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Although this manual covers all aspects of the CUP system, it is relatively brief, and assumes you have at least a little bit of knowledge of LR parsing. A working knowledge of YACC is also very helpful in understanding how CUP specifications work. A number of compiler construction textbooks (such as [2,3]) cover this material, and discuss the YACC system (which is quite similar to this one) as a specific example.
Using CUP involves creating a simple specification based on the grammar for which a parser is needed, along with construction of a scanner capable of breaking characters up into meaningful tokens (such as keywords, numbers, and special symbols).
As a simple example, consider a system for evaluating simple arithmetic expressions over integers. This system would read expressions from standard input (each terminated with a semicolon), evaluate them, and print the result on standard output. A grammar for the input to such a system might look like:
expr_list ::= expr_list expr_part | expr_part
expr_part ::= expr ';'
expr ::= expr '+' expr | expr '-' expr | expr '*' expr
| expr '/' expr | expr '%' expr | '(' expr ')'
| '-' expr | number
To specify a parser based on this grammar, our first step is to identify
and name the set of terminal symbols that will appear on input, and the set of
non-terminal symbols. In this case, the non-terminals are: expr_list, expr_part and expr .For terminal names we might choose:
SEMI, PLUS, MINUS, TIMES, DIVIDE, MOD, NUMBER, LPAREN, and RPARENThe experienced user will note a problem with the above grammar. It is ambiguous. An ambiguous grammar is a grammar which, given a certain input, can reduce the parts of the input in two different ways such as to give two different answers. Take the above grammar, for example. given the following input:
// CUP specification for a simple expression evaluator (no actions)
import java_cup.runtime.*;
/* Preliminaries to set up and use the scanner. */
init with {: scanner.init(); :};
scan with {: return scanner.next_token(); :};
/* Terminals (tokens returned by the scanner). */
terminal SEMI, PLUS, MINUS, TIMES, DIVIDE, MOD;
terminal UMINUS, LPAREN, RPAREN;
terminal Integer NUMBER;
/* Non terminals */
non terminal expr_list, expr_part;
non terminal Integer expr, term, factor;
/* Precedences */
precedence left PLUS, MINUS;
precedence left TIMES, DIVIDE, MOD;
precedence left UMINUS;
/* The grammar */
expr_list ::= expr_list expr_part |
expr_part;
expr_part ::= expr SEMI;
expr ::= expr PLUS expr
| expr MINUS expr
| expr TIMES expr
| expr DIVIDE expr
| expr MOD expr
| MINUS expr %prec UMINUS
| LPAREN expr RPAREN
| NUMBER
;
To produce a parser from this specification we use the CUP generator. If this specification were stored in a file parser.cup, then (on a Unix system at least) we might invoke CUP using a command like:
java java_cup.Main < parser.cupIn this case, the system will produce two Java source files containing parts of the generated parser: sym.java and parser.java. As you might expect, these two files contain declarations for the classes sym and parser. The sym class contains a series of constant declarations, one for each terminal symbol. This is typically used by the scanner to refer to symbols (e.g. with code such as "return new Symbol(sym.SEMI);" ). The parser class implements the parser itself.
The specification above, while constructing a full parser, does not perform any semantic actions &emdash; it will only indicate success or failure of a parse. To calculate and print values of each expression, we must embed Java code within the parser to carry out actions at various points. In CUP, actions are contained in code strings which are surrounded by delimiters of the form {: and :} (we can see examples of this in the init with and scan with clauses above). In general, the system records all characters within the delimiters, but does not try to check that it contains valid Java code.
