Chris@13: Walking the AST
Chris@13: ===============
Chris@13: 
Chris@13: The most common way to work with the AST is by using a node traverser and one or more node visitors.
Chris@13: As a basic example, the following code changes all literal integers in the AST into strings (e.g.,
Chris@13: `42` becomes `'42'`.)
Chris@13: 
Chris@13: ```php
Chris@13: use PhpParser\{Node, NodeTraverser, NodeVisitorAbstract};
Chris@13: 
Chris@13: $traverser = new NodeTraverser;
Chris@13: $traverser->addVisitor(new class extends NodeVisitorAbstract {
Chris@13:     public function leaveNode(Node $node) {
Chris@13:         if ($node instanceof Node\Scalar\LNumber) {
Chris@13:             return new Node\Scalar\String_((string) $node->value);
Chris@13:         }
Chris@13:     }
Chris@13: });
Chris@13: 
Chris@13: $stmts = ...;
Chris@13: $modifiedStmts = $traverser->traverse($stmts);
Chris@13: ```
Chris@13: 
Chris@13: Node visitors
Chris@13: -------------
Chris@13: 
Chris@13: Each node visitor implements an interface with following four methods:
Chris@13: 
Chris@13: ```php
Chris@13: interface NodeVisitor {
Chris@13:     public function beforeTraverse(array $nodes);
Chris@13:     public function enterNode(Node $node);
Chris@13:     public function leaveNode(Node $node);
Chris@13:     public function afterTraverse(array $nodes);
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: The `beforeTraverse()` and `afterTraverse()` methods are called before and after the traversal
Chris@13: respectively, and are passed the entire AST. They can be used to perform any necessary state
Chris@13: setup or cleanup.
Chris@13: 
Chris@13: The `enterNode()` method is called when a node is first encountered, before its children are
Chris@13: processed ("preorder"). The `leaveNode()` method is called after all children have been visited
Chris@13: ("postorder").
Chris@13: 
Chris@13: For example, if we have the following excerpt of an AST
Chris@13: 
Chris@13: ```
Chris@13: Expr_FuncCall(
Chris@13:     name: Name(
Chris@13:         parts: array(
Chris@13:             0: printLine
Chris@13:         )
Chris@13:     )
Chris@13:     args: array(
Chris@13:         0: Arg(
Chris@13:             value: Scalar_String(
Chris@13:                 value: Hello World!!!
Chris@13:             )
Chris@13:             byRef: false
Chris@13:             unpack: false
Chris@13:         )
Chris@13:     )
Chris@13: )
Chris@13: ```
Chris@13: 
Chris@13: then the enter/leave methods will be called in the following order:
Chris@13: 
Chris@13: ```
Chris@13: enterNode(Expr_FuncCall)
Chris@13: enterNode(Name)
Chris@13: leaveNode(Name)
Chris@13: enterNode(Arg)
Chris@13: enterNode(Scalar_String)
Chris@13: leaveNode(Scalar_String)
Chris@13: leaveNode(Arg)
Chris@13: leaveNode(Expr_FuncCall)
Chris@13: ```
Chris@13: 
Chris@13: A common pattern is that `enterNode` is used to collect some information and then `leaveNode`
Chris@13: performs modifications based on that. At the time when `leaveNode` is called, all the code inside
Chris@13: the node will have already been visited and necessary information collected.
Chris@13: 
Chris@13: As you usually do not want to implement all four methods, it is recommended that you extend
Chris@13: `NodeVisitorAbstract` instead of implementing the interface directly. The abstract class provides
Chris@13: empty default implementations.
