


=head1 NAME

Parse::Gnaw - Define a grammar and create a parser by calling 
nothing but perl subroutines.

=head1 SYNOPSIS

Gnaw is a perl module which implements full regular expressions and 
full text parsing grammars using nothing but pure perl code limited 
to subroutine closures and basic perl variables such as scalars, 
hashes, arrays, and references.

You write your grammar in pure perl. There is no intermediate 
"parser language" that then gets interpreted into something executable.

When you do a "use Parse::Gnaw", the Gnaw module will import a 
number of functions directly into your namespace. Yes, this is 
completely bad form for normal modules. But this is not a normal 
module. The imported subroutines include regular expression and 
parsing functions for matching, quantifiers, literals, 
alternations, character classes, and so on. You build up your 
grammar by calling these functions. The final call will return 
a code reference. This code reference is your grammar.

When you dereference that grammar, if it is a "match" function, 
then you pass in the string you want to parse.

	use Parse::Gnaw;

	# create the grammar
	my $grammar = match('hello');

	# apply the grammar to a string
	if($grammar->('hello world')) {
		print "match\n";
	} else {
		print "no match";
	}

You can also create the grammar and execute it in one step:

	my $texttoparse = "howdy partner";

	if(match('hello', 'world')->($texttoparse)) {
		print "match\n";
	} else {
		print "no match\n";
	}

Note the above example translated into perls regular expression syntax 
would look something like this:

	my $texttoparse = "howdy partner";

	if($texttoparse =~ m{hello\s*world}) {
		print "match\n";
	} else {
		print "no match\n";
	}


You can build up more complicated grammars fairly easily.
This one looks for a sentence about fruits.

	$grammar = match(ql('I would like to buy'), some('a', qa('banana apple pear peach')));

The "match" function looks for a match to the grammar in the string being parsed.

The "ql" function (quoted literal) allows you to put a sequence of 
literals into a single string. It the splits the string up into 
individual literals much like perls "qw" function does. Then it puts 
them into a grammar sequence for you. This saves you from putting 
quotes around each individual literal.

The "some" function is a quantifier looking for "1 or more" of whatever it surrounds, 
in this case, a sequence of the literal "a" followed by an alternation
of different possible fruit.

The "qa" function (quoted alternation) takes a single string and splits it 
into individual words, also similar to perls "qw" function. The "qa" function
then takes those individual words and creates an alternation that tries to 
match any individual word in the string as a valid alternate.

=head2 BETA

Please note that this is a beta release. This is more of a 
proof of concept than something ready for production code 
or for massive grammars. The interfaces may change completely 
in the future. When the interfaces have settled, I will release 
this as a version 1.0+ module. Until then, please do not use 
this to develop some gigantic parser when the grammar may have 
to completely change.

=head1 EXPORT

Currently, EVERYTHING is exported into the callers namespace. 
Yes, I know, bad programmer. No biscuit. 

The problem is I did not want grammars to be weighed down with 
a bunch of package prefixes at every step.

	my $grammar = Parse::Gnaw::match(Parse::Gnaw::a(Parse::Gnaw::l('hello'),Parse::Gnaw::l('howdy'));

Gets rather tedious.

Hopefully, no one will have collisions with subroutine names. 
If they do, then it might make sense to create a new package, 
declare the grammar in there, then simply pull out the code 
reference and use that in the rest of your code.

	package mynormalpackage;

	our $grammar;

	{
		package mylocalpackage;

		use Parse::Gnaw; # all subs imported into mylocalpackage

		$mynormalpackage::grammar = match('hello', 'world');

	}


	$grammar->('howdy');

Again, everything may change tomorrow as I sort this out.

=head1 ok function

In the examples provided, the "ok" function is used repeatedly.
This allows the examples to be used directly in the Parse::Gnaw
tests directly. The "ok" function is defined there by the 
Test::More package. Here is a stub for the "ok" function so that
you can use the examples independent of Test::More.

	use Parse::Gnaw;

	sub ok {
	 	my $boolean = shift(@_);
	
		if($boolean) {
			print "pass";
		} else {
			print "FAIL";
		}
		print " : ";
	
		my $msg = shift(@_);
	
		if(defined($msg)) {
			chomp($msg);
			print $msg;
		}
	
		print "\n";
	}

	my $grammar; # declare it here so other examples can just use it.

The above code is assumed to be contained in all the examples that follow.

=head1 OVERVIEW

So, again, when you do a

	use Parse::Gnaw;

in your code, a number of subroutines are imported into your namespace.
You can then call these subroutines to build up a grammar. The grammar
is held in the form of a code reference. You then dereference the coderef
to call the parser and provide it with the string you want to parse.
The dereferenced code reference that is the parser will return a boolean
to indicate whether a match was found or not.

