Chris@0: Lexer component documentation
Chris@0: =============================
Chris@0: 
Chris@0: The lexer is responsible for providing tokens to the parser. The project comes with two lexers: `PhpParser\Lexer` and
Chris@0: `PhpParser\Lexer\Emulative`. The latter is an extension of the former, which adds the ability to emulate tokens of
Chris@0: newer PHP versions and thus allows parsing of new code on older versions.
Chris@0: 
Chris@0: This documentation discusses options available for the default lexers and explains how lexers can be extended.
Chris@0: 
Chris@0: Lexer options
Chris@0: -------------
Chris@0: 
Chris@0: The two default lexers accept an `$options` array in the constructor. Currently only the `'usedAttributes'` option is
Chris@0: supported, which allows you to specify which attributes will be added to the AST nodes. The attributes can then be
Chris@0: accessed using `$node->getAttribute()`, `$node->setAttribute()`, `$node->hasAttribute()` and `$node->getAttributes()`
Chris@0: methods. A sample options array:
Chris@0: 
Chris@0: ```php
Chris@0: $lexer = new PhpParser\Lexer(array(
Chris@0:     'usedAttributes' => array(
Chris@0:         'comments', 'startLine', 'endLine'
Chris@0:     )
Chris@0: ));
Chris@0: ```
Chris@0: 
Chris@0: The attributes used in this example match the default behavior of the lexer. The following attributes are supported:
Chris@0: 
Chris@0:  * `comments`: Array of `PhpParser\Comment` or `PhpParser\Comment\Doc` instances, representing all comments that occurred
Chris@0:    between the previous non-discarded token and the current one. Use of this attribute is required for the
Chris@13:    `$node->getComments()` and `$node->getDocComment()` methods to work. The attribute is also needed if you wish the pretty
Chris@13:    printer to retain comments present in the original code.
Chris@0:  * `startLine`: Line in which the node starts. This attribute is required for the `$node->getLine()` to work. It is also
Chris@0:    required if syntax errors should contain line number information.
Chris@13:  * `endLine`: Line in which the node ends. Required for `$node->getEndLine()`.
Chris@13:  * `startTokenPos`: Offset into the token array of the first token in the node. Required for `$node->getStartTokenPos()`.
Chris@13:  * `endTokenPos`: Offset into the token array of the last token in the node. Required for `$node->getEndTokenPos()`.
Chris@13:  * `startFilePos`: Offset into the code string of the first character that is part of the node. Required for `$node->getStartFilePos()`.
Chris@13:  * `endFilePos`: Offset into the code string of the last character that is part of the node. Required for `$node->getEndFilePos()`.
Chris@0: 
Chris@0: ### Using token positions
Chris@0: 
Chris@13: > **Note:** The example in this section is outdated in that this information is directly available in the AST: While
Chris@13: > `$property->isPublic()` does not distinguish between `public` and `var`, directly checking `$property->flags` for
Chris@13: > the `$property->flags & Class_::VISIBILITY_MODIFIER_MASK) === 0` allows making this distinction without resorting to
Chris@13: > tokens. However the general idea behind the example still applies in other cases.
Chris@13: 
Chris@0: The token offset information is useful if you wish to examine the exact formatting used for a node. For example the AST
Chris@0: does not distinguish whether a property was declared using `public` or using `var`, but you can retrieve this
Chris@0: information based on the token position:
Chris@0: 
Chris@0: ```php
Chris@0: function isDeclaredUsingVar(array $tokens, PhpParser\Node\Stmt\Property $prop) {
Chris@0:     $i = $prop->getAttribute('startTokenPos');
Chris@0:     return $tokens[$i][0] === T_VAR;
Chris@0: }
Chris@0: ```
Chris@0: 
Chris@0: In order to make use of this function, you will have to provide the tokens from the lexer to your node visitor using
Chris@0: code similar to the following:
Chris@0: 
Chris@0: ```php
Chris@0: class MyNodeVisitor extends PhpParser\NodeVisitorAbstract {
Chris@0:     private $tokens;
Chris@0:     public function setTokens(array $tokens) {
Chris@0:         $this->tokens = $tokens;
Chris@0:     }
Chris@0: 
Chris@0:     public function leaveNode(PhpParser\Node $node) {
Chris@0:         if ($node instanceof PhpParser\Node\Stmt\Property) {
Chris@0:             var_dump(isDeclaredUsingVar($this->tokens, $node));
Chris@0:         }
Chris@0:     }
Chris@0: }
Chris@0: 
Chris@0: $lexer = new PhpParser\Lexer(array(
Chris@0:     'usedAttributes' => array(
Chris@0:         'comments', 'startLine', 'endLine', 'startTokenPos', 'endTokenPos'
Chris@0:     )
Chris@0: ));
Chris@13: $parser = (new PhpParser\ParserFactory)->create(PhpParser\ParserFactory::ONLY_PHP7, $lexer);
Chris@0: 
Chris@0: $visitor = new MyNodeVisitor();
Chris@0: $traverser = new PhpParser\NodeTraverser();
Chris@0: $traverser->addVisitor($visitor);
Chris@0: 
Chris@0: try {
Chris@0:     $stmts = $parser->parse($code);
Chris@0:     $visitor->setTokens($lexer->getTokens());
Chris@0:     $stmts = $traverser->traverse($stmts);
Chris@0: } catch (PhpParser\Error $e) {
Chris@0:     echo 'Parse Error: ', $e->getMessage();
Chris@0: }
Chris@0: ```
Chris@0: 
Chris@0: The same approach can also be used to perform specific modifications in the code, without changing the formatting in
Chris@0: other places (which is the case when using the pretty printer).
