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Parse::Yapp(3) |
User Contributed Perl Documentation |
Parse::Yapp(3) |
Parse::Yapp - Perl extension for generating and using LALR parsers.
yapp -m MyParser grammar_file.yp
...
use MyParser;
$parser=new MyParser();
$value=$parser->YYParse(yylex => \&lexer_sub, yyerror => \&error_sub);
$nberr=$parser->YYNberr();
$parser->YYData->{DATA}= [ 'Anything', 'You Want' ];
$data=$parser->YYData->{DATA}[0];
Parse::Yapp (Yet Another Perl Parser compiler) is a collection of modules that
let you generate and use yacc like thread safe (reentrant) parsers with perl
object oriented interface.
The script yapp is a front-end to the Parse::Yapp module and let
you easily create a Perl OO parser from an input grammar file.
- "Comments"
- Through all your files, comments are either Perl style, introduced by
# up to the end of line, or C style, enclosed between /* and
*/.
- "Tokens and string literals"
- Through all the grammar files, two kind of symbols may appear:
Non-terminal symbols, called also left-hand-side symbols,
which are the names of your rules, and Terminal symbols, called
also Tokens.
Tokens are the symbols your lexer function will feed your
parser with (see below). They are of two flavours: symbolic tokens and
string literals.
Non-terminals and symbolic tokens share the same identifier
syntax:
[A-Za-z][A-Za-z0-9_]*
String literals are enclosed in single quotes and can contain
almost anything. They will be output to your parser file double-quoted,
making any special character as such. '"', '$' and '@' will be
automatically quoted with '\', making their writing more natural. On the
other hand, if you need a single quote inside your literal, just quote
it with '\'.
You cannot have a literal 'error' in your grammar as it
would confuse the driver with the error token. Use a symbolic
token instead. In case you inadvertently use it, this will produce a
warning telling you you should have written it error and will
treat it as if it were the error token, which is certainly NOT
what you meant.
- "Grammar file syntax"
- It is very close to yacc syntax (in fact, Parse::Yapp should
compile a clean yacc grammar without any modification, whereas the
opposite is not true).
This file is divided in three sections, separated by
"%%":
header section
%%
rules section
%%
footer section
- The Header Section section may optionally contain:
- One or more code blocks enclosed inside
"%{" and
"%}" just like in yacc. They may contain
any valid Perl code and will be copied verbatim at the very beginning of
the parser module. They are not as useful as they are in yacc, but you can
use them, for example, for global variable declarations, though you will
notice later that such global variables can be avoided to make a reentrant
parser module.
- Precedence declarations, introduced by %left,
%right and %nonassoc
specifying associativity, followed by the list of tokens or litterals
having the same precedence and associativity. The precedence being the
latter declared will be having the highest level. (see the yacc or bison
manuals for a full explanation of how they work, as they are implemented
exactly the same way in Parse::Yapp)
- %start followed by a rule's left hand side,
declaring this rule to be the starting rule of your grammar. The default,
when %start is not used, is the first rule in your
grammar section.
- %token followed by a list of symbols, forcing them
to be recognized as tokens, generating a syntax error if used in the left
hand side of a rule declaration. Note that in Parse::Yapp, you
don't need to declare tokens as in yacc: any symbol not appearing
as a left hand side of a rule is considered to be a token. Other yacc
declarations or constructs such as %type and
%union are parsed but (almost) ignored.
- %expect followed by a number, suppress warnings
about number of Shift/Reduce conflicts when both numbers match, a la
bison.
- The Rule Section contains your grammar rules:
- A rule is made of a left-hand-side symbol, followed by a
':' and one or more right-hand-sides separated by
'|' and terminated by a
';':
exp: exp '+' exp
| exp '-' exp
;
A right hand side may be empty:
input: #empty
| input line
;
(if you have more than one empty rhs, Parse::Yapp will issue a
warning, as this is usually a mistake, and you will certainly have a
reduce/reduce conflict)
A rhs may be followed by an optional
%prec directive, followed by a token, giving the
rule an explicit precedence (see yacc manuals for its precise meaning)
and optional semantic action code block (see below).
exp: '-' exp %prec NEG { -$_[1] }
| exp '+' exp { $_[1] + $_[3] }
| NUM
;
Note that in Parse::Yapp, a lhs cannot appear more than
once as a rule name (This differs from yacc).
- "The footer section"
- may contain any valid Perl code and will be appended at the very end of
your parser module. Here you can write your lexer, error report subs and
anything relevant to you parser.
- "Semantic actions"
- Semantic actions are run every time a reduction occurs in the
parsing flow and they must return a semantic value.
