Rats!, a parser generator supporting extensible syntax.
Grammars for Rats! build on the Parsing Expression Grammar
(PEG) formalism described in Brian
Ford's Parsing
Expression Grammars paper. However, since Rats! produces
working parsers, the syntax of Rats! grammars is somewhat
different from and more expressive than the PEG syntax described in
the paper. Additionally, to make grammars more easily reusable and
extensible, Rats organizes grammar fragments into modules. A
good starting point for learning how to use Rats!, in addition
to this introduction, are Rats!' own grammar in
package xtc.parser and the C and Java grammars in
package xtc.lang.
The rest of this document covers the following topics:
Syntax
Here is the syntax of Rats!' grammar modules, expressed in PEG
syntax:
Module <- Spacing Intro Production* EOF
Intro <- ModuleDecl Dependency* Header? Body? Footer? Option?
ModuleDecl <- "module" FSpacing ModuleRef SEMICOLON
Dependency <- Modification / Instantiation / Import
Modification <- "modify" FSpacing ModuleRef ModuleTarget? SEMICOLON
Instantiation <- "instantiate" FSpacing ModuleRef ModuleTarget? SEMICOLON
Import <- "import" FSpacing ModuleRef ModuleTarget? SEMICOLON
ModuleRef <- QName ModuleParams?
ModuleParams <- OPEN ( QName (COMMA QName)* )? CLOSE
ModuleTarget <- "as" FSpacing QName
Header <- "header" Spacing Action
Body <- "body" Spacing Action
Footer <- "footer" Spacing Action
Option <- "option" FSpacing Attribute (COMMA Attribute)* SEMICOLON
Production <- Full / Addition / Removal / Override
Full <- PAttributes QName Identifier EQUAL Choice SEMICOLON
Addition <- QName Identifier PLUSEQUAL
( SName ELLIPSIS SLASH Choice SEMICOLON
/ Choice SLASH SName ELLIPSIS SEMICOLON )
Removal <- QName Identifier MINUSEQUAL
SName ( COMMA SName )* SEMICOLON
Override <- QName Identifier COLONEQUAL Choice SEMICOLON
/ QName Identifier COLONEQUAL ELLIPSIS SLASH Choice SEMICOLON
/ QName Identifier COLONEQUAL Choice SLASH ELLIPSIS SEMICOLON
/ PAttributes QName Identifier COLONEQUAL ELLIPSIS SEMICOLON
PAttributes <- &(QName Identifier EQUAL) / Attribute PAttributes
Choice <- Sequence (SLASH Sequence)*
Sequence <- !(SName ELLIPSIS / ELLIPSIS) SName? Voided*
Voided <- ("void" Spacing COLON)? Prefix
Prefix <- (AND / NOT / CARET / Identifier COLON
/ StringLit Spacing COLON)? Suffix
Suffix <- Primary (QUESTION / STAR / PLUS)?
Primary <- NullLit / QName / Literal / NodeMarker / Action
/ OPEN Choice CLOSE
NullLit <- "null" Spacing
NodeMarker <- '@' Id Spacing
Action <- '{' ActionBody* '}' Spacing
ActionBody <- Action / CharLit / StringLit / MLComment / SLComment / !'}' .
Attribute <- Identifier (OPEN AttValue CLOSE)?
AttValue <- Integer / QName / StringLit Spacing
QName <- Id ('.' Id)* Spacing
SName <- LESS Id GREATER
Identifier <- Id Spacing
Id <- [a-zA-Z] [a-zA-Z0-9]*
Literal <- ('_' / CharLit / StringLit / Class) Spacing
CharLit <- ['] (Escape / !['\\] .) [']
StringLit <- ["] (Escape / !["\\] .)* ["]
Class <- '[' (Char '-' Char / Char)* ']'
Char <- Escape / ![-\]\\] .
Escape <- '\\' [btnfr"'\[\\\]-] / '\\' 'u' HexQuad / OctalEscape
OctalEscape <- '\\' ([0-3] OctDigit OctDigit / OctDigit OctDigit / OctDigit)
Integer <- (HexNumber / OctNumber / Number) Spacing
HexNumber <- '0' [xX] HexDigit+
HexQuad <- HexDigit HexDigit HexDigit HexDigit
HexDigit <- [0-9a-fA-F]
Number <- '0' / NZDigit Digit*
NZDigit <- [1-9]
Digit <- [0-9]
OctNumber <- '0' OctDigit+
OctDigit <- [0-7]
ELLIPSIS <- "..." Spacing
PLUSEQUAL <- "+=" Spacing
MINUSEQUAL <- "-=" Spacing
COLONEQUAL <- ":=" Spacing
COMMA <- ',' Spacing
EQUAL <- '=' Spacing
SLASH <- '/' ![/*] Spacing
AND <- '&' Spacing
NOT <- '!' Spacing
CARET <- '^' Spacing
COLON <- ':' Spacing
QUESTION <- '?' Spacing
STAR <- '*' Spacing
PLUS <- '+' Spacing
OPEN <- '(' Spacing
CLOSE <- ')' Spacing
SEMICOLON <- ';' Spacing
LESS <- '<'
GREATER >- '>' Spacing
Spacing <- (Space / SLComment / MLComment)*
FSpacing <- (Space / SLComment / MLComment)+
Space <- ' ' / '\t' / '\f' / EOL
SLComment <- "//" (![\n\r] .)* EOL
MLComment <- "/*" ('*' !'/' / !'*' .)* "*/"
EOL <- '\r' '\n' / '\r' / '\n'
EOF <- !.
Note that QName stands for "qualified
name," SName stands for "sequence
name," PAttributes for "production
attributes," AttValue for "attribute
value," NZDigit for "non-zero
digit," FSpacing for "forced
spacing," SLComment for "single-line comment,"
and MLComment for "multi-line comment."
Overview of Expressions and Operators
The biggest difference between parsing expression grammars and the
more familiar context-free grammars (CFGs) is the use
of ordered choices instead of symmetric choices. Hence, the
different options in a PEG choice (and also a Rats! choice) are
separated by a slash / instead of a vertical
bar |. Furthermore, to emphasize that PEGs define how
to parse a language instead of how to generate a language, PEGs
use a left arrow <- instead of the right
arrow -> used in CFGs. Note that Rats! uses an
equal sign instead.
