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