	$grammar = match('hello', 'world');
	
	ok($grammar->('hello world')==1), "match");
	ok($grammar->('howdy world')==0), "NO match");

	> pass : match
	> pass : NO match

If you want your grammar to match a literal string, simply pass that string
into the "parse" or "match" function. Strings passed into the Gnaw
functions are interpreted to mean that they are literals to look for in the
text being parsed.

=head2 Top Level and Low Level Functions

The Gnaw module provides a number of "top level" functions to create  your 
grammars. These top level functions return code references which are your
grammar. You then dereference these code references to call the grammar
and pass in the string you want to parse.

Only top level functions return grammar code references. They include:

	match	Similar to perls =~ m/// regexp function.
		Find the first match of the grammar in the string.
		Skip any text until we find a match.

	swap	Similar to perls =~s/// regexp function.
		Define a pattern, and define either a replacement string
		or a subroutine which will receive the original text and
		return a replacement string. 

	parse	Similar to perls =~ m/\A// regexp function.
		Parse the string from the beginning only. 
		Do not scan through text looking for a match.

	modify	Allow grammars to modify text in flight.
		Note that the "modify" function does not modify anything
		unless the user calls the "get" function and defines 
		what gets modified. Think of "modify" as "match" but
		with the "write protect" bit turned off.

These functions return code references that are the grammar, code references
you dereference to run the grammar on a string.

match examples:

	my $grammar = match('hello', 'world');
	ok($grammar->("these are not the droids you are looking for")==0, 'no match');

swap examples:

	# swap 'Alice' for 'Bob', use string literal
	my $input = "hello Alice ! How are you?";
	ok(swap('Alice', 'Bob') -> ($input)==1, "checking match was found");
	ok($input eq 'hello Bob ! How are you?', "checking substitution/swap took place");


	# swap a phone number for the same number, but with asterisks around it.
	my $input = "please call 555-1212 for more details";
	swap(some(ccdigit),'-',some(ccdigit), sub{return "***".shift(@_)."***";}) -> ($input);
	ok($input eq 'please call ***555-1212*** for more details');


Low level functions return code references that are called "stitchers". 
These are low level subroutines used by Parse::Gnaw to build up the grammar. 
Most of the low level and high level Gnaw functions know how to handle stitchers.
A "stitcher" is not a grammar, and if you try to dereference a 
coderef from a low level function, your script will die with strange errors.

	my $stitcher = a('hello', 'howdy');
	
	# WRONG! Don't do it!
	$stitcher->("these are not the droids you are looking for");  

Always make sure you use a top level function as your final function 
of your grammar.

=head2 Literals

Most of the Parse::Gnaw subroutines will recognize strings passed in as
parameters as string literals to become part of the grammar. We already
saw this in the above example:

	$grammar = match('hello', 'world');

This example will create a grammar that looks for the literal 'hello'
followed by the literal 'world'.

Actually, the grammar will look for the literal 'hello', it will then
Skip over any text that the Skip function tells it to Skip (by default,
it will skip over any whitespace), and then look for the literal 'world'.

See the "Skip" function for more information.

Strings passed into Parse::Gnaw functions are perl strings, meaning
that they follow the normal perl rules for string evaluation.
i.e. Single quotes do NOT get evaluated. Double quotes do.

If you want to match a tab character, then be sure to use "\t" 
instead of '\t'.

=head2 List Context

Before we go too far into talking about Gnaw, we need to address a meta issue
around Parse::Gnaw. 

The thing to keep in mind is that all Parse::Gnaw subroutines are... 

... perl subroutines. 

This means that they are susceptible to list context issues that occur 
in perl when you call the functions, and that they are susceptible to 
list context issues and return mechanism issues when you declare your 
own subroutines to wrap up a subrule of your grammar.

When you call any perl subroutine, the parameters passed in are flattend
into one gigantic list by the list context mechanism in perl.

	# all parameters are flattened. subroutine receives 6 parameters.
	my_perl_sub( (1,2,3),('a', 'b', 'c') ); 

When the my_perl_sub subroutine is called, perl takes all the parameters
and flattens them into a single list context. This means in the above 
example that my_perl_sub receives 6 parameters:

	1, 2, 3, 'a', 'b', 'c'

This also means that my_perl_sub has no idea that the numbers had been
grouped together in one set of parens and the letters had been grouped 
together in another set of parens. The my_perl_sub subroutine sees 
nothing but a flat list of parameters being passed in.

If you want to call a subroutine and have it treat parameters in groups,
then the most common way to do that is to group the parameters you want
grouped by putting them into arrays and passing references to those arrays.

	# my_perl_sub receives two parameters, two array references.
	my_perl_sub( [1,2,3] , ['a', 'b', 'c'] ); 

This exact same list context mechanism affects Parse::Gnaw. And the solution
within Parse::Gnaw is the same as for general perl: wrap groups of items
in anonymous arrays.

