Provided by: libparser-mgc-perl_0.21-1_all 
NAME
"Parser::MGC" - build simple recursive-descent parsers
SYNOPSIS
package My::Grammar::Parser;
use base qw( Parser::MGC );
sub parse
{
my $self = shift;
$self->sequence_of( sub {
$self->any_of(
sub { $self->token_int },
sub { $self->token_string },
sub { \$self->token_ident },
sub { $self->scope_of( "(", \&parse, ")" ) }
);
} );
}
my $parser = My::Grammar::Parser->new;
my $tree = $parser->from_file( $ARGV[0] );
...
DESCRIPTION
This base class provides a low-level framework for building recursive-descent parsers that
consume a given input string from left to right, returning a parse structure. It takes its
name from the "m//gc" regexps used to implement the token parsing behaviour.
It provides a number of token-parsing methods, which each extract a grammatical token from
the string. It also provides wrapping methods that can be used to build up a possibly-
recursive grammar structure, by applying a structure around other parts of parsing code.
Backtracking
Each method, both token and structural, atomically either consumes a prefix of the string
and returns its result, or fails and consumes nothing. This makes it simple to implement
grammars that require backtracking.
Several structure-forming methods have some form of "optional" behaviour; they can
optionally consume some amount of input or take some particular choice, but if the code
invoked inside that subsequently fails, the structure can backtrack and take some
different behaviour. This is usually what is required when testing whether the structure
of the input string matches some part of the grammar that is optional, or has multiple
choices.
However, once the choice of grammar has been made, it is often useful to be able to fix on
that one choice, thus making subsequent failures propagate up rather than taking that
alternative behaviour. Control of this backtracking is given by the "commit" method; and
careful use of this method is one of the key advantages that "Parser::MGC" has over more
simple parsing using single regexps alone.
Stall Detection
Most of the methods in this class have bounded execution time, but some methods ("list_of"
and "sequence_of") repeatedly recuse into other code to build up a list of results until
some ending condition is reached. A possible class of bug is that whatever they recurse
into might successfully match an empty string, and thus make no progress.
These methods will automatically detect this situation if they repeatedly encounter the
same string position more than a certain number of times (given by the "stallcount"
argument). If this count is reached, the entire parse attempt will be aborted by the "die"
method.
CONSTRUCTOR
new
$parser = Parser::MGC->new( %args )
Returns a new instance of a "Parser::MGC" object. This must be called on a subclass that
provides method of the name provided as "toplevel", by default called "parse".
Takes the following named arguments
toplevel => STRING
Name of the toplevel method to use to start the parse from. If not supplied, will
try to use a method called "parse".
patterns => HASH
Keys in this hash should map to quoted regexp ("qr//") references, to override the
default patterns used to match tokens. See "PATTERNS" below
accept_0o_oct => BOOL
If true, the "token_int" method will also accept integers with a "0o" prefix as
octal.
stallcount => INT
Since version 0.21.
The number of times that the stall-detector would have to see the same position
before it aborts the parse attempt. If not supplied, a default of 10 will apply.
PATTERNS
The following pattern names are recognised. They may be passed to the constructor in the
"patterns" hash, or provided as a class method under the name "pattern_name".
• ws
Pattern used to skip whitespace between tokens. Defaults to "/[\s\n\t]+/"
• comment
Pattern used to skip comments between tokens. Undefined by default.
• int
Pattern used to parse an integer by "token_int". Defaults to
"/-?(?:0x[[:xdigit:]]+|[[:digit:]]+)/". If "accept_0o_oct" is given, then this will be
expanded to match "/0o[0-7]+/" as well.
• float
Pattern used to parse a floating-point number by "token_float". Defaults to
"/-?(?:\d*\.\d+|\d+\.)(?:e-?\d+)?|-?\d+e-?\d+/i".
• ident
Pattern used to parse an identifier by "token_ident". Defaults to "/[[:alpha:]_]\w*/"
• string_delim
Pattern used to delimit a string by "token_string". Defaults to "/["']/".
SUBCLASSING METHODS
The following optional methods may be defined by subclasses, to customise their parsing.
on_parse_start
$parser->on_parse_start
Since version 0.21.