Chris@0: 
Chris@0: Lexer extension
Chris@0: ---------------
Chris@0: 
Chris@0: A lexer has to define the following public interface:
Chris@0: 
Chris@13: ```php
Chris@13: function startLexing(string $code, ErrorHandler $errorHandler = null): void;
Chris@13: function getTokens(): array;
Chris@13: function handleHaltCompiler(): string;
Chris@13: function getNextToken(string &$value = null, array &$startAttributes = null, array &$endAttributes = null): int;
Chris@13: ```
Chris@0: 
Chris@13: The `startLexing()` method is invoked whenever the `parse()` method of the parser is called and is passed the source
Chris@13: code that is to be lexed (including the opening tag). It can be used to reset state or preprocess the source code or tokens. The
Chris@13: passed `ErrorHandler` should be used to report lexing errors.
Chris@0: 
Chris@0: The `getTokens()` method returns the current token array, in the usual `token_get_all()` format. This method is not
Chris@0: used by the parser (which uses `getNextToken()`), but is useful in combination with the token position attributes.
Chris@0: 
Chris@0: The `handleHaltCompiler()` method is called whenever a `T_HALT_COMPILER` token is encountered. It has to return the
Chris@0: remaining string after the construct (not including `();`).
Chris@0: 
Chris@0: The `getNextToken()` method returns the ID of the next token (as defined by the `Parser::T_*` constants). If no more
Chris@0: tokens are available it must return `0`, which is the ID of the `EOF` token. Furthermore the string content of the
Chris@0: token should be written into the by-reference `$value` parameter (which will then be available as `$n` in the parser).
Chris@0: 
Chris@0: ### Attribute handling
Chris@0: 
Chris@0: The other two by-ref variables `$startAttributes` and `$endAttributes` define which attributes will eventually be
Chris@0: assigned to the generated nodes: The parser will take the `$startAttributes` from the first token which is part of the
Chris@0: node and the `$endAttributes` from the last token that is part of the node.
Chris@0: 
Chris@0: E.g. if the tokens `T_FUNCTION T_STRING ... '{' ... '}'` constitute a node, then the `$startAttributes` from the
Chris@0: `T_FUNCTION` token will be taken and the `$endAttributes` from the `'}'` token.
Chris@0: 
Chris@0: An application of custom attributes is storing the exact original formatting of literals: While the parser does retain
Chris@0: some information about the formatting of integers (like decimal vs. hexadecimal) or strings (like used quote type), it
Chris@0: does not preserve the exact original formatting (e.g. leading zeros for integers or escape sequences in strings). This
Chris@0: can be remedied by storing the original value in an attribute:
Chris@0: 
Chris@0: ```php
Chris@0: use PhpParser\Lexer;
Chris@0: use PhpParser\Parser\Tokens;
Chris@0: 
Chris@0: class KeepOriginalValueLexer extends Lexer // or Lexer\Emulative
Chris@0: {
Chris@0:     public function getNextToken(&$value = null, &$startAttributes = null, &$endAttributes = null) {
Chris@0:         $tokenId = parent::getNextToken($value, $startAttributes, $endAttributes);
Chris@0: 
Chris@0:         if ($tokenId == Tokens::T_CONSTANT_ENCAPSED_STRING   // non-interpolated string
Chris@0:             || $tokenId == Tokens::T_ENCAPSED_AND_WHITESPACE // interpolated string
Chris@0:             || $tokenId == Tokens::T_LNUMBER                 // integer
Chris@0:             || $tokenId == Tokens::T_DNUMBER                 // floating point number
Chris@0:         ) {
Chris@0:             // could also use $startAttributes, doesn't really matter here
Chris@0:             $endAttributes['originalValue'] = $value;
Chris@0:         }
Chris@0: 
Chris@0:         return $tokenId;
Chris@0:     }
Chris@0: }
Chris@0: ```