Example. You want a grammar with two possible alternates. The first alternate 
is "hello" followed by "world". The second alternate is "howdy" followed by
"planet". If you try to put them into the alternate function (the "a" function),
you might try this:

	$grammar = match( a( 
			"hello", "world",
			"howdy", "planet"
	));

The problem is that the "a" function has no way of knowing how it is supposed
to group the parameters. Instead, the above example will look for four possible
alternates:

	"hello"
	or
	"world"
	or
	"howdy"
	or
	"planet"

The solution is to group the parameters with an anonymous array reference.
The above example can be rewritten as follows:

	$grammar = match( a( ["hello", "world"],["howdy", "planet"]	));

This will look for two possible alternates:

	"hello" "world"
	or
	"howdy" "planet"

Most of the Parse::Gnaw functions (such as the "a" (alternate) function) are 
smart enough to recognize array references and treat them as a series of 
grammar components meant to be addressed as a single unit.

=head2 declare subroutines for your own sub-rules

The other issue around perl list context and Parse::Gnaw occurs around
perl and its "return" function in subroutines. In Parse::Gnaw, if you have
a complex grammar, you can break it up into rules which take the form
of perl subroutines you declare. The subroutine is some subset of the
whole grammar, and must return something that the rest of the grammar
can use.

This means you must be aware of perls "return" functionality when declaring
subroutines as your rules.

For example, you can declare the following subroutine in any perl code:

	sub mysub { 'hello', 'there' };

When you call a perl subroutine, it will return whatever was the last
expression evaluated. And in the above example, "there" is the last 
expression evaluated, and "there" is what mysub will return.

	my $retval = tester;
	print "retval is '$retval'\n";

	> retval is 'there'

Again this is an issue relating to perls list context. If you want a subroutine
to return a list of items, then enclose them in parenthesis to force 
list context on the return value.

	sub fruit { ('apple', 'banana', 'pear') } # parens force list context

You could then call this subroutine and pass it to the Parse::Gnaw "a" function
as a list of possible alternates.

	$grammar = match(a( fruit ));

However, if you want to declare a subroutine which is intended to contain
a sequence of items, then you will want to wrap them in an anonymous array
so that a single array reference is returned and is treated as a sequence
by the rest of the grammar.

	# square brackets groups return value
	sub greetworld { ['hello', 'world'] }	
	sub greetplanet{ ['howdy', 'planet'] }

	$grammar = match(a( greetworld, greetplanet ));

Note that the two subroutines define a sequence of literals that get
passed into the "a" function. To keep the "a" function from flattening
the return value of the two subroutines, the subroutines need to group
their sequences into array references.

Always keep in mind that Parse::Gnaw functions are perl functions and
are susceptible to list context issues relating to the things being
passed into a function as well as what is returned by the function.

As a general rule, when declaring subroutines to describe pieces of your
grammar, if that subroutine contains more than one grammar component,
you should wrap the return value in parenthesis if you intend to use
the components in list context (a list of different alternates to pass in
to "a"), or you should wrap the return value in square brackets if you
intend to use the components as one sequence of components.

If the subroutine contains ONLY one Parse::Gnaw grammar component, then
you do not have to wrap that component in parens or square brackets.
Parse::Gnaw low level functions always return one scalar, a stitcher
code reference that can be used by other Parse::Gnaw functions.

Since Parse::Gnaw grammar functions always return one scalar, you do not
have to worry about forcing scalar context with square brackets. Just
call the Gnaw function and perl will return the stitcher for you.

	# return value of 'a' function is a scalar, 
	# (a scalar containing a code ref that is a stitcher function)
	# therefore, subroutine "fruit" doesn't have to put 
	# the "a" function in square brackets to force scalar context on
	# return value.
	sub fruit { a('apple', 'banana', 'pear') }

=head2 summary of return values in subroutines/subrules 

Always using parens or square brackets in your subroutines will 
show how you intend the subroutine to be used in a larger grammar.

	# parens forces return in list context. items may be used in parallel
	sub fruit { ('apple', 'pear', 'banana') } 

	# square brackets forces return in scalar context. items are seqential.
	sub greeting { ['may', 'the', 'force', 'be', 'with', 'you'] }

Do not use a wrapper (no parens, no square brackets) if subroutine 
only calls a single Gnaw grammar component. 

	# 'name' subroutine only calls one function 'a'. don't have to wrap.
	sub name { a('Alice', 'Bob', 'Charlie') }

Now back to the grammar functions.

=head2 cc (character classes)

The Parse::Gnaw "cc" function is used to declare a character class.
The input to the "cc" function is a single string which lists all the
characters that are to be part of the character class. The class will
match any single character that is listed in the string.

	$grammar = match( cc('0123456789') ); # any single digit. 0,1,2,3,4,5,6,7,8,9

The "cc" function recognizes the '-' character as a meta character which
operates the same as the '-' in perl character classes, creating a character
class with all the characters between the two characters (inclusive) 
separated by the '-' character.

