PPI(3) User Contributed Perl Documentation PPI(3)
NAME
PPI - Parse, Analyze and Manipulate Perl (without perl)
SYNOPSIS
use PPI;
# Create a new empty document
my $Document = PPI::Document->new;
# Create a document from source
$Document = PPI::Document->new('print "Hello World!
"');
# Load a Document from a file
$Document = PPI::Document->new('Module.pm');
# Does it contain any POD?
if ( $Document->find_any('PPI::Token::Pod') ) {
print "Module contains POD
";
}
# Get the name of the main package
$pkg = $Document->find_first('PPI::Statement::Package')->namespace;
# Remove all that nasty documentation
$Document->prune('PPI::Token::Pod');
$Document->prune('PPI::Token::Comment');
# Save the file
$Document->save('Module.pm.stripped');
DESCRIPTION
About this Document
This is the PPI manual. It describes its reason for existing, its general structure, its use, an overview of the API, and provides a few
implementation samples.
Background
The ability to read, and manipulate Perl (the language) programmatically other than with perl (the application) was one that caused
difficulty for a long time.
The cause of this problem was Perl's complex and dynamic grammar. Although there is typically not a huge diversity in the grammar of most
Perl code, certain issues cause large problems when it comes to parsing.
Indeed, quite early in Perl's history Tom Christenson introduced the Perl community to the quote "Nothing but perl can parse Perl", or as
it is more often stated now as a truism:
"Only perl can parse Perl"
One example of the sorts of things the prevent Perl being easily parsed are function signatures, as demonstrated by the following.
@result = (dothis $foo, $bar);
# Which of the following is it equivalent to?
@result = (dothis($foo), $bar);
@result = dothis($foo, $bar);
The first line above can be interpreted in two different ways, depending on whether the &dothis function is expecting one argument, or two,
or several.
A "code parser" (something that parses for the purpose of execution) such as perl needs information that is not found in the immediate
vicinity of the statement being parsed.
The information might not just be elsewhere in the file, it might not even be in the same file at all. It might also not be able to
determine this information without the prior execution of a "BEGIN {}" block, or the loading and execution of one or more external modules.
Or worse the &dothis function may not even have been written yet.
When parsing Perl as code, you must also execute it
Even perl itself never really fully understands the structure of the source code after and indeed as it processes it, and in that sense
doesn't "parse" Perl source into anything remotely like a structured document. This makes it of no real use for any task that needs to
treat the source code as a document, and do so reliably and robustly.
For more information on why it is impossible to parse perl, see Randal Schwartz's seminal response to the question of "Why can't you parse
Perl".
<http://www.perlmonks.org/index.pl?node_id=44722>
The purpose of PPI is not to parse Perl Code, but to parse Perl Documents. By treating the problem this way, we are able to parse a single
file containing Perl source code "isolated" from any other resources, such as libraries upon which the code may depend, and without needing
to run an instance of perl alongside or inside the parser.
Historically, using an embedded perl parser was widely considered to be the most likely avenue for finding a solution to "Parse::Perl". It
was investigated from time to time and attempts have generally failed or suffered from sufficiently bad corner cases that they were
abandoned.
What Does PPI Stand For?
"PPI" is an acronym for the longer original module name "Parse::Perl::Isolated". And in the spirit or the silly acronym games played by
certain unnamed Open Source projects you may have hurd of, it also a reverse backronym of "I Parse Perl".
Of course, I could just be lying and have just made that second bit up 10 minutes before the release of PPI 1.000. Besides, all the cool
Perl packages have TLAs (Three Letter Acronyms). It's a rule or something.
Why don't you just think of it as the Perl Parsing Interface for simplicity.
The original name was shortened to prevent the author (and you the users) from contracting RSI by having to type crazy things like
"Parse::Perl::Isolated::Token::QuoteLike::Backtick" 100 times a day.
In acknowledgment that someone may some day come up with a valid solution for the grammar problem it was decided at the commencement of the
project to leave the "Parse::Perl" namespace free for any such effort.
Since that time I've been able to prove to my own satisfaction that it is truly impossible to accurately parse Perl as both code and
document at once. For the academics, parsing Perl suffers from the "Halting Problem".
