GraphQL\Utils::keyMap PHP Method

    public static function keyMap($traversable, callable $keyFn)
    {
        self::invariant(is_array($traversable) || $traversable instanceof \Traversable, __METHOD__ . ' expects array or Traversable');
        $map = [];
        foreach ($traversable as $key => $value) {
            $newKey = $keyFn($value, $key);
            if (is_scalar($newKey)) {
                $map[$newKey] = $value;
            }
        }
        return $map;
    }

 public function __invoke(ValidationContext $context)
 {
     return [Node::FIELD => function (Field $fieldAST) use($context) {
         $fieldDef = $context->getFieldDef();
         if (!$fieldDef) {
             return Visitor::skipNode();
         }
         $errors = [];
         $argASTs = $fieldAST->arguments ?: [];
         $argASTMap = Utils::keyMap($argASTs, function (Argument $arg) {
             return $arg->name->value;
         });
         foreach ($fieldDef->args as $argDef) {
             $argAST = isset($argASTMap[$argDef->name]) ? $argASTMap[$argDef->name] : null;
             if (!$argAST && $argDef->getType() instanceof NonNull) {
                 $errors[] = new Error(Messages::missingArgMessage($fieldAST->name->value, $argDef->name, $argDef->getType()), [$fieldAST]);
             }
         }
         $argDefMap = Utils::keyMap($fieldDef->args, function ($def) {
             return $def->name;
         });
         foreach ($argASTs as $argAST) {
             $argDef = $argDefMap[$argAST->name->value];
             if ($argDef && !DocumentValidator::isValidLiteralValue($argAST->value, $argDef->getType())) {
                 $errors[] = new Error(Messages::badValueMessage($argAST->name->value, $argDef->getType(), Printer::doPrint($argAST->value)), [$argAST->value]);
             }
         }
         return !empty($errors) ? $errors : null;
     }];
 }