They are (usually, but see below "In
rule actions") written at the very end of the rhs, enclosed
with "{ }", and are copied verbatim to
your parser file, inside of the rules table.
Be aware that matching braces in Perl is much more difficult
than in C: inside strings they don't need to match. While in C it is
very easy to detect the beginning of a string construct, or a single
character, it is much more difficult in Perl, as there are so many ways
of writing such literals. So there is no check for that today. If you
need a brace in a double-quoted string, just quote it
("\{" or
"\}"). For single-quoted strings, you
will need to make a comment matching it in th right order. Sorry
for the inconvenience.
{
"{ My string block }".
"\{ My other string block \}".
qq/ My unmatched brace \} /.
# Force the match: {
q/ for my closing brace } /
q/ My opening brace { /
# must be closed: }
}
All of these constructs should work.
In Parse::Yapp, semantic actions are called like normal Perl
sub calls, with their arguments passed in @_,
and their semantic value are their return values.
$_[1] to $_[n]
are the parameters just as $1 to
$n in yacc, while $_[0]
is the parser object itself.
Having $_[0] being the parser object
itself allows you to call parser methods. That's how the yacc macros are
implemented:
yyerrok is done by calling $_[0]->YYErrok
YYERROR is done by calling $_[0]->YYError
YYACCEPT is done by calling $_[0]->YYAccept
YYABORT is done by calling $_[0]->YYAbort
All those methods explicitly return undef, for
convenience.
YYRECOVERING is done by calling $_[0]->YYRecovering
Four useful methods in error recovery sub
$_[0]->YYCurtok
$_[0]->YYCurval
$_[0]->YYExpect
$_[0]->YYLexer
return respectivly the current input token that made the parse
fail, its semantic value (both can be used to modify their values too,
but know what you are doing ! See Error reporting routine
section for an example), a list which contains the tokens the parser
expected when the failure occurred and a reference to the lexer
routine.
Note that if
"$_[0]->YYCurtok" is declared as a
%nonassoc token, it can be included in
"$_[0]->YYExpect" list whenever the
input try to use it in an associative way. This is not a bug: the token
IS expected to report an error if encountered.
To detect such a thing in your error reporting sub, the
following example should do the trick:
grep { $_[0]->YYCurtok eq $_ } $_[0]->YYExpect
and do {
#Non-associative token used in an associative expression
};
Accessing semantics values on the left of your reducing rule
is done through the method
$_[0]->YYSemval( index )
where index is an integer. Its value being 1 .. n
returns the same values than $_[1] ..
$_[n], but -n .. 0 returns values on the left
of the rule being reduced (It is related to $-n .. $0
.. $n in yacc, but you cannot use
$_[0] or $_[-n] constructs in
Parse::Yapp for obvious reasons)
There is also a provision for a user data area in the parser
object, accessed by the method:
$_[0]->YYData
which returns a reference to an anonymous hash, which let you
have all of your parsing data held inside the object (see the Calc.yp or
ParseYapp.yp files in the distribution for some examples). That's how
you can make you parser module reentrant: all of your module states and
variables are held inside the parser object.
Note: unfortunately, method calls in Perl have a lot of
overhead,
and when YYData is used, it may be called a huge number
of times. If your are not a *real* purist and efficiency
is your concern, you may access directly the user-space
in the object: $parser->{USER} wich is a
reference to an
anonymous hash array, and then benchmark.
If no action is specified for a rule, the equivalant of a
default action is run, which returns the first parameter:
{ $_[1] }
- "In rule actions"
- It is also possible to embed semantic actions inside of a rule:
typedef: TYPE { $type = $_[1] } identlist { ... } ;
When the Parse::Yapp's parser encounter such an embedded
action, it modifies the grammar as if you wrote (although
@x-1 is not a legal lhs value):
@x-1: /* empty */ { $type = $_[1] };
typedef: TYPE @x-1 identlist { ... } ;
where x is a sequential number incremented for each
"in rule" action, and -1 represents the "dot
position" in the rule where the action arises.
In such actions, you can use $_[1]..$_[n]
variables, which are the semantic values on the left of your action.
Be aware that the way Parse::Yapp modifies your grammar
because of in rule actions can produce, in some cases, spurious
conflicts that wouldn't happen otherwise.
- "Generating the Parser Module"
- Now that you grammar file is written, you can use yapp on it to generate
your parser module:
yapp -v Calc.yp
will create two files Calc.pm, your parser module, and
Calc.output a verbose output of your parser rules, conflicts,
warnings, states and summary.
What your are missing now is a lexer routine.