With this in mind "Parse::Perl" has now been co-opted as the title for the SourceForge project that publishes PPI and a large collection of
other applications and modules related to the (document) parsing of Perl source code.
You can find this project at <http://sf.net/projects/parseperl>, however we no longer use the SourceForge CVS server. Instead, the current
development version of PPI is available via SVN at <http://svn.ali.as/cpan/trunk/PPI/>.
Why Parse Perl?
Once you can accept that we will never be able to parse Perl well enough to meet the standards of things that treat Perl as code, it is
worth re-examining "why" we want to "parse" Perl at all.
What are the things that people might want a "Perl parser" for.
Documentation
Analyzing the contents of a Perl document to automatically generate documentation, in parallel to, or as a replacement for, POD
documentation.
Allow an indexer to to locate and process all the comments and documentation from code for "full text search" applications.
Structural and Quality Analysis
Determine quality or other metrics across a body of code, and identify situations relating to particular phrases, techniques or
locations.
Index functions, variables and packages within Perl code, and doing search and graph (in the node/edge sense) analysis of large code
bases.
Refactoring
Make structural, syntax, or other changes to code in an automated manner, either independently or in assistance to an editor. This sort
of task list includes backporting, forward porting, partial evaluation, "improving" code, or whatever. All the sort of things you'd
want from a Perl::Editor.
Layout
Change the layout of code without changing its meaning. This includes techniques such as tidying (like perltidy), obfuscation,
compressing and "squishing", or to implement formatting preferences or policies.
Presentation
This includes methods of improving the presentation of code, without changing the content of the code. Modify, improve, syntax colour
etc the presentation of a Perl document. Generating "IntelliText"-like functions.
If we treat this as a baseline for the sort of things we are going to have to build on top of Perl, then it becomes possible to identify a
standard for how good a Perl parser needs to be.
How good is Good Enough(TM)
PPI seeks to be good enough to achieve all of the above tasks, or to provide a sufficiently good API on which to allow others to implement
modules in these and related areas.
However, there are going to be limits to this process. Because PPI cannot adapt to changing grammars, any code written using source filters
should not be assumed to be parsable.
At one extreme, this includes anything munged by Acme::Bleach, as well as (arguably) more common cases like Switch. We do not pretend to be
able to always parse code using these modules, although as long as it still follows a format that looks like Perl syntax, it may be
possible to extend the lexer to handle them.
The ability to extend PPI to handle lexical additions to the language is on the drawing board to be done some time post-1.0
The goal for success was originally to be able to successfully parse 99% of all Perl documents contained in CPAN. This means the entire
file in each case.
PPI has succeeded in this goal far beyond the expectations of even the author. At time of writing there are only 28 non-Acme Perl modules
in CPAN that PPI is incapable of parsing. Most of these are so badly broken they do not compile as Perl code anyway.
So unless you are actively going out of your way to break PPI, you should expect that it will handle your code just fine.
Internationalisation
PPI provides partial support for internationalisation and localisation.
Specifically, it allows the use characters from the Latin-1 character set to be used in quotes, comments, and POD. Primarily, this covers
languages from Europe and South America.
PPI does not currently provide support for Unicode, although there is an initial implementation available in a development branch from CVS.
If you need Unicode support, and would like to help stress test the Unicode support so we can move it to the main branch and enable it in
the main release should contact the author. (contact details below)
Round Trip Safe
When PPI parses a file it builds everything into the model, including whitespace. This is needed in order to make the Document fully "Round
Trip" safe.
The general concept behind a "Round Trip" parser is that it knows what it is parsing is somewhat uncertain, and so expects to get things
wrong from time to time. In the cases where it parses code wrongly the tree will serialize back out to the same string of code that was
read in, repairing the parser's mistake as it heads back out to the file.
The end result is that if you parse in a file and serialize it back out without changing the tree, you are guaranteed to get the same file
you started with. PPI does this correctly and reliably for 100% of all known cases.
What goes in, will come out. Every time.
The one minor exception at this time is that if the newlines for your file are wrong (meaning not matching the platform newline format),
PPI will localise them for you. (It isn't to be convenient, supporting arbitrary newlines would make some of the code more complicated)
Better control of the newline type is on the wish list though, and anyone wanting to help out is encouraged to contact the author.