A more complete CUP specification for our example system (with actions
embedded at various points in the grammar) is shown below:
// CUP specification for a simple expression evaluator (w/ actions)
import java_cup.runtime.*;
/* Preliminaries to set up and use the scanner. */
init with {: scanner.init(); :};
scan with {: return scanner.next_token(); :};
/* Terminals (tokens returned by the scanner). */
terminal SEMI, PLUS, MINUS, TIMES, DIVIDE, MOD;
terminal UMINUS, LPAREN, RPAREN;
terminal Integer NUMBER;
/* Non-terminals */
non terminal expr_list, expr_part;
non terminal Integer expr;
/* Precedences */
precedence left PLUS, MINUS;
precedence left TIMES, DIVIDE, MOD;
precedence left UMINUS;
/* The grammar */
expr_list ::= expr_list expr_part
|
expr_part;
expr_part ::= expr:e
{: System.out.println("= " + e); :}
SEMI
;
expr ::= expr:e1 PLUS expr:e2
{: RESULT = new Integer(e1.intValue() + e2.intValue()); :}
|
expr:e1 MINUS expr:e2
{: RESULT = new Integer(e1.intValue() - e2.intValue()); :}
|
expr:e1 TIMES expr:e2
{: RESULT = new Integer(e1.intValue() * e2.intValue()); :}
|
expr:e1 DIVIDE expr:e2
{: RESULT = new Integer(e1.intValue() / e2.intValue()); :}
|
expr:e1 MOD expr:e2
{: RESULT = new Integer(e1.intValue() % e2.intValue()); :}
|
NUMBER:n
{: RESULT = n; :}
|
MINUS expr:e
{: RESULT = new Integer(0 - e.intValue()); :}
%prec UMINUS
|
LPAREN expr:e RPAREN
{: RESULT = e; :}
;
expr:e1 PLUS expr:e2
{: RESULT = new Integer(e1.intValue() + e2.intValue()); :}
the first non-terminal expr has been labeled
with e1, and the second with e2. The left hand side value of
each production is always implicitly labeled as RESULT.
Each symbol appearing in a production is represented at runtime by an object of type Symbol on the parse stack. The labels refer to the instance variable value in those objects. In the expression expr:e1 PLUS expr:e2, e1 and e2 refer to objects of type Integer. These objects are in the value fields of the objects of type Symbol representing those non-terminals on the parse stack. RESULT is of type Integer as well, since the resulting non-terminal expr was declared as of type Integer. This object becomes the value instance variable of a new Symbol object.
For each label, two more variables accessible to the user are declared. A left and right value labels are passed to the code string, so that the user can find out where the left and right side of each terminal or non-terminal is in the input stream. The name of these variables is the label name, plus left or right. for example, given the right hand side of a production expr:e1 PLUS expr:e2 the user could not only access variables e1 and e2, but also e1left, e1right, e2left and e2right. these variables are of type int.
The final step in creating a working parser is to create a
scanner (also known as a lexical analyzer or simply a
lexer). This routine is responsible for reading individual characters,
removing things things like white space and comments, recognizing which terminal
symbols from the grammar each group of characters represents, then returning
Symbol objects representing these symbols to the parser. The terminals will be
retrieved with a call to the scanner function. In the example, the parser will
call scanner.next_token(). The scanner should return objects of type
java_cup.runtime.Symbol. This type is very different than older
versions of CUP's java_cup.runtime.symbol. These Symbol objects
contains the instance variable value of type Object, which should be
set by the lexer. This variable refers to the value of that symbol, and the type
of object in value should be of the same type as declared in the
terminal and non terminal declarations. In the above example,
if the lexer wished to pass a NUMBER token, it should create a Symbol
with the value instance variable filled with an object of type
Integer. The code contained in the init with clause of the specification will
be executed before any tokens are requested. Each token will be requested using
whatever code is found in the scan with clause. Beyond this, the exact
form the scanner takes is up to you; however note that each call to the scanner
function should return a new instance of In the next
section a more detailed and formal explanation of all parts of a CUP
specification will be given. Section
3 describes options for running the CUP system. Section
4 discusses the details of how to customize a CUP parser, while section
5 discusses the scanner interface added in CUP 0.10j. Section
6 considers error recovery. Finally, Section
7 provides a conclusion.
Symbol objects corresponding to terminals and
non-terminals with no value have a null value field.
java_cup.runtime.Symbol
(or a subclass). These symbol objects are annotated with parser information and
pushed onto a stack; reusing objects will result in the parser annotations being
scrambled. As of CUP 0.10j, Symbol reuse should be detected if it
occurs; the parser will throw an Error telling you to fix your
scanner.