Otherwise, PEGs have many of the same expressions and operators
as other syntax and grammar formalisms. For example, the any
character constant . (_ for Rats!)
matches any character in the input, and a character class, as defined
by the [] operator, matches any of the characters in the
class. The option operator ? makes an expression
optional, and the star * and plus +
operators indicate zero-or-more and one-or-more repetitions,
respectively. Somewhat less common are the and & and
not ! operators, which denote syntactic predicates.
Expressions in a syntactic predicate must match (for the
and & operator) or not match (for the not !
operator) the input, though the corresponding text in the input is
not consumed.
Rats! grammars differ from PEGs in that they have
additional expressions and operators necessary for generating actual
parsers. Most importantly, Rats! grammars include
actions that generate semantic values. Actions are surrounded
by curly brackets {}, just like blocks of code in C, C++,
or Java. To access such semantic values, Rats! grammars can
also include bindings. Bindings first specify the variable
name, followed by a colon :, and then the expression to
be bound. Additionally, Rats! grammars support semantic
predicates, which are denoted by the and
& operator directly followed by an action (which must be
an expression evaluating to a boolean value). Furthermore,
Rats! grammars support text matching expressions,
which first specify the text to be matched as a string literal,
followed by a colon :, and then the expression to be
matched. A text matching expression
"text":expr
is equivalent to the expression
fresh-variable:expr &{ "text".equals(fresh-variable) }
but implemented more efficiently.
Rats! grammars also support a voiding operator, which is
specified as "void:" followed by an expression, node
markers, which are specified as an at sign '@'
immediately followed by an identifier, and parser actions,
which are actions prefixed by a caret '^'. Voided
expressions and node markers help with the automatic generation of
abstract syntax trees and are explained here.
Parser actions provide a low-level interface for extending parsers
generated by Rats! and are
explained here.
Other differences between PEGs and Rats! grammars include
that, as already mentioned Rats! uses an underscore
'_' as the any character constant, while PEGs use a dot
'.'. Furthermore, Rats!, just like C/C++/Java,
requires that single-quoted literals specify exactly one character,
while double-quoted literals may specify an arbitrary number of
characters. Escape sequences include basic C/C++/Java escapes such as
'\n' and '\\', Java Unicode escapes such as
'\u00ff', and '\[', '\-', and
'\]' for character class specifications. Rats!
grammars also use standard C/C++/Java comments. Note that, while
Rats! supports Unicode, it only supports 16 bit characters in
the basic multilingual plane (i.e., with code points between U+0000
and U+FFFF). Expressions recognizing supplementary characters (i.e.,
with code points between U+10000 and U+10FFFF) need to
use Java's
encoding as a pair of char values in the surrogates
range.
Grammar Modules
A Rats! grammar consists of one top-level module, which is the
module specified on the command line when invoking Rats!, and
zero or more dependent modules. Each module starts with several
declarations, whose syntax follows from Intro in
the Rats! syntax specification above:
- The
module declaration specifies the fully qualified
name of a grammar module. Optionally, the module may have one or more
parameters, which are treated as module names and are replaced with
the actual arguments throughout the module, notably in module
dependency declarations and in qualified nonterminals (a qualified
nonterminal consists of the module name, followed by a dot
'.', followed by the unqualified nonterminal). Module
parameters are discussed in more
detail here.
- Zero or more module dependency declarations specify how the
current module depends on other modules. Rats! supports three
types of such dependency declarations. First, import dependencies
make another module's nonterminals referenceable from within the
current module. Second, modification dependencies make another
module's productions available for modification in the current module.
Each module can have at most one modification dependency. Finally,
instantiation dependencies instantiate other modules (typically,
modules that require arguments) and make their names available for use
in other dependencies. The effects of import and instantiation
declarations are discussed in more
detail here; module modifications are
discussed in detail here.
- The action code following an optional
header
declaration is copied into the parser just before the parser class
declaration. If a grammar spans several modules, all unique header
actions are copied into the parser class.
- The action code following an optional
body
declaration is copied into the body of the parser class. Again, if a
grammar spans several modules, all unique body actions are copied into
the parser class.
- The action code following an optional
footer
declaration is copied just after the parser class declaration. As for
header and body declarations, if a grammar spans several modules, all
unique footer actions are copied into the parser class.
- An optional
option declaration specifies parser
options. Each option is expressed as an attribute, which is an
identifier specifying the attribute's name followed by an optional
integer literal, qualified name, or string within
parentheses () specifying the attribute's value. The
currently recognized options are
discussed here.
A grammar's top-level module (i.e., the module specified on the
command line when invoking Rats!) is treated somewhat
differently from its dependent modules:
- By default, the Java package and class names of the generated
parser are automatically deduced from the fully qualified name of the
top-level module. For example, the default package and class names
for module
xtc.parser.PGrammar would be
xtc.parser and PGrammar, respectively.
However, this default can be overriden through
the parser option (and
is, in fact, overriden by that module).
- Option declarations are outside the module system, and only the
top-level module's options are observed by the parser generator. The
exceptions are the following options. First,
the
stateful option
specifies a global state object and must have the same value across
all of a grammar's modules (if it appears at all). Second,
the setOfString option specifies a
global set of strings and has the corresponding field's name as its
value; each unique value results in its own global field. Finally
the flag option specifies a global
boolean field and has the corresponding field's name as its value;
each unique value results in its own global field.
- The top-level module must have at least one production that is
marked as
public, which
indicates that that production is a top-level production and thus
visible outside the parser class. Other modules' public productions
are not treated as top-level production for the grammar.
- Finally, because the top-level module is directly instantiated, it
cannot have any parameters. Only dependent modules may have
parameters; the corresponding arguments are specified when importing,
instantiating, or modifying the module.
Similar to the Java compiler automatically loading dependent Java
classes, Rats! automatically loads dependent modules from the
file system. For example, when looking for the dependent
module xtc.util.Spacing, Rats! searches for the
file xtc/util/Spacing.rats on Unix-like systems. One or
more root directories for this search can be specified through
the -in command line option.
A grammar's modules can be visualized by
using Rats!' -html command line option combined
with either the -instantiated, -applied,
or -processed command line option to specify the stage
of Rats!' processing. In all three cases,
Rats! creates a hyperlinked version of the grammar at that
processing stage. Make sure that
the grammar.css
stylesheet is in the same directory as the generated HTML file(s).