	$grammar = match( cc('0-9') ); # any single digit. 0,1,2,3,4,5,6,7,8,9

If you want the '-' character to be part of your character class, make it 
the first character in the string.

	$grammar = match( cc('-aeiou') ); # a hyphen or any vowel

Character class shortcuts are provided for common character classes.

	ccdigit => 0-9
	ccword => a-zA-Z0-9_
	ccspace => \n \t \r \f or literal space

	$grammar = match( ccdigit ); # any single digit. 0,1,2,3,4,5,6,7,8,9

Note that the "cc" function and its related shortcuts are one of the functions
in Parse::Gnaw that do NOT handle array references, nor do they handle multiple
strings being passed into them in list context. The only valid parameter to
pass to the "cc" function is a single string containing all the characters 
that you want in the character class.

The cc shortcuts take no parameters at all.

=head2 CC (inverted character class)

The Parse::Gnaw "CC" function is an inverted character class. The input is a
single string which defines the characters that you do NOT want to be part of
the character class. Note that the "CC" function is uppercase and the "cc" 
function is lower case.

	$grammar = match( CC('0-9') ); # any single character EXCEPT a digit

Short cuts are provided for common inverted character classes:

	CCDIGIT => anything BUT 0-9
	CCWORD => anything BUT a-zA-Z0-9_
	CCSPACE => anything BUT \n \t \r \f or literal space

	$grammar = match( CCDIGIT ); # any single character EXCEPT a digit

Note that the "CC" function has the same parameter restrictions as "cc".
Only one string containing the characters defining the NOT character class.
No more than a single parameter. No array references. The shortcuts take
no parameters at all.

=head2 thing

The Parse::Gnaw "thing" function is a special character class that 
matches any single character. It is equivalent to the perl
regular expression '.' character. But perl does not allow '.' to be
the name of a subroutine, so I called it "thing". Part of the reason 
for this name was because of how it works well with quantifier 
shortcuts of "any" and "some", described below.

The "thing" function takes no parameters.

	$grammar=match('b', thing, 'b');

	ok($grammar->('bob')==1, "bob matches");
	ok($grammar->('bb')==0, "bb does not match");

=head2 a (alternation)

The "a" function is an alternation function. It interprets each
of its parameters to be one of several possible alternates.

The following will look for one of any possible fruit in your text.

	$grammar=match(a('apple', 'banana', 'pear', 'peach'));

	ok($grammar->("she is the apple of my eye")==1, "found apple");

Because listing a bunch of alternate literals is fairly common,
the "qa" function allows you to do that with fewer quotation marks.
The "qa" function takes a single string, breaks it up into individual
literals based on whitespace, and then treats each literal as an
alternate in the grammar.

The above example could be shortened to this:

	$grammar = match(qa('apple banana pear peach'));

	ok($grammar->("she is the apple of my eye")==1, "found apple");
	
Grouping of a sequence of components into one alternate is done
by putting the sequence in an array reference, see "list context"
issues described above for a more detailed explanation of why this
is.

The following example looks for two possible alternates. The first
alternate is the literal 'hello' followed by the literal 'world'.
The second alternate is the literal 'howdy' followed by 'partner'.

	$grammar = match(a( ['hello', 'world'], ['howdy', 'partner'] ));
	ok($grammar->("why howdy partner !")==1, "found alternate");

The alternates can be any sequence of grammar components wrapped in 
an array reference.

=head2 g and t (greedy and thrifty quantifiers)

The "g" and "t" functions are quantifier functions. They allow you 
to define a grammar that looks for some pattern that repeats for
some number of times.

The format for a quantifier call looks like this:

	g ( [min, max(?)], grammar component(, component)* )

The first parameter to the "g" or "t" function is an array reference.
This contains the parameters that control how the quantifier works.
The "min" value determines the minimum number of times the pattern
must match before considered a pass. The "max" value is optional. If 
max is not defined, it is assumed to be infinite, and the parser will 
attempt to fit as many patterns into memory as can fit.

The remaining one or more parameters define the pattern to which 
the quantifier will be applied. If more than one are supplied, 
they will be applied as one large sequence.

The difference between "g" and "t" is how quickly the quantifier
will try to match. The "g" function is greedy and will try to match
as many patterns as possible. If it fails to match, the greedy 
quantifier will start backing off pattern counts and try to let the
rest of the grammar find a match.

The "t" function is thrifty and will try to match as few patterns
as possible. If the rest of the grammar fails to match, the 
thrifty pattern will try to match another pattern, before trying
the rest of the grammar.

	# seventeen or more 'g' or 'a' or 't' or 'c' characters.
	$grammar = match( g([17], cc('gatc')) ); 

	# one or more "word" characters
	$grammar = match( g([1], ccword) );

	# zero or more "digit" characters
	$grammar = match( g([0], ccdigit) );

	# at least 3 and up to 8 letters
	$grammar = match( g([3,8], cc('a-zA-Z')));

Note that the "g" and "t" quantifiers are functions that can accept
one or more parameters passed into them. These parameters can be
other grammar components, string literals, or array references
for grouped bits of grammar.