Chris@13: 
Chris@13: Modifying the AST
Chris@13: -----------------
Chris@13: 
Chris@13: There are a number of ways in which the AST can be modified from inside a node visitor. The first
Chris@13: and simplest is to simply change AST properties inside the visitor:
Chris@13: 
Chris@13: ```php
Chris@13: public function leaveNode(Node $node) {
Chris@13:     if ($node instanceof Node\Scalar\LNumber) {
Chris@13:         // increment all integer literals
Chris@13:         $node->value++;
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: The second is to replace a node entirely by returning a new node:
Chris@13: 
Chris@13: ```php
Chris@13: public function leaveNode(Node $node) {
Chris@13:     if ($node instanceof Node\Expr\BinaryOp\BooleanAnd) {
Chris@13:         // Convert all $a && $b expressions into !($a && $b)
Chris@13:         return new Node\Expr\BooleanNot($node);
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: Doing this is supported both inside enterNode and leaveNode. However, you have to be mindful about
Chris@13: where you perform the replacement: If a node is replaced in enterNode, then the recursive traversal
Chris@13: will also consider the children of the new node. If you aren't careful, this can lead to infinite
Chris@13: recursion. For example, let's take the previous code sample and use enterNode instead:
Chris@13: 
Chris@13: ```php
Chris@13: public function enterNode(Node $node) {
Chris@13:     if ($node instanceof Node\Expr\BinaryOp\BooleanAnd) {
Chris@13:         // Convert all $a && $b expressions into !($a && $b)
Chris@13:         return new Node\Expr\BooleanNot($node);
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: Now `$a && $b` will be replaced by `!($a && $b)`. Then the traverser will go into the first (and
Chris@13: only) child of `!($a && $b)`, which is `$a && $b`. The transformation applies again and we end up
Chris@13: with `!!($a && $b)`. This will continue until PHP hits the memory limit.
Chris@13: 
Chris@13: Finally, two special replacement types are supported only by leaveNode. The first is removal of a
Chris@13: node:
Chris@13: 
Chris@13: ```php
Chris@13: public function leaveNode(Node $node) {
Chris@13:     if ($node instanceof Node\Stmt\Return_) {
Chris@13:         // Remove all return statements
Chris@13:         return NodeTraverser::REMOVE_NODE;
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: Node removal only works if the parent structure is an array. This means that usually it only makes
Chris@13: sense to remove nodes of type `Node\Stmt`, as they always occur inside statement lists (and a few
Chris@13: more node types like `Arg` or `Expr\ArrayItem`, which are also always part of lists).
Chris@13: 
Chris@13: On the other hand, removing a `Node\Expr` does not make sense: If you have `$a * $b`, there is no
Chris@13: meaningful way in which the `$a` part could be removed. If you want to remove an expression, you
Chris@13: generally want to remove it together with a surrounding expression statement:
Chris@13: 
Chris@13: ```php
Chris@13: public function leaveNode(Node $node) {
Chris@13:     if ($node instanceof Node\Stmt\Expression
Chris@13:         && $node->expr instanceof Node\Expr\FuncCall
Chris@13:         && $node->expr->name instanceof Node\Name
Chris@13:         && $node->expr->name->toString() === 'var_dump'
Chris@13:     ) {
Chris@13:         return NodeTraverser::REMOVE_NODE;
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: This example will remove all calls to `var_dump()` which occur as expression statements. This means
Chris@13: that `var_dump($a);` will be removed, but `if (var_dump($a))` will not be removed (and there is no
Chris@13: obvious way in which it can be removed).
Chris@13: 
Chris@13: Next to removing nodes, it is also possible to replace one node with multiple nodes. Again, this
Chris@13: only works inside leaveNode and only if the parent structure is an array.
Chris@13: 
Chris@13: ```php
Chris@13: public function leaveNode(Node $node) {
Chris@13:     if ($node instanceof Node\Stmt\Return_ && $node->expr !== null) {
Chris@13:         // Convert "return foo();" into "$retval = foo(); return $retval;"
Chris@13:         $var = new Node\Expr\Variable('retval');
Chris@13:         return [
Chris@13:             new Node\Stmt\Expression(new Node\Expr\Assign($var, $node->expr)),
Chris@13:             new Node\Stmt\Return_($var),
Chris@13:         ];
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: Short-circuiting traversal
Chris@13: --------------------------
Chris@13: 
Chris@13: An AST can easily contain thousands of nodes, and traversing over all of them may be slow,
Chris@13: especially if you have more than one visitor. In some cases, it is possible to avoid a full
Chris@13: traversal.