The output directory can be controlled with the -out
command line option.
Productions and Semantic Values
After the module declarations follow the productions, with each
production defining how to parse a nonterminal. Regular modules may
only contain full productions, whose syntax follows from
Full in the Rats! syntax
specification above. Module modifications may
also contain alternative additions, alternative removals, and
production overrides, which are
discussed below.
Each full production first lists zero or more attributes, then
declares the type of the semantic value it returns, and then specifies
the name of the nonterminal it defines (which must be unqualified).
The definition follows after the equals sign = and is
terminated by a semicolon ;. Each full production can
either be public, protected, or private, as indicated by the
corresponding attribute of the same name. A public production is a
top-level production for the grammar if it appears in the top-level
module. Otherwise, it is treated as a protected production, which is
the default, can be omitted, and makes the production referenceable
from other modules. Finally, a private production can only be
referenced from within the same module. Note that the use of
the transient attribute is
explained here. All per-production
attributes supported by Rats! are discussed
here. Further note that types cannot be
primitive types, such as int; though, as
discussed here, void is allowed.
A production's semantic value is usually defined in an action by
setting the yyValue variable (so named in deference to
the Yacc parser
generator) to the desired value. The action code can use characters
or strings matched by a literal as well as semantic values returned by
nested nonterminals through bindings. As specified above, each binding first declares the variable,
followed by a colon, and then the bound expression.
For example, this is the
production for the FullProduction nonterminal
from Rats!' own grammar:
FullProduction FullProduction =
atts:ProductionAttributes
type:Name nt:UnqualifiedNonTerminal "=":Symbol choice:Choice ";":Symbol
{ yyValue = new FullProduction(atts.list(), type, nt, choice); }
;
The production first declares the type of the semantic value it
returns to be FullProduction (which only coincidentally
has the same name as the production). In the definition, the semantic
value of recognizing the production's attributes is bound to the
variable atts, the semantic value returned by
the Name production is bound to the variable
type, the semantic value returned by the
UnqualifiedNonTerminal production is bound to the
variable nt, and the semantic value returned by the
Choice production is bound to the variable
choice. The action code then uses these variables to
create a new FullProduction object and assigns that
object to yyValue, thus making the newly created object
the production's semantic value.
In the generated parser, Rats! declares the types of
variables as following:
- Variables bound to the any character constant
_, to a
character literal, or a character class are declared
as char.
- Variables bound to a string literal are declared
as
String.
- Variables bound to a nonterminal are declared to be of the type of
the corresponding production. For example, the declared type of
the
Name production from the above example
is String and, consequently, type is
declared to be a String as well.
- Similarly,
yyValue is declared to be of the type of
the corresponding production.
Passing the Semantic Value Through
Sometimes, the semantic value of a production is the same as the
semantic value of one of the expressions appearing in that production.
To avoid first binding that expression to some variable and then
assigning that variable to yyValue , it is possible to
directly bind yyValue.
For example, the production for the Header nonterminal
from Rats!' own grammar makes use of this feature:
Action Header = "header":Word yyValue:Action ;
An equivalent, but more verbose definition might look like this:
Action Header = "header":Word a:Action { yyValue = a; } ;
Options, Repetitions, and Nested Choices
In general, Rats! lifts options, repetitions, and nested
choices into their own, newly generated productions. It also desugars
options and repetitions into equivalent productions without the option
or repetition operators. Note that nested choices are not
lifted if they are the last element in a sequence. Further note that
repetitions are not desugared if they appear in
a transient production. Finally, options are
not desugared if they are not bound or if Rats can
automatically deduce the value of the bound optional expression.
This lifting and desugaring raises several questions:
- What is the semantic value of an expression before applying the
option or repetition operator? Similarly, what is the semantic value
of a nested choice?
- For options and repetitions, what is the semantic value after
applying the option or repetition operator?
- Finally, for each of these expressions, what is the type of the
newly generated production?
We now answer these questions in turn.
To determine the semantic value of an expression before applying
the option or repetition operator and that of a nested
choice, Rats! generally requires that each expression has
its own, embedded action that defines the corresponding value. As a
result, the definition of a production may contain several actions
that all assign values to yyValue, even in nested
expressions.
However, for some expressions, Rats! is able to
automatically deduce their semantic values. Notably, if a nonterminal
is optional or repeated, the semantic value of the expression before
applying the option or repetition operator simply is the semantic
value of the nonterminal's production. Similarly, if a nonterminal is
the only expression appearing in an option, the semantic value for
that option simply is the semantic value of the nonterminal's
production. In these cases, no embedded actions are necessary.
For options, the semantic value after applying the option
operator is the semantic value of the expression, if that expression
can be matched in the input. Otherwise, the overall semantic value is
simply null. For repetitions, the semantic value after
applying the repetition operator is a {@link xtc.util.Pair}. Pairs
are used to implement singly-linked lists and contain the semantic
values of the repeated expressions. In the case of zero matches, the
pair is the special {@link xtc.util.Pair#EMPTY empty pair}, which is
also accessible through the type-safe {@link xtc.util.Pair#empty()}
method. Rats! uses pairs as they allow for efficient addition
of an element to the front of a sequence of pairs. Note that pairs
can easily be converted into a Java Collections Framework list by
calling {@link xtc.util.Pair#list()} (as illustrated in the production
for Rats! productions
shown above).
The type of a newly generated production representing a desugared
option generally is Object. However,
if Rats! can automatically determine the semantic value of
an optional expression, it uses the more specific type (which, in the
case of desugared optional nonterminals, is the type of the
corresponding production). The type of a newly generated production
representing a desugared repetition always
is xtc.util.Pair. Finally, the type of a newly generated
production representing a lifted choice always is Object.
To illustrate the general case of repetitions and nested choices,
consider the following snippet from Rats!' own grammar,
which is taken from the Terminal production and parses
character class specifications:
/ '['
l:( c1:ClassChar '-' c2:ClassChar
{
yyValue = new CharRange(Utilities.unescape(c1).charAt(0),
Utilities.unescape(c2).charAt(0));
}
/ c1:ClassChar
{
yyValue = new CharRange(Utilities.unescape(c1).charAt(0));
}
)*
']' Spacing
{ yyValue = new CharClass(l.list()); }
The nested choice has its own actions, which create character ranges
as their semantic values. The parser collects these character ranges
into a sequence of pairs, which is then bound to the l
variable. The outer action uses this sequence of pairs to create a
new character class as the overall semantic value (converting the
sequence of pairs into a Java Collections Framework list along the
way).
