public $description;
 
     /**
-     * @var callback|InputType
+     * @var callable|InputType
      */
     public $type;
 
diff --git a/src/Validator/Rules/VariablesDefaultValueAllowed.php b/src/Validator/Rules/VariablesDefaultValueAllowed.php
index fcbbef4..92e84d1 100644
--- a/src/Validator/Rules/VariablesDefaultValueAllowed.php
+++ b/src/Validator/Rules/VariablesDefaultValueAllowed.php
@@ -49,10 +49,10 @@ class VariablesDefaultValueAllowed extends AbstractValidationRule
 
                 return Visitor::skipNode();
             },
-            NodeKind::SELECTION_SET => function(SelectionSetNode $node) use ($context) {
+            NodeKind::SELECTION_SET => function(SelectionSetNode $node) {
                 return Visitor::skipNode();
             },
-            NodeKind::FRAGMENT_DEFINITION => function(FragmentDefinitionNode $node) use ($context) {
+            NodeKind::FRAGMENT_DEFINITION => function(FragmentDefinitionNode $node) {
                 return Visitor::skipNode();
             },
         ];