IMPLEMENTATION
General Layout
PPI is built upon two primary "parsing" components, PPI::Tokenizer and PPI::Lexer, and a large tree of about 50 classes which implement the
various the Perl Document Object Model (PDOM).
The PDOM is conceptually similar in style and intent to the regular DOM or other code Abstract Syntax Trees (ASTs), but contains some
differences to handle perl-specific cases, and to assist in treating the code as a document. Please note that it is not an implementation
of the official Document Object Model specification, only somewhat similar to it.
On top of the Tokenizer, Lexer and the classes of the PDOM, sit a number of classes intended to make life a little easier when dealing with
PDOM trees.
Both the major parsing components were hand-coded from scratch with only plain Perl code and a few small utility modules. There are no
grammar or patterns mini-languages, no YACC or LEX style tools and only a small number of regular expressions.
This is primarily because of the sheer volume of accumulated cruft that exists in Perl. Not even perl itself is capable of parsing Perl
documents (remember, it just parses and executes it as code).
As a result, PPI needed to be cruftier than perl itself. Feel free to shudder at this point, and hope you never have to understand the
Tokenizer codebase. Speaking of which...
The Tokenizer
The Tokenizer takes source code and converts it into a series of tokens. It does this using a slow but thorough character by character
manual process, rather than using a pattern system or complex regexes.
Or at least it does so conceptually. If you were to actually trace the code you would find it's not truly character by character due to a
number of regexps and optimisations throughout the code. This lets the Tokenizer "skip ahead" when it can find shortcuts, so it tends to
jump around a line a bit wildly at times.
In practice, the number of times the Tokenizer will actually move the character cursor itself is only about 5% - 10% higher than the number
of tokens contained in the file. This makes it about as optimal as it can be made without implementing it in something other than Perl.
In 2001 when PPI was started, this structure made PPI quite slow, and not really suitable for interactive tasks. This situation has
improved greatly with multi-gigahertz processors, but can still be painful when working with very large files.
The target parsing rate for PPI is about 5000 lines per gigacycle. It is currently believed to be at about 1500, and main avenue for making
it to the target speed has now become PPI::XS, a drop-in XS accelerator for PPI.
Since PPI::XS has only just gotten off the ground and is currently only at proof-of-concept stage, this may take a little while. Anyone
interested in helping out with PPI::XS is highly encouraged to contact the author. In fact, the design of PPI::XS means it's possible to
port one function at a time safely and reliably. So every little bit will help.
The Lexer
The Lexer takes a token stream, and converts it to a lexical tree. Because we are parsing Perl documents this includes whitespace,
comments, and all number of weird things that have no relevance when code is actually executed.
An instantiated PPI::Lexer consumes PPI::Tokenizer objects and produces PPI::Document objects. However you should probably never be working
with the Lexer directly. You should just be able to create PPI::Document objects and work with them directly.
The Perl Document Object Model
The PDOM is a structured collection of data classes that together provide a correct and scalable model for documents that follow the
standard Perl syntax.
The PDOM Class Tree
The following lists all of the 67 current PDOM classes, listing with indentation based on inheritance.