Chris@13: 
Chris@13: If you are looking for all class declarations in a file (and assuming you're not interested in
Chris@13: anonymous classes), you know that once you've seen a class declaration, there is no point in also
Chris@13: checking all it's child nodes, because PHP does not allow nesting classes. In this case, you can
Chris@13: instruct the traverser to not recurse into the class node:
Chris@13: 
Chris@13: ```
Chris@13: private $classes = [];
Chris@13: public function enterNode(Node $node) {
Chris@13:     if ($node instanceof Node\Stmt\Class_) {
Chris@13:         $this->classes[] = $node;
Chris@13:         return NodeTraverser::DONT_TRAVERSE_CHILDREN;
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: Of course, this option is only available in enterNode, because it's already too late by the time
Chris@13: leaveNode is reached.
Chris@13: 
Chris@13: If you are only looking for one specific node, it is also possible to abort the traversal entirely
Chris@13: after finding it. For example, if you are looking for the node of a class with a certain name (and
Chris@13: discounting exotic cases like conditionally defining a class two times), you can stop traversal
Chris@13: once you found it:
Chris@13: 
Chris@13: ```
Chris@13: private $class = null;
Chris@13: public function enterNode(Node $node) {
Chris@13:     if ($node instanceof Node\Stmt\Class_ &&
Chris@17:         $node->namespacedName->toString() === 'Foo\Bar\Baz'
Chris@13:     ) {
Chris@13:         $this->class = $node;
Chris@13:         return NodeTraverser::STOP_TRAVERSAL;
Chris@13:     }
Chris@13: }
Chris@13: ```
Chris@13: 
Chris@13: This works both in enterNode and leaveNode. Note that this particular case can also be more easily
Chris@13: handled using a NodeFinder, which will be introduced below.
Chris@13: 
Chris@13: Multiple visitors
Chris@13: -----------------
Chris@13: 
Chris@13: A single traverser can be used with multiple visitors:
Chris@13: 
Chris@13: ```php
Chris@13: $traverser = new NodeTraverser;
Chris@13: $traverser->addVisitor($visitorA);
Chris@13: $traverser->addVisitor($visitorB);
Chris@17: $stmts = $traverser->traverse($stmts);
Chris@13: ```
Chris@13: 
Chris@13: It is important to understand that if a traverser is run with multiple visitors, the visitors will
Chris@13: be interleaved. Given the following AST excerpt
Chris@13: 
Chris@13: ```
Chris@13: Stmt_Return(
Chris@13:     expr: Expr_Variable(
Chris@13:         name: foobar
Chris@13:     )
Chris@13: )
Chris@13: ```
Chris@13: 
Chris@13: the following method calls will be performed:
Chris@13: 
Chris@13: ```
Chris@13: $visitorA->enterNode(Stmt_Return)
Chris@13: $visitorB->enterNode(Stmt_Return)
Chris@13: $visitorA->enterNode(Expr_Variable)
Chris@13: $visitorB->enterNode(Expr_Variable)
Chris@13: $visitorA->leaveNode(Expr_Variable)
Chris@13: $visitorB->leaveNode(Expr_Variable)
Chris@13: $visitorA->leaveNode(Stmt_Return)
Chris@13: $visitorB->leaveNode(Stmt_Return)
Chris@13: ```
Chris@13: 
Chris@13: That is, when visiting a node, enterNode and leaveNode will always be called for all visitors.