	# one or more 'hello' followed by 'world'
	$grammar = match( g([1], 'hello', 'world') )

Shorcuts are provided for g([0],...) and g([1], ...), called 
"any" and "some" respectively

=head2 any and some functions

Because greedy quantifiers of "zero or more" and "one or more" are fairly 
common, shortcuts 'any' and 'some' have been provided.

	$grammar = match( any(ccdigit) ); # zero or more digits

	$grammar = match( some(ccword) ); # one or more word characters.

The "any" and "some" functions take any number of input parameters,
and the parameters can be string literals, array references, and other
grammar components.

	# one or more of "hello" followed by "world"
	$grammar = match( some('hello', 'world') );

=head2 anything and something functions

The character set "thing" matches any single character. The "any" and "some" 
functions are quantifiers of zero or more and one or more. These two functions
are commonly combined, therefore a shortcut has been provided:

	anything  => g([0], thing)
	something => g([1], thing)

You can use these in your grammars to make them slightly more readable.

	# literal 'hello' followed by 1 or more characters, followed by
	# literal 'how', literal 'are', and literal 'you'
	$grammar = match( 'hello', something, 'how', 'are', 'you' );

=head2 get

The "get" function is used to capture the text that matches a specific portion
of the grammar and pass it into a user defined subroutine.

The first parameter of the "get" function can be one of several possible things. 

(1) a scalar reference

(2) an array reference

(3) a subroutine reference. 

If the first parameter is a scalar reference, the scalar will be used 
to hold whatever matched the "get" function.

	my $name;

	$grammar = match('hello', get(\$name, some(ccword)), '!' );

	$grammar->('hello Alice !');

	print "name is '$name'\n";

	> name is 'Alice'

If your grammar gets more than one match while it parses,
the scalar reference only holds the last match. If you use the
same grammar to parse different strings, scalars are initialized
to whatever value they held when the grammar was generated, and
this value is overwritten only if a match occurs.

	my $name;

	$grammar = match('hello', get(\$name, some(ccword)), '!' );
	$grammar->('hello Alice !');
	$grammar->('hello Bob !');
	$grammar->('hello Charlie !');

	print "name is '$name'\n";

	> name is 'Charlie'

If the first parameter is an array reference, then every match
by the "get" function will get pushed into the array. If you call
the same grammar multiple times, the array is emptied at the start
of parsing. Note this behaviour is different from scalars.

	my @name;

	$grammar = match(some('hello', get(\@name, some(ccword)), '!') );
	$grammar->('hello Alice ! hello Bob ! hello Charlie !');

	print "name is ". Dumper \@name;

	> name is $VAR1 = [
	>	'Alice',
	> 	'Bob',
	> 	'Charlie'
	> ];

If the first parameter is a subroutine reference, the parser will
call this subroutine on every match and pass in as the first parameter
the value of the string that matched. You may then do whatever you
wish with this string inside the subroutine you define.	

	my $name = sub{
		my ($string) = @_;
		# write $string to a file, or whatever
		print "called subroutine, received '$string'\n";
	};

	$grammar = match('hello', get($name, some(ccword)), '!' );

	$grammar->('hello Alice !');

	> called subroutine, received 'Alice'


The remaining parameters of the "get" function are one or more 
grammar components. Literals, character classes, alternations, 
quantifiers, array refs used for grouping sequences of grammar
components, etc.

The "get" function can be nested, like nested capturing parens
in perl regular expressions.

	my ($g1, $g2, $g3, $g4, $g5, $g6, $g7, $g8);

	$grammar = parse( 
		get(\$g8,
			get(\$g1, get(\$g2, thing), get(\$g3, thing) ),
			get(\$g4, get(\$g5, thing), get(\$g6, thing), get(\$g7, thing) ),
		) 
	
	);

	$grammar->('abcdefghijklmnop');
	
	ok($g2 eq 'a', "g2");
	ok($g3 eq 'b', "g3");
	ok($g1 eq 'ab', "g1");
	ok($g5 eq 'c', "g5");
	ok($g6 eq 'd', "g6");
	ok($g7 eq 'e', "g7");
	ok($g4 eq 'cde', "g4");
	ok($g8 eq 'abcde', "g8");
	

=head2 c (capture)

For short grammars or regular expressions, the "get" function might be
a bit of needless overhead. For those who want to save a bit of typing, 
the "c" function is available.

The "c" function is somewhat like "capture parens" in perl regular 
expressions in that it captures whatever that matches inside the
parens and puts it into a numbered variable. The "c" function uses
numbered variables that look like this:

	$c1 $c2 $c3 and so on.

You can have up to 99 capture variables. The variables are declared
and imported into your namespace when you use the Parse::Gnaw package.