PPI::Element
PPI::Node
PPI::Document
PPI::Document::Fragment
PPI::Statement
PPI::Statement::Package
PPI::Statement::Include
PPI::Statement::Sub
PPI::Statement::Scheduled
PPI::Statement::Compound
PPI::Statement::Break
PPI::Statement::Given
PPI::Statement::When
PPI::Statement::Data
PPI::Statement::End
PPI::Statement::Expression
PPI::Statement::Variable
PPI::Statement::Null
PPI::Statement::UnmatchedBrace
PPI::Statement::Unknown
PPI::Structure
PPI::Structure::Block
PPI::Structure::Subscript
PPI::Structure::Constructor
PPI::Structure::Condition
PPI::Structure::List
PPI::Structure::For
PPI::Structure::Given
PPI::Structure::When
PPI::Structure::Unknown
PPI::Token
PPI::Token::Whitespace
PPI::Token::Comment
PPI::Token::Pod
PPI::Token::Number
PPI::Token::Number::Binary
PPI::Token::Number::Octal
PPI::Token::Number::Hex
PPI::Token::Number::Float
PPI::Token::Number::Exp
PPI::Token::Number::Version
PPI::Token::Word
PPI::Token::DashedWord
PPI::Token::Symbol
PPI::Token::Magic
PPI::Token::ArrayIndex
PPI::Token::Operator
PPI::Token::Quote
PPI::Token::Quote::Single
PPI::Token::Quote::Double
PPI::Token::Quote::Literal
PPI::Token::Quote::Interpolate
PPI::Token::QuoteLike
PPI::Token::QuoteLike::Backtick
PPI::Token::QuoteLike::Command
PPI::Token::QuoteLike::Regexp
PPI::Token::QuoteLike::Words
PPI::Token::QuoteLike::Readline
PPI::Token::Regexp
PPI::Token::Regexp::Match
PPI::Token::Regexp::Substitute
PPI::Token::Regexp::Transliterate
PPI::Token::HereDoc
PPI::Token::Cast
PPI::Token::Structure
PPI::Token::Label
PPI::Token::Separator
PPI::Token::Data
PPI::Token::End
PPI::Token::Prototype
PPI::Token::Attribute
PPI::Token::Unknown
To summarize the above layout, all PDOM objects inherit from the PPI::Element class.
Under this are PPI::Token, strings of content with a known type, and PPI::Node, syntactically significant containers that hold other
Elements.
The three most important of these are the PPI::Document, the PPI::Statement and the PPI::Structure classes.
The Document, Statement and Structure
At the top of all complete PDOM trees is a PPI::Document object. It represents a complete file of Perl source code as you might find it on
disk.
There are some specialised types of document, such as PPI::Document::File and PPI::Document::Normalized but for the purposes of the PDOM
they are all just considered to be the same thing.
Each Document will contain a number of Statements, Structures and Tokens.
A PPI::Statement is any series of Tokens and Structures that are treated as a single contiguous statement by perl itself. You should note
that a Statement is as close as PPI can get to "parsing" the code in the sense that perl-itself parses Perl code when it is building the
op-tree.
Because of the isolation and Perl's syntax, it is provably impossible for PPI to accurately determine precedence of operators or which
tokens are implicit arguments to a sub call.
So rather than lead you on with a bad guess that has a strong chance of being wrong, PPI does not attempt to determine precedence or sub
parameters at all.
At a fundamental level, it only knows that this series of elements represents a single Statement as perl sees it, but it can do so with
enough certainty that it can be trusted.
However, for specific Statement types the PDOM is able to derive additional useful information about their meaning. For the best, most
useful, and most heavily used example, see PPI::Statement::Include.
A PPI::Structure is any series of tokens contained within matching braces. This includes code blocks, conditions, function argument
braces, anonymous array and hash constructors, lists, scoping braces and all other syntactic structures represented by a matching pair of
braces, including (although it may not seem obvious at first) "<READLINE>" braces.
Each Structure contains none, one, or many Tokens and Structures (the rules for which vary for the different Structure subclasses)
Under the PDOM structure rules, a Statement can never directly contain another child Statement, a Structure can never directly contain
another child Structure, and a Document can never contain another Document anywhere in the tree.
Aside from these three rules, the PDOM tree is extremely flexible.
The PDOM at Work
To demonstrate the PDOM in use lets start with an example showing how the tree might look for the following chunk of simple Perl code.
#!/usr/bin/perl
print( "Hello World!" );
exit();
Translated into a PDOM tree it would have the following structure (as shown via the included PPI::Dumper).
PPI::Document
PPI::Token::Comment '#!/usr/bin/perl
'
PPI::Token::Whitespace '
'
PPI::Statement::Expression
PPI::Token::Bareword 'print'
PPI::Structure::List ( ... )
PPI::Token::Whitespace ' '
PPI::Statement::Expression
PPI::Token::Quote::Double '"Hello World!"'
PPI::Token::Whitespace ' '
PPI::Token::Structure ';'
PPI::Token::Whitespace '
'
PPI::Token::Whitespace '
'
PPI::Statement::Expression
PPI::Token::Bareword 'exit'
PPI::Structure::List ( ... )
PPI::Token::Structure ';'
PPI::Token::Whitespace '
'
Please note that in this this example, strings are only listed for the actual PPI::Token that contains that string. Structures are listed
with the type of brace characters it represents noted.
