Provided by: libmakefile-dom-perl_0.008-3_all 
NAME
Makefile::DOM - Simple DOM parser for Makefiles
VERSION
This document describes Makefile::DOM 0.008 released on 18 November 2014.
DESCRIPTION
This library can serve as an advanced lexer for (GNU) makefiles. It parses makefiles as
"documents" and the parsing is lossless. The results are data structures similar to DOM
trees. The DOM trees hold every single bit of the information in the original input files,
including white spaces, blank lines and makefile comments. That means it's possible to
reproduce the original makefiles from the DOM trees. In addition, each node of the DOM
trees is modifiable and so is the whole tree, just like the PPI module used for Perl
source parsing and the HTML::TreeBuilder module used for parsing HTML source.
If you're looking for a true GNU make parser that generates an AST, please see
Makefile::Parser::GmakeDB instead.
The interface of "Makefile::DOM" mimics the API design of PPI. In fact, I've directly
stolen the source code and POD documentation of PPI::Node, PPI::Element, and PPI::Dumper,
with the full permission from the author of PPI, Adam Kennedy.
"Makefile::DOM" tries to be independent of specific makefile's syntax. The same set of DOM
node types is supposed to get shared by different makefile DOM generators. For example,
MDOM::Document::Gmake parses GNU makefiles and returns an instance of MDOM::Document,
i.e., the root of the DOM tree while the NMAKE makefile lexer in the future,
"MDOM::Document::Nmake", also returns instances of the MDOM::Document class. Later, I'll
also consider adding support for dmake and bsdmake.
Structure of the DOM
Makefile DOM (MDOM) is a structured set of a series of data types. They provide a flexible
document model conformed to the makefile syntax. Below is a complete list of the 19 MDOM
classes in the current implementation where the indentation indicates the class
inheritance relationships.
MDOM::Element
MDOM::Node
MDOM::Unknown
MDOM::Assignment
MDOM::Command
MDOM::Directive
MDOM::Document
MDOM::Document::Gmake
MDOM::Rule
MDOM::Rule::Simple
MDOM::Rule::StaticPattern
MDOM::Token
MDOM::Token::Bare
MDOM::Token::Comment
MDOM::Token::Continuation
MDOM::Token::Interpolation
MDOM::Token::Modifier
MDOM::Token::Separator
MDOM::Token::Whitespace
It's not hard to see that all of the MDOM classes inherit from the MDOM::Element class.
MDOM::Token and MDOM::Node are its direct children. The former represents a string token
which is atomic from the perspective of the lexer while the latter represents a structured
node, which usually has one or more children, and serves as the container for other
DOM::Element objects.
Next we'll show a few examples to demonstrate how to map DOM trees to particular
makefiles.
Case 1
Consider the following simple "hello, world" makefile:
all : ; echo "hello, world"
We can use the MDOM::Dumper class provided by Makefile::DOM to dump out the internal
structure of its corresponding MDOM tree:
MDOM::Document::Gmake
MDOM::Rule::Simple
MDOM::Token::Bare 'all'
MDOM::Token::Whitespace ' '
MDOM::Token::Separator ':'
MDOM::Token::Whitespace ' '
MDOM::Command
MDOM::Token::Separator ';'
MDOM::Token::Whitespace ' '
MDOM::Token::Bare 'echo "hello, world"'
MDOM::Token::Whitespace '\n'
In this example, speparators ":" and ";" are all instances of the
MDOM::Token::Separator class while spaces and new line characters are all represented
as MDOM::Token::Whitespace. The other two leaf nodes, "all" and "echo "hello, world""
both belong to MDOM::Token::Bare.
It's worth mentioning that, the space characters in the rule command "echo "hello,
world"" were not represented as MDOM::Token::Whitespace. That's because in makefiles,
the spaces in commands do not make any sense to "make" in syntax; those spaces are
usually sent to shell programs verbatim. Therefore, the DOM parser does not try to
recognize those spaces specifially so as to reduce memory use and the number of nodes.
However, leading spaces and trailing new lines will still be recognized as
MDOM::Token::Whitespace.
On a higher level, it's a MDOM::Rule::Simple instance holding several "Token" and one
MDOM::Command. On the highest level, it's the root node of the whole DOM tree, i.e.,
an instance of MDOM::Document::Gmake.
Case 2
Below is a relatively complex example:
a: foo.c bar.h $(baz) # hello!
@echo ...