An exmaple using the C function might look like this:

	$grammar = parse( 
		c( 
			c( c(thing),c(thing) ),
			c( c(thing), c(thing), c(thing) ),
		) 	
	);

	$grammar->('abcdefghijklmnop');

	ok($c1 eq 'abcde', 	"checking c1 ");
	ok($c2 eq 'ab',    	"checking c2 ");
	ok($c3 eq 'a', 		"checking c3 ");
	ok($c4 eq 'b', 		"checking c4 ");
	ok($c5 eq 'cde', 	"checking c5 ");
	ok($c6 eq 'c', 		"checking c6 ");
	ok($c7 eq 'd', 		"checking c7 ");
	ok($c8 eq 'e', 		"checking c8 ");
	

One significant difference between "c" numbered variables and 
perl regular expression capture parens is that the "c" function
puts the captured results into the numbered variable that 
corresponds to the number of left parens that actually match
the particular string being parsed.

This shows up when your grammar uses the "a" (alternation) function.
In the below example, we have a grammar that uses an alternation to
look for one of two possible sentences, and then capture different
bits of each sentence. 


	$grammar = parse( 
		a(  
			['hello', c('world'), 'my', 'name', 'is', c(something)],
			['howdy', c(something)]
		) 
	
	);

	ok($grammar->('hello world my name is Alice')==1, "1 match");

	ok($c1 eq 'world', 'c1 is world');
	ok($c2 eq 'Alice', 'c2 is alice');

	ok($grammar->('howdy Bob')==1, "1 match");
		
	ok($c1 eq 'Bob', 'c1 is Bob');

With the first string we parse, the first alternate matches, therefore
the first "c" function is assigned to "$c1" and the second "c" function
is assigned to "$c2". 

With the second string we parse, the second alternate matches and 
the c(something) function after 'howdy' is assigned to "$c1".
The two "c" functions in the first alternate were not part of the 
correct interpretation, and therefore were not used to count the
numbered variables.

=head2 x (exchange inline)

The "x" function is used to exchange text in the string being parsed with
some other text supplied by the user. The user provides a subroutine reference
as the first parameter. If the grammar matches, the "x" function will take 
the text that matched, pass it into the users code reference, and then
whatever that subroutine call returns will replace the matching text.

Note that "x" is a lower level function, so it can be used within a grammar
just like "get" or "a" or "g" and so on.

The following example looks for 'hello', name, '!' and exchanges name with 
an all upper case version of name.

	


=head2 now versus defer

The "now" function allows you to define a subroutine to be called every time
the parser hits that part of the grammar. The code subroutine will be called whether
that part of the grammar eventually matches the text being parsed or not.

The "defer" function allows you to define a subroutine to be called only if
that branch of the grammar matches the text being parsed. The subroutine
handed to the "defer" function is called at the end of parsing when a match occurs
or when the "commit" function is called.

This example uses the "now" function:

	sub howdy { [ now(sub{print"trying howdy\n"}), 'howdy' ] }

	sub hi    { [ now(sub{print"trying hi   \n"}), 'hi' ] }

	$grammar = match(a(howdy, hi);

	$grammar->("hi");

The above example will print 

	trying howdy
	trying hi

Even though the alternative "howdy" ended up failing.


The next example uses the "defer" function instead:

	sub howdy { [ defer(sub{print"trying howdy\n"}), 'howdy' ] }

	sub hi    { [ defer(sub{print"trying hi   \n"}), 'hi' ] }

	$grammar = match(a(howdy, hi);

	$grammar->("hi");

The above example with print only

	trying hi

Because "howdy" ended up failing, the defered subroutine for "howdy" did not get called.

Because the "defer" function is more likely to be useful in more grammars, the short
cut to do a "defer" function is to simply pass in a subroutine to any of the other 
grammar functions. Any code reference will be assumed to be a "defer".

Rewriting the "defer" example above but with the shorthand would look like this:

	sub howdy { [ sub{print"trying howdy\n"}, 'howdy' ] }

	sub hi    { [ sub{print"trying hi   \n"}, 'hi' ] }

	$grammar = match(a(howdy, hi);

	$grammar->("hi");

As before, the above example with print only

	trying hi


The "defer" function is useful for doing things only when the parser knows it has
a correct interpretation of the text being parsed. The "get" function is a "defer"
function that has been wrapped.

The "now" function is useful for doing something even if the interpretation might 
fail later. One example is printing debug statements of a grammar to understand 
where ther parser is going as it works through a piece of text. The "commit" function
is a "now" function that has been wrapped with extra features.

=head2 commit

The "commit" function is used to commit the parser to a particular interpretation
of a grammar. If the parser is working a grammar and applying it to a text,
and the parser hits a "commit" function, then that path in the grammar must
match. If the grammar fails to match after a "commit", rather than try any other 
possible interpretations, the parser will quit with an error.

The "commit" function will cause any currently scheduled "defer" functions 
to be executed by the parser. The "commit" function can also trigger the
text being parsed to be deleted up to the current character being parsed.

