153 lines
6.9 KiB
Markdown
153 lines
6.9 KiB
Markdown
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Lexer component documentation
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=============================
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The lexer is responsible for providing tokens to the parser. The project comes with two lexers: `PhpParser\Lexer` and
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`PhpParser\Lexer\Emulative`. The latter is an extension of the former, which adds the ability to emulate tokens of
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newer PHP versions and thus allows parsing of new code on older versions.
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This documentation discusses options available for the default lexers and explains how lexers can be extended.
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Lexer options
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-------------
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The two default lexers accept an `$options` array in the constructor. Currently only the `'usedAttributes'` option is
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supported, which allows you to specify which attributes will be added to the AST nodes. The attributes can then be
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accessed using `$node->getAttribute()`, `$node->setAttribute()`, `$node->hasAttribute()` and `$node->getAttributes()`
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methods. A sample options array:
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```php
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$lexer = new PhpParser\Lexer(array(
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'usedAttributes' => array(
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'comments', 'startLine', 'endLine'
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)
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));
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```
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The attributes used in this example match the default behavior of the lexer. The following attributes are supported:
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* `comments`: Array of `PhpParser\Comment` or `PhpParser\Comment\Doc` instances, representing all comments that occurred
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between the previous non-discarded token and the current one. Use of this attribute is required for the
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`$node->getDocComment()` method to work. The attribute is also needed if you wish the pretty printer to retain
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comments present in the original code.
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* `startLine`: Line in which the node starts. This attribute is required for the `$node->getLine()` to work. It is also
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required if syntax errors should contain line number information.
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* `endLine`: Line in which the node ends.
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* `startTokenPos`: Offset into the token array of the first token in the node.
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* `endTokenPos`: Offset into the token array of the last token in the node.
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* `startFilePos`: Offset into the code string of the first character that is part of the node.
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* `endFilePos`: Offset into the code string of the last character that is part of the node.
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### Using token positions
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The token offset information is useful if you wish to examine the exact formatting used for a node. For example the AST
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does not distinguish whether a property was declared using `public` or using `var`, but you can retrieve this
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information based on the token position:
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```php
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function isDeclaredUsingVar(array $tokens, PhpParser\Node\Stmt\Property $prop) {
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$i = $prop->getAttribute('startTokenPos');
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return $tokens[$i][0] === T_VAR;
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}
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```
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In order to make use of this function, you will have to provide the tokens from the lexer to your node visitor using
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code similar to the following:
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```php
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class MyNodeVisitor extends PhpParser\NodeVisitorAbstract {
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private $tokens;
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public function setTokens(array $tokens) {
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$this->tokens = $tokens;
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}
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public function leaveNode(PhpParser\Node $node) {
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if ($node instanceof PhpParser\Node\Stmt\Property) {
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var_dump(isDeclaredUsingVar($this->tokens, $node));
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}
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}
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}
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$lexer = new PhpParser\Lexer(array(
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'usedAttributes' => array(
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'comments', 'startLine', 'endLine', 'startTokenPos', 'endTokenPos'
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)
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));
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$parser = (new PhpParser\ParserFactory)->create(PhpParser\ParserFactory::PREFER_PHP7, $lexer);
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$visitor = new MyNodeVisitor();
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$traverser = new PhpParser\NodeTraverser();
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$traverser->addVisitor($visitor);
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try {
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$stmts = $parser->parse($code);
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$visitor->setTokens($lexer->getTokens());
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$stmts = $traverser->traverse($stmts);
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} catch (PhpParser\Error $e) {
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echo 'Parse Error: ', $e->getMessage();
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}
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```
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The same approach can also be used to perform specific modifications in the code, without changing the formatting in
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other places (which is the case when using the pretty printer).
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Lexer extension
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---------------
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A lexer has to define the following public interface:
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void startLexing(string $code, ErrorHandler $errorHandler = null);
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array getTokens();
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string handleHaltCompiler();
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int getNextToken(string &$value = null, array &$startAttributes = null, array &$endAttributes = null);
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The `startLexing()` method is invoked with the source code that is to be lexed (including the opening tag) whenever the
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`parse()` method of the parser is called. It can be used to reset state or preprocess the source code or tokens. The
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passes `ErrorHandler` should be used to report lexing errors.
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The `getTokens()` method returns the current token array, in the usual `token_get_all()` format. This method is not
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used by the parser (which uses `getNextToken()`), but is useful in combination with the token position attributes.
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The `handleHaltCompiler()` method is called whenever a `T_HALT_COMPILER` token is encountered. It has to return the
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remaining string after the construct (not including `();`).
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The `getNextToken()` method returns the ID of the next token (as defined by the `Parser::T_*` constants). If no more
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tokens are available it must return `0`, which is the ID of the `EOF` token. Furthermore the string content of the
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token should be written into the by-reference `$value` parameter (which will then be available as `$n` in the parser).
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### Attribute handling
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The other two by-ref variables `$startAttributes` and `$endAttributes` define which attributes will eventually be
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assigned to the generated nodes: The parser will take the `$startAttributes` from the first token which is part of the
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node and the `$endAttributes` from the last token that is part of the node.
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E.g. if the tokens `T_FUNCTION T_STRING ... '{' ... '}'` constitute a node, then the `$startAttributes` from the
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`T_FUNCTION` token will be taken and the `$endAttributes` from the `'}'` token.
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An application of custom attributes is storing the exact original formatting of literals: While the parser does retain
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some information about the formatting of integers (like decimal vs. hexadecimal) or strings (like used quote type), it
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does not preserve the exact original formatting (e.g. leading zeros for integers or escape sequences in strings). This
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can be remedied by storing the original value in an attribute:
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```php
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use PhpParser\Lexer;
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use PhpParser\Parser\Tokens;
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class KeepOriginalValueLexer extends Lexer // or Lexer\Emulative
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{
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public function getNextToken(&$value = null, &$startAttributes = null, &$endAttributes = null) {
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$tokenId = parent::getNextToken($value, $startAttributes, $endAttributes);
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if ($tokenId == Tokens::T_CONSTANT_ENCAPSED_STRING // non-interpolated string
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|| $tokenId == Tokens::T_ENCAPSED_AND_WHITESPACE // interpolated string
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|| $tokenId == Tokens::T_LNUMBER // integer
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|| $tokenId == Tokens::T_DNUMBER // floating point number
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) {
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// could also use $startAttributes, doesn't really matter here
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$endAttributes['originalValue'] = $value;
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}
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return $tokenId;
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}
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}
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```
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