To illustrate the automatic
deduction of a semantic value, consider the production for sequences
from Rats!' own grammar:
Sequence Sequence =
!Ellipsis n:SequenceName? l:Voided*
{ yyValue = new Sequence(n, l.list()); }
;
A sequence essentially consists of zero or more repetitions of the
Voided nonterminal; no embedded action is necessary for
the repetition. Rats! automatically collects the individual
semantic values into a sequence of pairs. The optional sequence name
preceding the repeated Voided nonterminal simply is an
identifier between less-than < and
greater-than > signs and is used
when modifying productions. It can also
be used for documentation, as Rats! tries to preserve it
throughout its internal processing, so that it will be included in the
generated parser (within a comment). The corresponding optional
expression does not need an embedded action either. The syntactic
predicate !Ellipsis excludes sequence names followed by
an ellipsis ... or an ellipsis by itself from sequences,
as they are used to modify productions.
Void and Text-Only Productions
Adding actions to create a production's semantic value can seem
tedious and may make grammars unnecessarily verbose. To reduce the
need for explicit actions, Rats! supports special types of
productions, which need no semantic actions at all. We now
discuss void and text-only productions, which are especially useful
for recognizing lexical syntax (such as identifiers, punctuation,
numbers, and so on).
A void production is a production that declares the type
of its semantic value to be void. Such a production does
not require any actions, but it may not be bound to an identifier
either. A void production is typically used for ignored spacing or
punctuation elements. Void productions also improve the accuracy of
Rats!' deduction of a compound expression's semantic
value. If the compound expression only contains a single non-void
nonterminal, that nonterminal's semantic value is used as the semantic
value of the entire expression.
Here is an example void production from Rats!' own
grammar:
transient void Spacing =
(Space / LineTerminator / TraditionalComment/ EndOfLineComment)*
;
This production matches space characters, line terminators, and
comments. Note that the transient keyword is explained
here.
The fact the Spacing is declared
as void is then used in the production
for Symbol:
String Symbol = SymbolCharacters Spacing ;
No action is necessary because only the SymbolCharacters
nonterminal produces a semantic value. Rats! automatically
detects this case and passes the value
of SymbolCharacters through.
In general, Rats! tries to deduce a compound expression's
value, even if it is nested within another expression, ignoring
nonterminals referencing a void production or explicitly voided
expressions. Furthermore, for repetitions, options, and nested
choices that do not appear as the last expression in a sequence, it
treats the entire repetition, option, or nested choice as
void, if all component expressions are nonterminals referencing void
productions, explicitly voided expressions, or predicates. For
example, if nt references a void production, then the
expression "nt*" is equivalent to
"void:(nt*)" (note that the parentheses are not strictly
necessary).
A text-only production is a production that declares the
type of its semantic value to be a String, does not
contain any bindings to yyValue nor actions that
reference yyValue and references only other text-only
productions (if any). The semantic value of a text-only production is
the text matched in the input, represented as a String.
Note that the above Symbol production is not text-only
because it references a void production. Also note that bindings
(besides to yyValue) and semantic predicates may appear
in text-only productions.
To illustrate text-only
productions, consider the following productions from Rats!' own
grammar:
String StringLiteral = ["] ( EscapeSequence / !["\\] _ )* ["] ;
transient String EscapeSequence =
'\\' ( [btnfr\"\'\-\[\\\]] / 'u' HexNumber ) ;
transient String HexNumber = HexDigit HexDigit HexDigit HexDigit ;
transient String HexDigit = [0-9a-fA-F] ;
The StringLiteral production only references terminals
and other productions that are also text-only. As a result, its
semantic value is the text matched in the input, exactly what we
expect from a production that matches string literals. Note that the
transient keyword is explained here.
A note on bindings within text-only productions: When
binding to a character terminal (the any character constant, a
character class, or a character literal), the bound variable is a
char (just as for all other productions). However, for
all other expressions (including options, repetitions, and
nested choices), the value of the bound variable always is a string
representing the text matched by that expression in the input. For
options that cannot be matched in the input, the value is the empty
string (and not null).
Generic Productions
Void and text-only productions typically help with recognizing lexical
syntax; though, they are of limited use for productions that recognize
hierarchical syntax, such as the productions for
FullProduction, Header,
or Sequence shown above. Generic productions help
simplify productions for hierarchical syntax. Just as for void and
text-only productions, no explicit semantic actions are required.
A generic production is a production that declares
generic as its type and returns a semantic value that is
a {@link xtc.tree.GNode generic node}. Rats!
automatically creates the production's semantic value: its name is the
name of the production, and its children are the semantic values of
the component expressions in the recognized sequence, with the
exception of any voided expressions (using the void:
operator), void nonterminals, and character terminals, which are
ignored.
For example, we could rewrite the production
for FullProduction as a
generic production:
generic FullProduction =
ProductionAttributes
Name UnqualifiedNonTerminal void:"=":Symbol Choice void:";":Symbol ;
;
The rewritten production has a generic node as its semantic value; the
grammar author thus does not need to define
a FullProduction class to represent the semantic value.
Furthermore, the rewritten production is equivalent to the following
production with an explicit semantic action:
Node FullProduction =
v1:ProductionAttributes
v2:Name v3:UnqualifiedNonTerminal "=":Symbol v4:Choice ";":Symbol
{ yyValue = GNode.create("Production", v1, v2, v3, v4); }
;
The two symbol references are not bound because they are explicitly
voided.
Options, repetitions, and nested choices in generic productions
are treated just like the corresponding expressions in regular
productions. In other words, they may be desugared and lifted into
their own productions. As a result, they require explicit actions to
create their semantic values, unless, of course, Rats! can
automatically determine the corresponding values.
List-Valued Productions
If a grammar has the flatten attribute, pairs
resulting from a repeated expression are treated somewhat differently
in generic productions: the values on the list are added to the
production's generic node as individual children. For example,
consider this production for recognizing parameter lists in C:
generic ParameterList =
ParameterDeclaration (void:",":Symbol ParameterDeclaration)* ;
Because the semantic values of the embedded repetition are added as
invidividual children to the production's generic node, that node's
children are all parameter declarations, which is exactly the desired
behavior.