Use "commit" if your grammar contains a reserved keyword and you want to force
the parser to interpret the text as a keyword. Use the "commit" function if
you are parsing a large chunk of text and you want to be able to remove the
previous text from memory.

	$grammar = match(some('keyword', commit, 'blah', 'blah', 'blah'));

=head1 EXAMPLES

=head2 trekisms

The following grammar shows how one might identify dialogue and 
attribute it to a particular fictional character. 

The "attributeline" subroutine allowed us to bundle the functionality
of the output into one spot and then easily use that functionality 
at several different points in the grammar.

	my $captured="";

	sub attributeline { 
		my ($name, $restofgrammar) = @_;
		my $callback = sub{$captured.=sprintf ("%8s says: %s\n", $name,shift(@_));};
		my $stitcher = get($callback, $restofgrammar);
		return $stitcher;
	}

	sub trekname { qa('Jim Captain Spock Bones Doctor Scotty') } 
	sub occupation {a('ditch digger', 'bricklayer', 'mechanic')}
	sub mccoy_job { [ql("I'm a doctor, not a"), occupation, a('!', '.')] }
	sub mccoy_diag { [ "He's", 'dead', ',', trekname, a('!', '.') ] }
	sub mccoy_rant1 { [ql('You green-blooded Vulcan'), a('!', '.') ] }
	sub mccoy_isms {
		attributeline('McCoy', a(mccoy_job, mccoy_diag, mccoy_rant1)) 
	}

	sub spock_awe {['Fascinating', ',', trekname, '.']}
	sub spock_logic {['Highly', 'illogical',',', trekname, '.']}
	sub spock_sensors { [ql("It's life ,"), trekname, ql(', but not as we know it .')]}
	sub spock_isms {
		attributeline('Spock', a(spock_awe, spock_logic, spock_sensors))
	}
	
	sub kirk_dipolomacy1 {ql('We come in peace .')}
	sub kirk_dipolomacy2 {ql('Shoot to kill .')}
	sub kirk_to_scotty {ql('I need warp speed now, Scotty !')}
	sub kirk_to_spock {ql('What is it , Spock ?')}
	sub kirk_to_bones {ql('Just fix him , Bones')}
	sub kirk_solution {ql('Activate ship self-destruct mechanism .')}
	sub kirk_isms {
		attributeline('Kirk', a(
			kirk_dipolomacy1, 
			kirk_dipolomacy2,
			kirk_to_scotty,
			kirk_to_spock,	
			kirk_to_bones,	
			kirk_solution
		))
	}

	sub scotty_phy101 {ql('Ya kenna change the laws of physics .')}
	sub time_units {qa('minutes hours days weeks')}
	sub scotty_estimate {[ ql("I'll have it ready for you in three"), time_units, '.' ]}
	
	sub scotty_isms {attributeline('Scotty', a(scotty_phy101, scotty_estimate))}
	
	
	sub alien_isms {attributeline('alien', 'weeboo')}
	
	
	sub trek_isms {a(mccoy_isms, spock_isms, kirk_isms, scotty_isms, alien_isms )}
	sub trek_script {some(trek_isms)}	
	
	$grammar = parse(  trek_script );
		
	my $script = <<'SCRIPT';
	What is it, Spock?
	It's life, Jim, but not as we know it.
	We come in peace.
	weeboo
	Shoot to kill.
	weeboo
	I need warp speed now, Scotty!
	I'll have it ready for you in three minutes.
	weeboo
	I need warp speed now, Scotty!
	Ya kenna change the laws of physics.	
	weeboo
	weeboo
	Shoot to kill.
	Shoot to kill.
	I'm a doctor, not a bricklayer.
	Highly illogical, Doctor.
	You green-blooded Vulcan.
	Shoot to kill.
	Shoot to kill.
	He's dead, Jim.
	Activate ship self-destruct mechanism.
	Highly illogical, Captain.
	SCRIPT
	;

	#print "script is '$script'\n";

	ok($grammar->( $script )==1, "1 match");

	my $expected =  <<'EXPECTED';
	    Kirk says: What is it, Spock?
	   Spock says: It's life, Jim, but not as we know it.
	    Kirk says: We come in peace.
	   alien says: weeboo
	    Kirk says: Shoot to kill.
	   alien says: weeboo
	    Kirk says: I need warp speed now, Scotty!
	  Scotty says: I'll have it ready for you in three minutes.
	   alien says: weeboo
	    Kirk says: I need warp speed now, Scotty!
	  Scotty says: Ya kenna change the laws of physics.
	   alien says: weeboo
	   alien says: weeboo
	    Kirk says: Shoot to kill.
	    Kirk says: Shoot to kill.
	   McCoy says: I'm a doctor, not a bricklayer.
	   Spock says: Highly illogical, Doctor.
	   McCoy says: You green-blooded Vulcan.
	    Kirk says: Shoot to kill.
	    Kirk says: Shoot to kill.
	   McCoy says: He's dead, Jim.
	    Kirk says: Activate ship self-destruct mechanism.
	   Spock says: Highly illogical, Captain.
	EXPECTED
	;
	
		
	
	ok($captured eq $expected, "checking captured string matches expected");

The above example is included as a test in the t directory of the tarball from CPAN.