Chris@13: Running multiple visitors in parallel improves performance, as the AST only has to be traversed
Chris@13: once. However, it is not always possible to write visitors in a way that allows interleaved
Chris@13: execution. In this case, you can always fall back to performing multiple traversals:
Chris@13: 
Chris@13: ```php
Chris@13: $traverserA = new NodeTraverser;
Chris@13: $traverserA->addVisitor($visitorA);
Chris@13: $traverserB = new NodeTraverser;
Chris@13: $traverserB->addVisitor($visitorB);
Chris@13: $stmts = $traverserA->traverser($stmts);
Chris@13: $stmts = $traverserB->traverser($stmts);
Chris@13: ```
Chris@13: 
Chris@13: When using multiple visitors, it is important to understand how they interact with the various
Chris@13: special enterNode/leaveNode return values:
Chris@13: 
Chris@13:  * If *any* visitor returns `DONT_TRAVERSE_CHILDREN`, the children will be skipped for *all*
Chris@13:    visitors.
Chris@17:  * If *any* visitor returns `DONT_TRAVERSE_CURRENT_AND_CHILDREN`, the children will be skipped for *all*
Chris@17:    visitors, and all *subsequent* visitors will not visit the current node.
Chris@13:  * If *any* visitor returns `STOP_TRAVERSAL`, traversal is stopped for *all* visitors.
Chris@13:  * If a visitor returns a replacement node, subsequent visitors will be passed the replacement node,
Chris@13:    not the original one.
Chris@13:  * If a visitor returns `REMOVE_NODE`, subsequent visitors will not see this node.
Chris@13:  * If a visitor returns an array of replacement nodes, subsequent visitors will see neither the node
Chris@13:    that was replaced, nor the replacement nodes.
Chris@13: 
Chris@13: Simple node finding
Chris@13: -------------------
Chris@13: 
Chris@13: While the node visitor mechanism is very flexible, creating a node visitor can be overly cumbersome
Chris@13: for minor tasks. For this reason a `NodeFinder` is provided, which can find AST nodes that either
Chris@13: satisfy a certain callback, or which are instanced of a certain node type. A couple of examples are
Chris@13: shown in the following:
Chris@13: 
Chris@13: ```php
Chris@13: use PhpParser\{Node, NodeFinder};
Chris@13: 
Chris@13: $nodeFinder = new NodeFinder;
Chris@13: 
Chris@13: // Find all class nodes.
Chris@13: $classes = $nodeFinder->findInstanceOf($stmts, Node\Stmt\Class_::class);
Chris@13: 
Chris@13: // Find all classes that extend another class
Chris@17: $extendingClasses = $nodeFinder->find($stmts, function(Node $node) {
Chris@13:     return $node instanceof Node\Stmt\Class_
Chris@13:         && $node->extends !== null;
Chris@13: });
Chris@13: 
Chris@13: // Find first class occuring in the AST. Returns null if no class exists.
Chris@13: $class = $nodeFinder->findFirstInstanceOf($stmts, Node\Stmt\Class_::class);
Chris@13: 
Chris@13: // Find first class that has name $name
Chris@13: $class = $nodeFinder->findFirst($stmts, function(Node $node) use ($name) {
Chris@13:     return $node instanceof Node\Stmt\Class_
Chris@13:         && $node->resolvedName->toString() === $name;
Chris@13: });
Chris@13: ```
Chris@13: 
Chris@13: Internally, the `NodeFinder` also uses a node traverser. It only simplifies the interface for a
Chris@13: common use case.
Chris@13: 
Chris@13: Parent and sibling references
Chris@13: -----------------------------
Chris@13: 
Chris@13: The node visitor mechanism is somewhat rigid, in that it prescribes an order in which nodes should
Chris@13: be accessed: From parents to children. However, it can often be convenient to operate in the
Chris@13: reverse direction: When working on a node, you might want to check if the parent node satisfies a
Chris@13: certain property.
Chris@13: 
Chris@13: PHP-Parser does not add parent (or sibling) references to nodes by itself, but you can easily
Chris@17: emulate this with a visitor. See the [FAQ](FAQ.markdown) for more information.