Alternatively, Rats! can automatically deduce the semantic
value of productions with a Pair type. The production's
value is a list of all of a sequence's component expressions. For
example, consider this rewritten production for recognizing parameter
lists:
Pair<Node> ParameterList =
ParameterDeclaration (void:",":Symbol ParameterDeclaration)* ;
Beacuse the declared type
is Pair<Node>, Rats! automatically deduces
the semantic value of the production. It is a list containing the
value of the first parameter declaration followed by the value of the
repetition. The production is equivalent to the following production
using an explicit semantic action:
Pair<Node> ParameterList =
head:ParamterDeclaration tail:(void:",":Symbol ParameterDeclaration)*
{ yyValue = new Pair<Node>(head, tail); } ;
As illustrated by this version, Rats! recognizes when the last
component expression in a list-valued production already has a list
value and turns that value into the overall list's tail. If the last
component expression does not have a list value, the overall list's
tail is the empty list.
Whether to (1) use the flatten attribute so that
list elements are added as individual children to generic nodes or (2)
use list-valued productions to create lists and preserve them as
generic nodes' children is a trade-off. With the flatten
attribute, ASTs are more uniform, typically consisting of only generic
nodes and strings. In contrast, without the flatten
attribute, generic nodes may also contain lists of, say, nodes or
strings. However, flattening lists may result in the loss of
information. For example, consider this production for Java
compilation units:
public generic CompilationUnit =
Spacing
PackageDeclaration? ImportDeclaration* Declaration*
EndOfFile ;
When lists are flattened, there is no way to distinguish import
declarations from regular declarations without looking at the
actual children. In contrast, when lists are not flattened, the
generic node for compilation units has exactly three children,
corresponding to package, import, and regular declarations
respectively. Additionally, the preservation of lists in generic
nodes enables more exact typing of ASTs
through Rats!' -ast option. Consequently, the
recommended practice is not to use the flatten
attribute.
Left-Recursive Productions
To simplify the specification of productions that recognize
expressions at different precedence levels, void, text-only, and
generic productions may contain direct left-recursions; though
arbitrary left-recursions are illegal in Rats! grammars just
as for PEGs. Directly left-recursive productions are automatically
transformed into equivalent right-iterative productions, while
preserving the left-recursive structure of the original production's
semantic value. As an example, consider the following generic
production for recognizing logical and expressions in C:
generic LogicalAndExpression =
LogicalAndExpression void:"&&":Symbol BitwiseOrExpression
/ BitwiseOrExpression
;
This directly left-recursive production is equivalent to the following
two productions, which leverage so-called actions (i.e., promises) to
create the semantic values:
Node LogicalAndExpression =
seed:BitwiseOrExpression actions:LogicalAndExpressionTail*
{ yyValue = apply(actions, seed); }
;
Action<Node> LogicalAndExpressionTail =
"&&":Symbol right:BitwiseOrExpression
{
yyValue = new Action<Node>() {
public Node run(Node left) {
Node result = GNode.create("LogicalAndExpression", left, right);
result.setLocation(location(yyStart));
return result;
}};
}
;
The LogicalAndExpressionTail* expression creates a list
of {@link xtc.util.Action actions}, with each action on the list
creating a generic node that is annotated with the source location
corresponding to the start of the production.
The LogicalAndExpression production uses {@link
xtc.parser.ParserBase#apply(Pair,Object)} to apply each action in the
list onto the result of the previous action, starting with
the seed value for the first action. If the list is
empty, the seed value is simply passed through, which is the desired
behavior. By using actions and thus delaying the creation of generic
nodes, the rewritten productions ensure that the resulting abstract
syntax tree preserves left-associativity. Note that actions can also
be used outside of generic productions to create left-associative
abstract syntax trees.
In the previous example, the use of a list of actions results in
the correct treatment of the base case: no new generic node is
created, rather the semantic value of the bitwise or expression is
simply passed through. When specifying productions that do not use
actions, grammar writers can achieve similar results by explicitly
binding yyValue. For alternatives in a generic
production that assign yyValue either through a binding
or a semantic action, Rats! does not create a new generic node
but rather uses the explicitly specified semantic value.
Node Markers in Generic Productions
Also in the previous example, there is only one recursive
alternative; consequently, the production's name, i.e.,
"LogicalAndExpression", provides a meaningful name for
the newly created generic node. However, languages such as C contain
more than one left-recursive operator at the same precedence level.
Using the features described so far, grammar developers can either
write one directly left-recursive generic production for all
operators, thus using the same name for all operators' generic nodes,
or they can write several right-iterative productions that create the
generic nodes through explicit semantic actions. Neither option is
particularly attractive and node markers provide a more elegant
solution. They are written as an at sign '@'
immediately followed by an identifier and specify a generic node's
name for nodes generated by the sequence they appear in.
As an example, consider the following generic production for
recognizing postfix expressions in C:
generic PostfixExpression =
<Subscript> PostfixExpression void:"[":Symbol Expression
void:"]":Symbol
@SubscriptExpression
/ <DirectSelection> PostfixExpression void:".":Symbol Identifier
@DirectComponentSelection
/ <IndirectSelection> PostfixExpression void:"->":Symbol Identifier
@IndirectComponentSelection
/ <FunctionCall> PostfixExpression void:"(":Symbol ExpressionList?
void:")":Symbol
@FunctionCall
/ <Increment> PostfixExpression void:"++":Symbol
@PostincrementExpression
/ <Decrement> PostfixExpression void:"--":Symbol
@PostdecrementExpression
/ <Compound> CompoundLiteral
/ <Primary> PrimaryExpression
;
In a single directly left-recursive production, the example specifies
all of C's postfix expressions. Yet, each recursive alternative
contains a distinct node marker, thus resulting in a differently named
generic node. Internally (and as described above), Rats!
converts the directly left-recursive production into the corresponding
right-iterative version, which uses actions to create left-recursive
AST nodes. Note that the last two alternatives do not require node
markers (nor bindings to yyValue) because they provide
the base cases for the recursion.