It's corresponding DOM structure is
MDOM::Document::Gmake
MDOM::Rule::Simple
MDOM::Token::Bare 'a'
MDOM::Token::Separator ':'
MDOM::Token::Whitespace ' '
MDOM::Token::Bare 'foo.c'
MDOM::Token::Whitespace ' '
MDOM::Token::Bare 'bar.h'
MDOM::Token::Whitespace '\t'
MDOM::Token::Interpolation '$(baz)'
MDOM::Token::Whitespace ' '
MDOM::Token::Comment '# hello!'
MDOM::Token::Whitespace '\n'
MDOM::Command
MDOM::Token::Separator '\t'
MDOM::Token::Modifier '@'
MDOM::Token::Bare 'echo ...'
MDOM::Token::Whitespace '\n'
Compared to the previous example, here appears several new node types.
The variable interpolation "$(baz)" on the first line of the original makefile
corresponds to a MDOM::Token::Interpolation node in its MDOM tree. Similarly, the
comment "# hello" corresponds to a MDOM::Token::Comment node.
On the second line, the rule command indented by a tab character is still represented
by a MDOM::Command object. Its first child node (or its first element) is also an
MDOM::Token::Seperator instance corresponding to that tab. The command modifier "@"
follows the "Separator" immediately, which is of type MDOM::Token::Modifier.
Case 3
Now let's study a sample makefile with various global structures:
a: b
foo = bar
# hello!
Here on the top level, there are three language structures: one rule ""a: b"", one
assignment statement "foo = bar", and one comment "# hello!".
Its MDOM tree is shown below:
MDOM::Document::Gmake
MDOM::Rule::Simple
MDOM::Token::Bare 'a'
MDOM::Token::Separator ':'
MDOM::Token::Whitespace ' '
MDOM::Token::Bare 'b'
MDOM::Token::Whitespace '\n'
MDOM::Assignment
MDOM::Token::Bare 'foo'
MDOM::Token::Whitespace ' '
MDOM::Token::Separator '='
MDOM::Token::Whitespace ' '
MDOM::Token::Bare 'bar'
MDOM::Token::Whitespace '\n'
MDOM::Token::Whitespace '\t'
MDOM::Token::Comment '# hello!'
MDOM::Token::Whitespace '\n'
We can see that below the root node MDOM::Document::Gmake, there are
MDOM::Rule::Simple, MDOM::Assignment, and MDOM::Comment three elements, as well as two
MDOM::Token::Whitespace objects.
It can be observed from the examples above that the MDOM representation for the makefile's
lexical elements is rather loose. It only provides very limited structural representation
instead of making a bad guess.
OPERATIONS FOR MDOM TREES
Generating an MDOM tree from a GNU makefile only requires two lines of Perl code:
use MDOM::Document::Gmake;
my $dom = MDOM::Document::Gmake->new('Makefile');
If the makefile source code being parsed is already stored in a Perl variable, say, $var,
then we can construct an MDOM via the following code:
my $dom = MDOM::Document::Gmake->new(\$var);
Now $dom becomes the reference to the root of the MDOM tree and its type is now
MDOM::Document::Gmake, which is also an instance of the MDOM::Node class.
Just as mentioned above, "MDOM::Node" is the container for other MDOM::Element instances.
So we can retrieve some element node's value via its "child" method:
$node = $dom->child(3);
# or $node = $dom->elements(0);
And we may also use the "elements" method to obtain the values of all the nodes:
@elems = $dom->elements;
For every MDOM node, its corresponding makefile source can be generated by invoking its
"content" method.
BUGS AND TODO
The current implementation of the MDOM::Document::Gmake lexer is based on a hand-written
state machie. Although the efficiency of the engine is not bad, the code is rather
complicated and messy, which hurts both extensibility and maintanabilty. So it's expected
to rewrite the parser using some grammatical tools like the Perl 6 regex engine
Pugs::Compiler::Rule or a yacc-style one like Parse::Yapp.
SOURCE REPOSITORY
You can always get the latest source code of this module from its GitHub repository:
<http://github.com/agentzh/makefile-dom-pm>
If you want a commit bit, please let me know.
AUTHOR
Yichun "agentzh" Zhang (章亦春) <agentzh@gmail.com>
COPYRIGHT
Copyright 2006-2014 by Yichun "agentzh" Zhang (章亦春).
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
SEE ALSO
MDOM::Document, MDOM::Document::Gmake, PPI, Makefile::Parser::GmakeDB, makesimple.