- "The Lexer sub"
- is called each time the parser need to read the next token.
It is called with only one argument that is the parser object
itself, so you can access its methods, specially the
$_[0]->YYData
data area.
It is its duty to return the next token and value to the
parser. They "must" be returned as a
list of two variables, the first one is the token known by the parser
(symbolic or literal), the second one being anything you want (usually
the content of the token, or the literal value) from a simple scalar
value to any complex reference, as the parsing driver never use it but
to call semantic actions:
( 'NUMBER', $num )
or
( '>=', '>=' )
or
( 'ARRAY', [ @values ] )
When the lexer reach the end of input, it must return the
'' empty token with an undef value:
( '', undef )
Note that your lexer should never return
'error' as token value: for the driver, this is
the error token used for error recovery and would lead to odd
reactions.
Now that you have your lexer written, maybe you will need to
output meaningful error messages, instead of the default which is to
print 'Parse error.' on STDERR.
So you will need an Error reporting sub.
- "Error reporting routine"
- If you want one, write it knowing that it is passed as parameter the
parser object. So you can share information with the lexer routine quite
easily.
You can also use the
"$_[0]->YYErrok" method in it,
which will resume parsing as if no error occurred. Of course, since the
invalid token is still invalid, you're supposed to fix the problem by
yourself.
The method
"$_[0]->YYLexer" may help you, as
it returns a reference to the lexer routine, and can be called as
($tok,$val)=&{$_[0]->Lexer}
to get the next token and semantic value from the input
stream. To make them current for the parser, use:
($_[0]->YYCurtok, $_[0]->YYCurval) = ($tok, $val)
and know what you're doing...
- "Parsing"
- Now you've got everything to do the parsing.
First, use the parser module:
use Calc;
Then create the parser object:
$parser=new Calc;
Now, call the YYParse method, telling it where to find the
lexer and error report subs:
$result=$parser->YYParse(yylex => \&Lexer,
yyerror => \&ErrorReport);
(assuming Lexer and ErrorReport subs have been written in your
current package)
The order in which parameters appear is unimportant.
Et voila.
The YYParse method will do the parse, then return the last
semantic value returned, or undef if error recovery cannot recover.
If you need to be sure the parse has been successful (in case
your last returned semantic value is undef) make a call to:
$parser->YYNberr()
which returns the total number of time the error reporting sub
has been called.
- "Error Recovery"
- in Parse::Yapp is implemented the same way it is in yacc.
- "Debugging Parser"
- To debug your parser, you can call the YYParse method with a debug
parameter:
$parser->YYParse( ... , yydebug => value, ... )
where value is a bitfield, each bit representing a specific
debug output:
Bit Value Outputs
0x01 Token reading (useful for Lexer debugging)
0x02 States information
0x04 Driver actions (shifts, reduces, accept...)
0x08 Parse Stack dump
0x10 Error Recovery tracing
To have a full debugging output, use
debug => 0x1F
Debugging output is sent to STDERR, and be aware that it can
produce "huge" outputs.
- "Standalone Parsers"
- By default, the parser modules generated will need the Parse::Yapp module
installed on the system to run. They use the Parse::Yapp::Driver which can
be safely shared between parsers in the same script.
In the case you'd prefer to have a standalone module
generated, use the "-s" switch with
yapp: this will automagically copy the driver code into your module so
you can use/distribute it without the need of the Parse::Yapp module,
making it really a "Standalone
Parser".
If you do so, please remember to include Parse::Yapp's
copyright notice in your main module copyright, so others can know about
Parse::Yapp module.
- "Source file line numbers"
- by default will be included in the generated parser module, which will
help to find the guilty line in your source file in case of a syntax
error. You can disable this feature by compiling your grammar with yapp
using the "-n" switch.
If you find bugs, think of anything that could improve Parse::Yapp or have any
questions related to it, feel free to contact the author.
William N. Braswell, Jr. <wbraswell_cpan@NOSPAM.nym.hush.com> (Remove
"NOSPAM".)
yapp(1) perl(1) yacc(1) bison(1).
The Parse::Yapp module and its related modules and shell scripts are copyright:
Copyright © 1998, 1999, 2000, 2001, Francois Desarmenien. Copyright
© 2017 William N. Braswell, Jr.
You may use and distribute them under the terms of either the GNU
General Public License or the Artistic License, as specified in the Perl
README file.
If you use the "standalone parser" option so people
don't need to install Parse::Yapp on their systems in order to run you
software, this copyright noticed should be included in your software
copyright too, and the copyright notice in the embedded driver should be
left untouched.
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