Modules, Name Spaces, and Parameters
The intent behind Rats!' module system is to facilitate the
re-use and extensibility of grammar specifications. Even without
parameters and modifications, basic modules help, as they allow a
grammar writer to break up a grammar into several units. All modules
exist in the same global name space. Without parameters and
modifications, this name space consists simply of the module names as
specified by the corresponding module declarations. With
parameters and modifications, this name space consists of the module
names of all instantiated modules (with instantiation being
the process of providing arguments to parameterized modules).
In contrast to module names, nonterminals are only meaningful in
relation to a specific module. Without any import
or modify declarations, a module can only reference the
nonterminals defined in that module. In the presence
of import or modify declarations, a module
can also reference the public and protected nonterminals defined by
imported modules and all nonterminals defined by modified modules.
Nonterminals may be ambiguous, e.g., the same nonterminal may be
defined within a module and an imported module or in several imported
modules. Rats! gives precedence to nonterminals defined in the
same module as the nonterminal reference. For example, if
module Foo defines nonterminal Name and
imports module Bar, which also defines nonterminal
Name, then a reference to Name appearing in
one of Foo's productions is interpreted to
mean Foo's Name. Bar's
Name can still be referenced by using the qualified
notation Bar.Name. Furthermore, if
module Foo does not define Name but
imports Bar and Baz, which both
define Name, any reference to Name
in Foo must be qualified, writing Bar.Name and
Baz.Name respectively. Note that, while qualified
nonterminals are globally unique and thus always unambiguous, a
nonterminal, whether qualified or not, can only be used if the
corresponding production's module is the same module, imported by the
referencing module, or modified by the referencing module.
Parameterized modules improve re-use when compared to basic
modules, as the same parameterized module can be used with different
actual dependencies. For example, module xtc.util.Symbol
takes one parameter for a module defining spacing:
module xtc.util.Symbol(Spacing);
import Spacing;
String Symbol = SymbolCharacters Spacing ;
transient String SymbolCharacters = ..elided.. ;
As a result, this module can be instantiated with different forms of
spacing, without changing xtc.util.Symbol. At the same
time, an instantiation must provide a module that specifies a void
nonterminal Spacing for the resulting module to be
meaningful.
A parameterized module is instantiated by specifying the
corresponding arguments in the corresponding import
declaration:
import xtc.util.Symbol(xtc.util.Spacing);
This import declaration without a target name is
equivalent to the following declaration with a target name:
import xtc.util.Symbol(xtc.util.Spacing) as xtc.util.Symbol;
In either case, all occurrences of the parameter module name
Spacing are replaced by the specified argument module
name xtc.util.Spacing. Furthermore, all occurrences of
the module name itself are replaced by the specified target name
(which, in this example, is the same as the module name). Finally,
the instantiated module becomes the sole module of that name in the
global module name space. No other instantiation with the same target
name is possible, unless it instantiates exactly the same module with
the same arguments.
Note that the nonterminal Spacing in the production
for Symbol is not renamed during instantiation, as it
denotes a nonterminal and not a module. However, if the nonterminal
was written as Spacing.Spacing, it would be renamed
to xtc.util.Spacing.Spacing. Further note
that xtc.util.Symbol could be instantiated with a
different argument within the same grammar, as long as that
instantiation's target name is different. Finally, note that once
instantiated, other modules can reference the instantiated module
through its target name. For example, after
module xtc.util.Symbol has been instantiated through
either of the above import declarations, it can also be
imported as following:
import xtc.util.Symbol;
This import declaration, when appearing in a dependent module,
references the same instantiated module.
While module parameters and arguments can only be module names
and never more complex expressions, sometimes more complex arguments
are desirable. For example, a grammar writer may want to
instantiate xtc.util.Symbol with a more complex
module my.Spacing, which requires its own
argument my.Argument. In this case, the grammar writer
can use an instantiate declaration, which instantiates a
module but does not make its nonterminals accessible from within the
instantiating module. In the example, the grammar writer would use
the following declarations:
instantiate my.Spacing(my.Argument) as Spacing;
import xtc.util.Symbol(Spacing);
Module dependencies are resolved through a breadth-first search
starting with the top-level module. In other words, a dependent
module's dependencies are only processed after all the
dependencies of the depending module have been resolved. As a result,
the order of import, instantiate, and
modify declarations in a module does not matter and
mutually dependent modules can be instantiated within the same module.
For example, xtc.lang.CSpacing has a parameter for a
constant module and xtc.lang.CConstant has a parameter
for a spacing module, with each module importing the parameterized
module. To instantiate these two mutually dependent modules,
module xtc.lang.C simply declares (with some intermediate
declarations omitted and ignoring the
additional xtc.lang.CState argument):
instantiate xtc.lang.CConstant(xtc.lang.CSpacing);
instantiate xtc.lang.CSpacing(xtc.lang.CState, xtc.lang.CConstant);
Because it uses breadth-first search, Rats! processes these
two instantiate declarations before processing the
corresponding
import declarations in xtc.lang.CConstant
and xtc.lang.CSpacing. By the time Rats reaches
the import declarations in the dependent modules, the
modules have been instantiated and no arguments are necessary.
Module Modifications
To further facilitate re-use and extensibility, Rats! supports
module modifications, which concisely express extensions to a grammar
and, in addition to full productions, can also contain alternative
additions, which add new alternatives to a production's top-level
choice, alternative removals, which remove alternatives from
a production's top-level choice, and production overrides,
which can override an entire production, specific alternatives, or a
production's attributes. All three types of partial production, whose
syntax is specified above and illustrated below,
depend on the different alternatives in a production's top-level
choice having sequence names. Consequently, it is good practice to
always name your sequences!
A module modification must contain a single modify
declaration, which specifies the module to be modified. For example,
the following declaration specifies that
module xtc.lang.JavaSymbol modifies
module Symbol:
module xtc.lang.JavaSymbol(Symbol);
modify Symbol;
The example uses a module parameter to ensure that
module xtc.lang.JavaSymbol can be applied to different
versions of Symbol. The resulting module contains all
full productions appearing in both the modifying and modified modules
(in the example, xtc.lang.JavaSymbol
and Symbol, respectively). Furthermore, all productions
in the resulting module are modified as specified by the alternative
additions, alternative removals, and production overrides in the
modifying module. Modifications are applied in the same order as they
appear in the modifying module (in the
example, xtc.lang.JavaSymbol). The resulting productions
are the only versions; i.e., all nonterminals, whether they originally
appear in the modifying module or the modified module reference the
modified productions.
