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!\n"');
# 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\n";
}
# 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 Christiansen 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 parsing Perl. It has
been investigated from time to time, but 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".
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 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.
Perl::Critic, based on PPI, is a large, thriving tool for bug
detection and style analysis of Perl code.
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. If you need Unicode
support and would like to help, 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\n'
PPI::Token::Whitespace '\n'
PPI::Statement
PPI::Token::Word '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 '\n'
PPI::Token::Whitespace '\n'
PPI::Statement
PPI::Token::Word 'exit'
PPI::Structure::List ( ... )
PPI::Token::Structure ';'
PPI::Token::Whitespace '\n'
Please note that in 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\n'
PPI::Statement
PPI::Token::Word 'print'
PPI::Structure::List ( ... )
PPI::Statement::Expression
PPI::Token::Quote::Double '"Hello World!"'
PPI::Token::Structure ';'
PPI::Statement
PPI::Token::Word '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 its 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 use by PPI itself. 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 the PPIx::
namespace, you should consider contacting the PPI maintainers on GitHub
first, as what you want may already be in progress, or you may wish to
consider contributing to PPI 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
The most recent version of PPI is available at the following address.
<http://search.cpan.org/~mithaldu/PPI/>
PPI source is maintained in a GitHub repository at the following
address.
<https://github.com/adamkennedy/PPI>
Contributions via GitHub pull request are welcome.
Bug fixes in the form of pull requests or bug reports with new (failing)
unit tests have the best chance of being addressed by busy maintainers,
and are strongly encouraged.
If you cannot provide a test or fix, or don't have time to do so, then
regular bug reports are still accepted and appreciated via the GitHub
bug tracker.
<https://github.com/adamkennedy/PPI/issues>
The "ppidump" utility that is part of the Perl::Critic distribution is a
useful tool for demonstrating how PPI is parsing (or misparsing) small
code snippets, and for providing information for bug reports.
For other issues, questions, or commercial or media-related enquiries,
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.