If defined, is invoked by the "from_*" method that begins a new parse operation, just
before invoking the toplevel structure method.
on_parse_end
$result = $parser->on_parse_end( $result )
Since version 0.21.
If defined, is invoked by the "from_*" method once it has finished the toplevel structure
method. This is passed the tentative result from the structure method, and whatever it
returns becomes the result of the "from_*" method itself.
METHODS
from_string
$result = $parser->from_string( $str )
Parse the given literal string and return the result from the toplevel method.
from_file
$result = $parser->from_file( $file, %opts )
Parse the given file, which may be a pathname in a string, or an opened IO handle, and
return the result from the toplevel method.
The following options are recognised:
binmode => STRING
If set, applies the given binmode to the filehandle before reading. Typically this
can be used to set the encoding of the file.
$parser->from_file( $file, binmode => ":encoding(UTF-8)" )
filename
$filename = $parser->filename
Since version 0.20.
Returns the name of the file currently being parsed, if invoked from within "from_file".
from_reader
$result = $parser->from_reader( \&reader )
Since version 0.05.
Parse the input which is read by the "reader" function. This function will be called in
scalar context to generate portions of string to parse, being passed the $parser object.
The function should return "undef" when it has no more string to return.
$reader->( $parser )
Note that because it is not generally possible to detect exactly when more input may be
required due to failed regexp parsing, the reader function is only invoked during
searching for skippable whitespace. This makes it suitable for reading lines of a file in
the common case where lines are considered as skippable whitespace, or for reading lines
of input interactively from a user. It cannot be used in all cases (for example, reading
fixed-size buffers from a file) because two successive invocations may split a single
token across the buffer boundaries, and cause parse failures.
pos
$pos = $parser->pos
Since version 0.09.
Returns the current parse position, as a character offset from the beginning of the file
or string.
take
$str = $parser->take( $len )
Since version 0.16.
Returns the next $len characters directly from the input, prior to any whitespace or
comment skipping. This does not take account of any end-of-scope marker that may be
pending. It is intended for use by parsers of partially-binary protocols, or other
situations in which it would be incorrect for the end-of-scope marker to take effect at
this time.
where
( $lineno, $col, $text ) = $parser->where
Returns the current parse position, as a line and column number, and the entire current
line of text. The first line is numbered 1, and the first column is numbered 0.
fail
fail_from
$parser->fail( $message )
$parser->fail_from( $pos, $message )
"fail_from" since version 0.09.
Aborts the current parse attempt with the given message string. The failure message will
include the line and column position, and the line of input that failed at the current
parse position ("fail"), or a position earlier obtained using the "pos" method
("fail_from").
This failure will propagate up to the inner-most structure parsing method that has not
been committed; or will cause the entire parser to fail if there are no further options to
take.
die
die_from
$parser->die( $message )
$parser->die_from( $pos, $message )
Since version 0.20.
Throws an exception that propagates as normal for "die", entirely out of the entire parser
and to the caller of the toplevel "from_*" method that invoked it, bypassing all of the
back-tracking logic.
This is much like using core's "die" directly, except that the message string will include
the line and column position, and the line of input that the parser was working on, as it
does in the "fail" method.
This method is intended for reporting fatal errors where the parsed input was correctly
recognised at a grammar level, but is requesting something that cannot be fulfilled
semantically.
at_eos
$eos = $parser->at_eos
Returns true if the input string is at the end of the string.
scope_level
$level = $parser->scope_level
Since version 0.05.
Returns the number of nested "scope_of" calls that have been made.
include_string
$result = $parser->include_string( $str, %opts )
Since version 0.21.
Parses a given string into the existing parser object.
The current parser state is moved aside from the duration of this method, and is replaced
by the given string. Then the toplevel parser method (or a different as specified) is
invoked over it. Its result is returned by this method.
This would typically be used to handle some sort of "include" or "macro expansion"
ability, by injecting new content in as if the current parse location had encountered it.
Other than the internal parser state, other object fields are not altered, so whatever
effects the invoked parsing methods will have on it can continue to inspect and alter it
as required.