The resulting module's options are the options of the modifying
module, with exception of
any stateful, setOfString,
and flag options, which are preserved
if they appear in the modified module's options. Furthermore, the
modifying module's header, body, and footer actions are combined with
the modified module's actions (if they exist).
An alternative addition adds an expression before or after an
existing sequence. For example, the following addition adds
sequence s after sequence <Bar>
in production Foo:
Type Foo += <Bar> ... / <NewSequence> s ;
Similarly, the following addition adds the new sequence before
sequence <Bar>:
Type Foo += <NewSequence> s / <Bar> ... ;
Note that the new expression may actually consist of several sequences
and not only one as suggested by the examples. Further note that the
type and nonterminal must both be specified and match an existing full
production.
An alternative removal removes one or more sequences from a
production. For example, the following removal eliminates sequences
<Bar> and <Baz> from production
Foo:
Type Foo -= <Bar>, <Baz> ;
As before, the type and nonterminal must both be specified and match
an existing full production.
A production override can replace all alternatives, only specific
alternatives, or a production's attributes. For example, the
following override replaces all alternatives of
production Foo with expression e:
Type Foo := e ;
In contrast, this override only replaces
alternatives <Bar> and <Baz<:
Type Foo := ... / <Bar> s1 / <Baz> s2 ;
Finally, this override replaces Foo's attributes with
the public attribute:
public Type Foo := ... ;
Note that the list of attributes may be empty, thus removing all
attributes from the full production.
Memoization and Transient Productions
Packrat parsers process their input in time linear to the size of the
input, even if they need to backtrack. The reason they can do this is
that they store, or memoize, the result of trying each
nonterminal at every input position – hence the name "packrat"
parser.
The benefits of memoization are best illustrated with a production
from Rats!' own grammar:
Element Suffix =
( p:Primary "?":Symbol { yyValue = new Option(p); }
/ p:Primary "*":Symbol { yyValue = new Repetition(false, p); }
/ p:Primary "+":Symbol { yyValue = new Repetition(true, p); }
/ Primary
)
;
The Suffix production matches possible suffix expressions
in the input, with each of the four alternatives in the choice first
trying to parse a primary expression. Assume that the current input
contains a primary expression without a suffix (i.e., the
fourth alternative is the alternative that will succeed). If the
parser did not memoize its results, the primary expression
would be parsed four times, once for each alternative,
resulting in suboptimal performance. However, because parsers
generated by Rats! memoize their results, the primary
expression is parsed exactly once. Each subsequent alternative simply
looks up the result of the first parse.
Note that it is possible to rewrite the production
for Suffix to not require backtracking. An alternative
expression, without semantic actions, might look like this:
Primary ( Question / Star / Plus / /* Empty */ )
Further note that other top-down parsers, such as JavaCC and ANTLR,
may not require backtracking, even with a comparable production to the
one used in Rats!' own grammar as they support (limited)
lookahead. However, the original form clearly and concisely expresses
the intent behind this production and, because parsers generated
by Rats! memoize their results, does not require any
fiddling with lookahead (as would be necessary for other top-down
parsers).
In theory, packrat parsers create a two-dimensional array, with
characters in the input stream along the x-axis and nonterminals along
the y-axis. In practice, not all values in this array are calculated
(many of them represent mismatched nonterminals anyway) and the array
is sparse. To avoid allocating memory for the entire array, parsers
generated by Rats! break each column into several chunks.
Memory for the different chunks is allocated independently and on
demand (i.e., if a parsing result needs to be stored). Furthermore,
for productions that are only referenced once in the entire grammar
the corresponding row is never allocated.
Grammar writers have further control over memory allocation by
declaring productions to be transient: if a production is
transient, the corresponding row is not allocated either. The
transient keyword can thus reduce a parser's memory footprint and also
improve its performance. However, it is also dangerous
because, if overused, the resulting parser may need to reparse the
same input for the same nonterminal several times and thus perform in
time superlinear to the size of the input.
So, which productions should be declared transient? In other
parsers, the parser processes tokens generated by a separate
lexer instead of accessing the character stream directly. Packrat
parser grammars thus need to include productions that build up tokens.
Examples include the Escape, HexNumber, and
HexDigit productions in Rats!' own grammar
(shown above). Such productions can
be declared transient, as the parser typically does not backtrack at
the level of individual characters. Furthermore, productions that
cover ignored input characters, notably all productions that cover
white space and comments, can be declared transient as well. However,
when in doubt, it is best to not declare a production as
transient, or to perform performance studies on several inputs to see
how such declarations impact the parser's memory footprint and
performance.
Grammar and Production Attributes
Rats! supports a number of options, which are either specified
as a grammar-wide option in the top-level module's introduction or as
per-production attributes:
public, protected,
and private instruct Rats! to make a production a
top-level production, referenceable from other modules, and
referenceable only from the same module, respectively. These
attributes do not have values. Furthermore, these attributes can only
be specified as per-production attributes. The default
is protected and can be omitted from productions.transient instructs Rats! not to memoize the
corresponding production. The attribute does not have a value and can
only be specified as a per-production attribute.memoized instructs Rats! to always memoize the
corresponding production. The attribute does not have a value and can
only be specified as a per-production attribute.inline instructs Rats! not to memoize the
corresponding production. For productions that are neither void nor
text-only, it also instructs Rats! to inline the production if
the corresponding nonterminal appears as the only element in an
ordered choice's alternative. The attribute does not have a value and
can only be specified as a per-production attribute.noinline instructs Rats! not to inline the
production, even if it is (recognized as) transient. The attribute
does not have a value and can only be specified as a per-production
attribute.constant instructs Rats! to make all bindings
constant and thus unmodifiable. The attribute does not have a value.
This attribute can be specified either as a grammar-wide attribute or
as a per-production attribute.withLocation instructs Rats! to annotate all
semantic values that are instances of {@link xtc.tree.Node} with their
locations in the source file. The attribute does not have a value.