The PPI::Dumper module can be used to generate similar trees yourself.
We can make that PDOM dump a little easier to read if we strip out all the whitespace. Here it is again, sans the distracting whitespace
tokens.
PPI::Document
PPI::Token::Comment '#!/usr/bin/perl
'
PPI::Statement::Expression
PPI::Token::Bareword 'print'
PPI::Structure::List ( ... )
PPI::Statement::Expression
PPI::Token::Quote::Double '"Hello World!"'
PPI::Token::Structure ';'
PPI::Statement::Expression
PPI::Token::Bareword 'exit'
PPI::Structure::List ( ... )
PPI::Token::Structure ';'
As you can see, the tree can get fairly deep at time, especially when every isolated token in a bracket becomes its own statement. This is
needed to allow anything inside the tree the ability to grow. It also makes the search and analysis algorithms much more flexible.
Because of the depth and complexity of PDOM trees, a vast number of very easy to use methods have been added wherever possible to help
people working with PDOM trees do normal tasks relatively quickly and efficiently.
Overview of the Primary Classes
The main PPI classes, and links to their own documentation, are listed here in alphabetical order.
PPI::Document
The Document object, the root of the PDOM.
PPI::Document::Fragment
A cohesive fragment of a larger Document. Although not of any real current use, it is needed for use in certain internal tree
manipulation algorithms.
For example, doing things like cut/copy/paste etc. Very similar to a PPI::Document, but has some additional methods and does not
represent a lexical scope boundary.
A document fragment is also non-serializable, and so cannot be written out to a file.
PPI::Dumper
A simple class for dumping readable debugging versions of PDOM structures, such as in the demonstration above.
PPI::Element
The Element class is the abstract base class for all objects within the PDOM
PPI::Find
Implements an instantiable object form of a PDOM tree search.
PPI::Lexer
The PPI Lexer. Converts Token streams into PDOM trees.
PPI::Node
The Node object, the abstract base class for all PDOM objects that can contain other Elements, such as the Document, Statement and
Structure objects.
PPI::Statement
The base class for all Perl statements. Generic "evaluate for side-effects" statements are of this actual type. Other more interesting
statement types belong to one of its children.
See it's own documentation for a longer description and list of all of the different statement types and sub-classes.
PPI::Structure
The abstract base class for all structures. A Structure is a language construct consisting of matching braces containing a set of other
elements.
See the PPI::Structure documentation for a description and list of all of the different structure types and sub-classes.
PPI::Token
A token is the basic unit of content. At its most basic, a Token is just a string tagged with metadata (its class, and some additional
flags in some cases).
PPI::Token::_QuoteEngine
The PPI::Token::Quote and PPI::Token::QuoteLike classes provide abstract base classes for the many and varied types of quote and quote-
like things in Perl. However, much of the actual quote login is implemented in a separate quote engine, based at
PPI::Token::_QuoteEngine.
Classes that inherit from PPI::Token::Quote, PPI::Token::QuoteLike and PPI::Token::Regexp are generally parsed only by the Quote
Engine.
PPI::Tokenizer
The PPI Tokenizer. One Tokenizer consumes a chunk of text and provides access to a stream of PPI::Token objects.
The Tokenizer is very very complicated, to the point where even the author treads carefully when working with it.
Most of the complication is the result of optimizations which have tripled the tokenization speed, at the expense of maintainability.
We cope with the spaghetti by heavily commenting everything.
PPI::Transform
The Perl Document Transformation API. Provides a standard interface and abstract base class for objects and classes that manipulate
Documents.
INSTALLING
The core PPI distribution is pure Perl and has been kept as tight as possible and with as few dependencies as possible.
It should download and install normally on any platform from within the CPAN and CPANPLUS applications, or directly using the distribution
tarball. If installing by hand, you may need to install a few small utility modules first. The exact ones will depend on your version of
perl.
There are no special install instructions for PPI, and the normal "Perl Makefile.PL", "make", "make test", "make install" instructions
apply.