The following options are recognised:
filename => STRING
If set, provides a filename (or other descriptive text) to pretend for the source
of this string. It need not be a real file on the filesystem; it could for example
explain the source of the string in some other way. It is the value reported by
the "filename" method and printed in failure messages.
toplevel => STRING | CODE
If set, provides the toplevel parser method to use within this inclusion,
overriding the object's defined default.
STRUCTURE-FORMING METHODS
The following methods may be used to build a grammatical structure out of the defined
basic token-parsing methods. Each takes at least one code reference, which will be passed
the actual $parser object as its first argument.
Anywhere that a code reference is expected also permits a plain string giving the name of
a method to invoke. This is sufficient in many simple cases, such as
$self->any_of(
'token_int',
'token_string',
...
);
maybe
$ret = $parser->maybe( $code )
Attempts to execute the given $code in scalar context, and returns what it returned,
accepting that it might fail. $code may either be a CODE reference or a method name given
as a string.
If the code fails (either by calling "fail" itself, or by propagating a failure from
another method it invoked) before it has invoked "commit", then none of the input string
will be consumed; the current parsing position will be restored. "undef" will be returned
in this case.
If it calls "commit" then any subsequent failure will be propagated to the caller, rather
than returning "undef".
This may be considered to be similar to the "?" regexp qualifier.
sub parse_declaration
{
my $self = shift;
[ $self->parse_type,
$self->token_ident,
$self->maybe( sub {
$self->expect( "=" );
$self->parse_expression
} ),
];
}
scope_of
$ret = $parser->scope_of( $start, $code, $stop )
Expects to find the $start pattern, then attempts to execute the given $code, then expects
to find the $stop pattern. Returns whatever the code returned. $code may either be a CODE
reference of a method name given as a string.
While the code is being executed, the $stop pattern will be used by the token parsing
methods as an end-of-scope marker; causing them to raise a failure if called at the end of
a scope.
sub parse_block
{
my $self = shift;
$self->scope_of( "{", 'parse_statements', "}" );
}
If the $start pattern is undefined, it is presumed the caller has already checked for
this. This is useful when the stop pattern needs to be calculated based on the start
pattern.
sub parse_bracketed
{
my $self = shift;
my $delim = $self->expect( qr/[\(\[\<\{]/ );
$delim =~ tr/([<{/)]>}/;
$self->scope_of( undef, 'parse_body', $delim );
}
This method does not have any optional parts to it; any failures are immediately
propagated to the caller.
committed_scope_of
$ret = $parser->committed_scope_of( $start, $code, $stop )
Since version 0.16.
A variant of "scope_of" that calls "commit" after a successful match of the start pattern.
This is usually what you want if using "scope_of" from within an "any_of" choice, if no
other alternative following this one could possibly match if the start pattern has.
list_of
$ret = $parser->list_of( $sep, $code )
Expects to find a list of instances of something parsed by $code, separated by the $sep
pattern. Returns an ARRAY ref containing a list of the return values from the $code. A
single trailing delimiter is allowed, and does not affect the return value. $code may
either be a CODE reference or a method name given as a string. It is called in list
context, and whatever values it returns are appended to the eventual result - similar to
perl's "map".
This method does not consider it an error if the returned list is empty; that is, that the
scope ended before any item instances were parsed from it.
sub parse_numbers
{
my $self = shift;
$self->list_of( ",", 'token_int' );
}
If the code fails (either by invoking "fail" itself, or by propagating a failure from
another method it invoked) before it has invoked "commit" on a particular item, then the
item is aborted and the parsing position will be restored to the beginning of that failed
item. The list of results from previous successful attempts will be returned.
If it calls "commit" within an item then any subsequent failure for that item will cause
the entire "list_of" to fail, propagating that to the caller.
sequence_of
$ret = $parser->sequence_of( $code )
A shortcut for calling "list_of" with an empty string as separator; expects to find at
least one instance of something parsed by $code, separated only by skipped whitespace.