This attribute can be specified either as a grammar-wide attribute or
as a per-production attribute.stateful
instructs Rats! to include code for managing global parsing
state through a {@link xtc.util.State state object}. This attribute
is used both as a grammar-wide attribute and a per-production
attribute. The grammar-wide attribute value specifies the class
managing the global state (which must have a no-argument
constructor). Rats! includes a static final
field yyState, which references an instance of that
class, in the generated parser. For a production with this attribute,
the attribute does not have a value and Rats! includes the
appropriate calls to {@link xtc.util.State#start()}, {@link
xtc.util.State#commit()}, and {@link xtc.util.State#abort()}. Note
that the state object must be reset explicitly with the
per-production resetting attribute. Further note that,
if a grammar spans several modules, all the module's stateful options
must specify the same class name.resetting instructs Rats! to include code to
{@link xtc.util.State#reset(String) reset} the global state object.
The attribut does not have a value. This attribute can only be
specified as a per-production attribute and requires a grammar-wide
stateful attribute.ignoringCase instructs Rats! to perform
comparisons for string matches in a case-insensitive manner. The
attribute does not have a value. This attribute can be specified
either as a grammar-wide attribute or as a per-production
attribute.flatten instructs Rats! to add the elements of
list values (i.e., instances of {@link xtc.util.Pair}) to generic
nodes instead of adding the list values themselves. This attribute
does not have a value and can only be specified as a grammar-wide
attribute.variant instructs Rats! that all generic nodes
returned by the production as a semantic value are variants of the
same type. This attribute does not have a value and can only be
specified as a per-production attribute.withParseTree instructs Rats! to generate a
parse tree instead of an abstract syntax tree. This attribute is
specified as a grammar-wide attribute and does not have a value. For
a grammar with this attribute, Rats! rewrites all generic,
text-only, and void productions as well as productions that pass the
semantic value through to preserve the input in the form of {@link
xtc.tree.Formatting} annotations. The embedded AST generally has the
same structure as for the grammr without the attribute. The exception
are strings, which are represented as instances of {@link
xtc.tree.Token}. Furthermore, generic nodes include additional
children consisting of Formatting
annotating null values if voided expressions or void
nonterminals appear between two non-void repetitions.verbose instructs Rats! to print debugging
information to the console for each parser method invocation. The
attribute does not have a value. This attribute can be specified
either as a grammar-wide attribute or as a per-production
attribute.nowarn instructs Rats! to suppress warnings
during parser generation. The attribute does not have a value; it can
be specified either as a grammar-wide attribute or as a per-production
attribute.parser
instructs Rats! to use the fully qualified class name specified
by the attribute's value as the Java class name for the generated
parser, instead of deducing that name from the module's name. This
attribute can only be specified as a grammar-wide attribute.factory instructs Rats! to use the (optionally
qualified) class name specified by the attribute's value as the
factory for creating generic nodes instead of
using xtc.tree.GNode. This attribute can only be
specified as a grammar-wide attribute.visibility instructs Rats! to make the
generated parser class either public or package private, depending on
the attribute's value (public
or packagePrivate). This attribute can only be specified
as a grammar-wide attribute. The default for modules without this
attribute is public.rawTypes instructs Rats! to use raw types
instead of generic types. Besides a
"@SuppressWarnings("unchecked")" annotation for the
parser class, the generated code is compatible with Java 1.4. The
attribute does not have a value and can only be specified as a
grammar-wide attribute.main instructs Rats! to include a static main
method that parses one or more input files and prints the
corresponding results. The value of this attribute must be the name
of the top-level nonterminal to parse. This attribute can only be
specified as a grammar-wide attribute.printer instructs Rats! to use the visitor
specified by the attribute's value for printing the semantic value
returned by a successful parse. This attribute can only be specified
as a grammar-wide attribute and requires that the grammar also has the
main attribute.setOfString instructs Rats!
to include a static final set of strings with the attribute's value as
its name. Rats! also includes a convience method for filling
sets add(Set,T[]). This attribute can only be
specified as a grammar-wide attribute.flag instructs Rats! to
include a public static final boolean field with the attribute's value
as its name. The field's value is true. This attribute
can only be specified as a grammar-wide attribute.genericAsVoid instructs Rats! to treat generic
productions as void productions. The attribute does not have a value
and can only be specified as a grammar-wide attribute.dump instructs Rats! to include a method for
dumping a plain-text representation of the parser's memoization table
to a printer: dump({@link xtc.tree.Printer}). The
attribute does not have a value and can only be specified as a
grammar-wide attribute.
A note on licensing: Parsers generated with the
main, printer, and dump
attributes link with code licensed under the GNU GPL version 2. As a
result, parsers generated with these attributes may not be
used in software that is not compatible with the GPL version 2. To
generate parsers that are not restricted by the GPL, use
the -lgpl option when running Rats!.
Parser Actions
Some languages cannot be expressed by PEGs or Rats! grammars.
For example, the on-the-wire format
for SPKI/SDSI
represents byte strings as one or more digits, followed by a colon,
followed by the number of characters specified by the sequence of
digits (when interpreted as a decimal integer); but parsing a specific
number of characters cannot be expressed by PEGs. Parser actions
allow grammar writers to still use Rats! for such languages by
providing a low-level extensibility mechanism.
Parser actions work as follows. Before executing the code in a
parser action, the variable yyBase is assigned the index
of the current parser position. The parser action can then use this
index together with the parser's methods to further consume the input,
parsing the expression not expressible by PEGs. When finished, it
assigns the corresponding result to yyResult, which must
either be a {@link xtc.parser.SemanticValue} object indicating a
successful parse or a {@link xtc.parser.ParseError} object indicating
a parse error. In the case of a semantic value object, that object's
semantic value becomes the semantic value of the production, unless
another action after the parser action changes it.
The following grammar illustrates the use of a parser action for
parsing byte strings as used by SPKI/SDSI. Note that
the yyStart reference in the parser action is the parser
position at the beginning of the production and is used for indicating
the position of an error.
module ByteString;
body {
Result parseChars(String number, int start, int base) throws IOException {
int n;
try {
n = Integer.parseInt(number);
} catch (NumberFormatException x) {
return new ParseError("Malformed length", start);
}
StringBuilder buf = new StringBuilder(n);
for (int i=0; i<n; i++) {
int c = character(base + i);
if (c != -1) {
buf.append((char)c);
} else {
return new ParseError("Unexpected end of bytestring", base + i);
}
}
return new SemanticValue(buf.toString(), base + n);
}
}
option main(ByteString);
public String ByteString =
n:Integer ':' ^{ yyResult = parseChars(n, yyStart, yyBase); } ;
String Integer = [0-9]+ ;
|