	
=head1 NOT IMPLEMENTED YET



=head2 recursive

The "recursive" function has not been implemented yet.

If you define a grammar which is recursive, you must
wrap the recursive call in the "recursive" function. 

If you declare a recursive grammar without using a 
"recursive" function, then perl will recursively call 
the subroutine until it dies a horrible death.

	sub expression;
	sub expression{ a([ '(' expression ')'], 'a+b') }

	# this call to "expression" will recursively call itself
	# until perl crashes.
	my $grammar = match(expression); 

The above example will crash. Note that it crashes during the
"generate" stage of creating the grammar. The grammar isnt
even being used to try to match a string yet. The mere act of 
declaring a recursive grammar without the "recursive" function will
cause a crash.

What you need to do is wrap any recursive call to a rule in
the "recursive" function.

	sub expression;
	sub expression{ a([ '(' recursive(\&expression) ')'], 'a+b') }

	my $grammar = match(expression);



=cut 





=head1 INTERNALS

These are notes to developers who may need to understand how
the internals of Parse::Gnaw operate so they can create 
advanced grammars.

Also, these are notes to myself to remember how things function.

=head2 Text being parsed.

The text being parsed is held internally in a linked list. 
This allows Parse::Gnaw to operate on a small subset of the 
entire string being parsed. Theoretically, Parse::Gnaw can 
parse an infinite amount of text as long as it can be 
parsed in chunks small enough that the rules can disambiguate 
text that fits in memory.

Each element in the linked list is an array. 

Array stores the following data: 

	0: numeric indicator of what type of element this is
	1: payload
	2: previous element pointer
	3: next element pointer
	4: used for debugging

There are several "types" of elements as indicated by the
value in index 0 of an element:

	0: element has been deleted. If code tries to use 
		a deleted element, this should act as a 
		flag that something went wrong
	1: element holds a single text character. 
		letter is stored in payload index.
	2: element holds a "marker". Markers are placed between
		text elements by the parser to keep track of
		where it is in the code, where captured strings
		are located, where to fallback to in the text
		if a particular branch of a parse fails, etc.
		A marker contains no payload.
	3: element contains a callback to be executed on
		successful completion of parsing or a commit.
		Payload contains code ref to callback.
	4: element is a head/tail endpoint

The linked list starts and ends with two signpost elements:
'head' and 'tail'. No text is ever stored in these signpost
elements. Any text being parsed is always contained between
these two signpost elements.

The 'current' pointer always points to the text that is about
to be processed.

When the __gnaw__parse_failed function is called, it will
pop the last fallback position off the fallback array, 
which will change the next instruction to be executed and
will also move the text pointer back to teh fallback marker.
The last thing the __gnaw__parse_failed function does is
delete the fallback marker and any other elements in the
text linked list that are not text.

When any instruction moves the text pointer forward, it
will also delete any old markers it encounters.

The current pointer may point to the tail signpost element.
This may mean that we are at the end of the string, or it
may mean that the next time we try to get the current letter
that we need to insert more text from a file or something.

All interfaces to the linked list should be through the 
subroutines designed for working with the linked list.
These start with the prefix "__gnaw__" to help prevent
subroutine colisions.

All parse level commands should manipulate text based on markers.

No user-level or grammar-level commands are currently supported.
You can always call the __gnaw__* functions, but they are
currently subject to change.

=head2 Generating the Parser

tbd



=head1 AUTHOR

Greg London, C<< <email at greglondon.com> >>

=head1 BUGS

Please report any bugs or feature requests to C<bug-parse-gnaw at rt.cpan.org>, or through
the web interface at L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Parse-Gnaw>.  I will be notified, and then you will
automatically be notified of progress on your bug as I make changes.




=head1 SUPPORT

You can find documentation for this module with the perldoc command.

    perldoc Parse::Gnaw


You can also look for information at:

=over 4

=item * RT: CPANs request tracker

L<http://rt.cpan.org/NoAuth/Bugs.html?Dist=Parse-Gnaw>

=item * AnnoCPAN: Annotated CPAN documentation

L<http://annocpan.org/dist/Parse-Gnaw>

=item * CPAN Ratings

L<http://cpanratings.perl.org/d/Parse-Gnaw>

=item * Search CPAN

L<http://search.cpan.org/dist/Parse-Gnaw>

=back


=head1 ACKNOWLEDGEMENTS


=head1 COPYRIGHT & LICENSE

Copyright 2008 Greg London, all rights reserved.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.


=cut