This may be considered to be similar to the "+" or "*" regexp qualifiers.
sub parse_statements
{
my $self = shift;
$self->sequence_of( 'parse_statement' );
}
The interaction of failures in the code and the "commit" method is identical to that of
"list_of".
any_of
$ret = $parser->any_of( @codes )
Since version 0.06.
Expects that one of the given code instances can parse something from the input, returning
what it returned. Each code instance may indicate a failure to parse by calling the "fail"
method or otherwise propagating a failure. Each code instance may either be a CODE
reference or a method name given as a string.
This may be considered to be similar to the "|" regexp operator for forming alternations
of possible parse trees.
sub parse_statement
{
my $self = shift;
$self->any_of(
sub { $self->parse_declaration; $self->expect(";") },
sub { $self->parse_expression; $self->expect(";") },
sub { $self->parse_block },
);
}
If the code for a given choice fails (either by invoking "fail" itself, or by propagating
a failure from another method it invoked) before it has invoked "commit" itself, then the
parsing position restored and the next choice will be attempted.
If it calls "commit" then any subsequent failure for that choice will cause the entire
"any_of" to fail, propagating that to the caller and no further choices will be attempted.
If none of the choices match then a simple failure message is printed:
Found nothing parseable
As this is unlikely to be helpful to users, a better message can be provided by the final
choice instead. Don't forget to "commit" before printing the failure message, or it won't
count.
$self->any_of(
'token_int',
'token_string',
...,
sub { $self->commit; $self->fail( "Expected an int or string" ) }
);
commit
$parser->commit
Calling this method will cancel the backtracking behaviour of the innermost "maybe",
"list_of", "sequence_of", or "any_of" structure forming method. That is, if later code
then calls "fail", the exception will be propagated out of "maybe", no further list items
will be attempted by "list_of" or "sequence_of", and no further code blocks will be
attempted by "any_of".
Typically this will be called once the grammatical structure alter has been determined,
ensuring that any further failures are raised as real exceptions, rather than by
attempting other alternatives.
sub parse_statement
{
my $self = shift;
$self->any_of(
...
sub {
$self->scope_of( "{",
sub { $self->commit; $self->parse_statements; },
"}" ),
},
);
}
Though in this common pattern, "committed_scope_of" may be used instead.
TOKEN PARSING METHODS
The following methods attempt to consume some part of the input string, to be used as part
of the parsing process.
expect
$str = $parser->expect( $literal )
$str = $parser->expect( qr/pattern/ )
@groups = $parser->expect( qr/pattern/ )
Expects to find a literal string or regexp pattern match, and consumes it. In scalar
context, this method returns the string that was captured. In list context it returns the
matching substring and the contents of any subgroups contained in the pattern.
This method will raise a parse error (by calling "fail") if the regexp fails to match.
Note that if the pattern could match an empty string (such as for example "qr/\d*/"), the
pattern will always match, even if it has to match an empty string. This method will not
consider a failure if the regexp matches with zero-width.
maybe_expect
$str = $parser->maybe_expect( ... )
@groups = $parser->maybe_expect( ... )
Since version 0.10.
A convenient shortcut equivalent to calling "expect" within "maybe", but implemented more
efficiently, avoiding the exception-handling set up by "maybe". Returns "undef" or an
empty list if the match fails.
substring_before
$str = $parser->substring_before( $literal )
$str = $parser->substring_before( qr/pattern/ )
Since version 0.06.
Expects to possibly find a literal string or regexp pattern match. If it finds such,
consume all the input text before but excluding this match, and return it. If it fails to
find a match before the end of the current scope, consumes all the input text until the
end of scope and return it.
This method does not consume the part of input that matches, only the text before it. It
is not considered a failure if the substring before this match is empty. If a non-empty
match is required, use the "fail" method:
sub token_nonempty_part
{
my $self = shift;
my $str = $parser->substring_before( "," );
length $str or $self->fail( "Expected a string fragment before ," );
return $str;
}
Note that unlike most of the other token parsing methods, this method does not consume
either leading or trailing whitespace around the substring. It is expected that this
method would be used as part a parser to read quoted strings, or similar cases where
whitespace should be preserved.
nonempty_substring_before
$str = $parser->nonempty_substring_before( $literal )
$str = $parser->nonempty_substring_before( qr/pattern/ )
Since version 0.20.