EXTENDING
The PPI namespace itself is reserved for the sole use of the modules under the umbrella of the "Parse::Perl" SourceForge project.
<http://sf.net/projects/parseperl>
You are recommended to use the PPIx:: namespace for PPI-specific modifications or prototypes thereof, or Perl:: for modules which provide a
general Perl language-related functions.
If what you wish to implement looks like it fits into PPIx:: namespace, you should consider contacting the "Parse::Perl" mailing list
(detailed on the SourceForge site) first, as what you want may already be in progress, or you may wish to consider joining the team and
doing it within the "Parse::Perl" project itself.
TO DO
- Many more analysis and utility methods for PDOM classes
- Creation of a PPI::Tutorial document
- Add many more key functions to PPI::XS
- We can always write more and better unit tests
- Complete the full implementation of ->literal (1.200)
- Full understanding of scoping (due 1.300)
SUPPORT
This module is stored in an Open Repository at the following address.
<http://svn.ali.as/cpan/trunk/PPI>
Write access to the repository is made available automatically to any published CPAN author, and to most other volunteers on request.
If you are able to submit your bug report in the form of new (failing) unit tests, or can apply your fix directly instead of submitting a
patch, you are strongly encouraged to do so, as the author currently maintains over 100 modules and it can take some time to deal with
non-"Critical" bug reports or patches.
This will also guarentee that your issue will be addressed in the next release of the module.
For large changes though, please consider creating a branch so that they can be properly reviewed and trialed before being applied to the
trunk.
If you cannot provide a direct test or fix, or don't have time to do so, then regular bug reports are still accepted and appreciated via
the CPAN bug tracker.
<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=PPI>
For other issues or questions, contact the "Parse::Perl" project mailing list.
For commercial or media-related enquiries, or to have your SVN commit bit enabled, contact the author.
AUTHOR
Adam Kennedy <adamk@cpan.org>
ACKNOWLEDGMENTS
A huge thank you to Phase N Australia (<http://phase-n.com/>) for permitting the original open sourcing and release of this distribution
from what was originally several thousand hours of commercial work.
Another big thank you to The Perl Foundation (<http://www.perlfoundation.org/>) for funding for the final big refactoring and completion
run.
Also, to the various co-maintainers that have contributed both large and small with tests and patches and especially to those rare few who
have deep-dived into the guts to (gasp) add a feature.
- Dan Brook : PPIx::XPath, Acme::PerlML
- Audrey Tang : "Line Noise" Testing
- Arjen Laarhoven : Three-element ->location support
- Elliot Shank : Perl 5.10 support, five-element ->location
And finally, thanks to those brave ( and foolish :) ) souls willing to dive in and use, test drive and provide feedback on PPI before
version 1.000, in some cases before it made it to beta quality, and still did extremely distasteful things (like eating 50 meg of RAM a
second).
I owe you all a beer. Corner me somewhere and collect at your convenience. If I missed someone who wasn't in my email history, thank you
too :)
# In approximate order of appearance
- Claes Jacobsson
- Michael Schwern
- Jeff T. Parsons
- CPAN Author "CHOCOLATEBOY"
- Robert Rotherberg
- CPAN Author "PODMASTER"
- Richard Soderberg
- Nadim ibn Hamouda el Khemir
- Graciliano M. P.
- Leon Brocard
- Jody Belka
- Curtis Ovid
- Yuval Kogman
- Michael Schilli
- Slaven Rezic
- Lars Thegler
- Tony Stubblebine
- Tatsuhiko Miyagawa
- CPAN Author "CHROMATIC"
- Matisse Enzer
- Roy Fulbright
- Dan Brook
- Johnny Lee
- Johan Lindstrom
And to single one person out, thanks go to Randal Schwartz who spent a great number of hours in IRC over a critical 6 month period
explaining why Perl is impossibly unparsable and constantly shoving evil and ugly corner cases in my face. He remained a tireless devil's
advocate, and without his support this project genuinely could never have been completed.
So for my schooling in the Deep Magiks, you have my deepest gratitude Randal.
COPYRIGHT
Copyright 2001 - 2011 Adam Kennedy.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.
perl v5.16.3 2011-02-26 PPI(3)