A variant of "substring_before" which fails if the matched part is empty.
The example above could have been written:
sub token_nonempty_part
{
my $self = shift;
return $parser->nonempty_substring_before( "," );
}
This is often useful for breaking out of repeating loops; e.g.
sub token_escaped_string
{
my $self = shift;
$self->expect( '"' );
my $ret = "";
1 while $self->any_of(
sub { $ret .= $self->nonempty_substring_before( qr/%|$/m ); 1 }
sub { my $escape = ( $self->expect( qr/%(.)/ ) )[1];
$ret .= _handle_escape( $escape );
1 },
sub { 0 },
)
return $ret;
}
generic_token
$val = $parser->generic_token( $name, $re, $convert )
Since version 0.08.
Expects to find a token matching the precompiled regexp $re. If provided, the $convert
CODE reference can be used to convert the string into a more convenient form. $name is
used in the failure message if the pattern fails to match.
If provided, the $convert function will be passed the parser and the matching substring;
the value it returns is returned from "generic_token".
$convert->( $parser, $substr )
If not provided, the substring will be returned as it stands.
This method is mostly provided for subclasses to define their own token types. For
example:
sub token_hex
{
my $self = shift;
$self->generic_token( hex => qr/[0-9A-F]{2}h/, sub { hex $_[1] } );
}
token_int
$int = $parser->token_int
Expects to find an integer in decimal, octal or hexadecimal notation, and consumes it.
Negative integers, preceeded by "-", are also recognised.
token_float
$float = $parser->token_float
Since version 0.04.
Expects to find a number expressed in floating-point notation; a sequence of digits
possibly prefixed by "-", possibly containing a decimal point, possibly followed by an
exponent specified by "e" followed by an integer. The numerical value is then returned.
token_number
$number = $parser->token_number
Since version 0.09.
Expects to find a number expressed in either of the above forms.
token_string
$str = $parser->token_string
Expects to find a quoted string, and consumes it. The string should be quoted using """ or
"'" quote marks.
The content of the quoted string can contain character escapes similar to those accepted
by C or Perl. Specifically, the following forms are recognised:
\a Bell ("alert")
\b Backspace
\e Escape
\f Form feed
\n Newline
\r Return
\t Horizontal Tab
\0, \012 Octal character
\x34, \x{5678} Hexadecimal character
C's "\v" for vertical tab is not supported as it is rarely used in practice and it
collides with Perl's "\v" regexp escape. Perl's "\c" for forming other control characters
is also not supported.
token_ident
$ident = $parser->token_ident
Expects to find an identifier, and consumes it.
token_kw
$keyword = $parser->token_kw( @keywords )
Expects to find a keyword, and consumes it. A keyword is defined as an identifier which is
exactly one of the literal values passed in.
EXAMPLES
Accumulating Results Using Variables
Although the structure-forming methods all return a value, obtained from their nested
parsing code, it can sometimes be more convenient to use a variable to accumulate a result
in instead. For example, consider the following parser method, designed to parse a set of
"name: "value"" assignments, such as might be found in a configuration file, or
YAML/JSON-style mapping value.
sub parse_dict
{
my $self = shift;
my %ret;
$self->list_of( ",", sub {
my $key = $self->token_ident;
exists $ret{$key} and $self->fail( "Already have a mapping for '$key'" );
$self->expect( ":" );
$ret{$key} = $self->parse_value;
} );
return \%ret
}
Instead of using the return value from "list_of", this method accumulates values in the
%ret hash, eventually returning a reference to it as its result. Because of this, it can
perform some error checking while it parses; namely, rejecting duplicate keys.
TODO
• Make unescaping of string constants more customisable. Possibly consider instead a
"parse_string_generic" using a loop over "substring_before".
• Easy ability for subclasses to define more token types as methods. Perhaps provide a
class method such as
__PACKAGE__->has_token( hex => qr/[0-9A-F]+/i, sub { hex $_[1] } );
• Investigate how well "from_reader" can cope with buffer splitting across other tokens
than simply skippable whitespace
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>