2. Specification Syntax
Now that we have seen a small example, we
present a complete description of all parts of a CUP specification. A
specification has four sections with a total of eight specific parts (however,
most of these are optional). A specification consists of:
package name;where name name is a Java package identifier, possibly in several parts separated by ".". In general, CUP employs Java lexical conventions. So for example, both styles of Java comments are supported, and identifiers are constructed beginning with a letter, dollar sign ($), or underscore (_), which can then be followed by zero or more letters, numbers, dollar signs, and underscores.
After an optional package declaration, there can be zero or more import declarations. As in a Java program these have the form:
import package_name.class_name;or
import package_name.*;The package declaration indicates what package the sym and parser classes that are generated by the system will be in. Any import declarations that appear in the specification will also appear in the source file for the parser class allowing various names from that package to be used directly in user supplied action code.
action code {: ... :};
where {: ... :} is a code string whose contents will be placed
directly within the action class class declaration.
After the action code declaration is an optional parser code declaration. This declaration allows methods and variable to be placed directly within the generated parser class. Although this is less common, it can be helpful when customizing the parser &emdash; it is possible for example, to include scanning methods inside the parser and/or override the default error reporting routines. This declaration is very similar to the action code declaration and takes the form:
parser code {: ... :};
Again, code from the code string is placed directly into the generated
parser class definition.
Next in the specification is the optional init declaration which has the form:
init with {: ... :};This declaration provides code that
will be executed by the parser before it asks for the first token. Typically,
this is used to initialize the scanner as well as various tables and other data
structures that might be needed by semantic actions. In this case, the code
given in the code string forms the body of a void method inside the
parser class.
The final (optional) user code section of the specification indicates how the parser should ask for the next token from the scanner. This has the form:
scan with {: ... :};As with the init clause,
the contents of the code string forms the body of a method in the generated
parser. However, in this case the method returns an object of type
java_cup.runtime.Symbol. Consequently the code found in the scan
with clause should return such a value. See section
5 for information on the default behavior if the scan with
section is omitted.
As of CUP 0.10j the action code, parser code, init code, and scan with sections may appear in any order. They must, however, precede the symbol lists.
terminal classname name1, name2, ...;
non terminal classname name1, name2, ...;
terminal name1, name2, ...;
and non terminal name1, name2, ...;where classname can be a multiple part name separated with "."s. The classname specified represents the type of the value of that terminal or non-terminal. When accessing these values through labels, the users uses the type declared. the classname can be of any type. If no classname is given, then the terminal or non-terminal holds no value. a label referring to such a symbol with have a null value. As of CUP 0.10j, you may specify non-terminals the declaration "
nonterminal" (note, no space) as well as the original "non
terminal" spelling.
Names of terminals and non-terminals cannot be CUP reserved words; these include "code", "action", "parser", "terminal", "non", "nonterminal", "init", "scan", "with", "start", "precedence", "left", "right", "nonassoc", "import", and "package".
precedence left terminal[, terminal...]; precedence right terminal[, terminal...]; precedence nonassoc terminal[, terminal...];The comma separated list indicates that those terminals should have the associativity specified at that precedence level and the precedence of that declaration. The order of precedence, from highest to lowest, is bottom to top. Hence, this declares that multiplication and division have higher precedence than addition and subtraction:
precedence left ADD, SUBTRACT; precedence left TIMES, DIVIDE;Precedence resolves shift reduce problems. For example, given the input to the above example parser 3 + 4 * 8, the parser doesn't know whether to reduce 3 + 4 or shift the '*' onto the stack. However, since '*' has a higher precedence than '+', it will be shifted and the multiplication will be performed before the addition.
CUP assigns each one of its terminals a precedence according to these declarations. Any terminals not in this declaration have lowest precedence. CUP also assigns each of its productions a precedence. That precedence is equal to the precedence of the last terminal in that production. If the production has no terminals, then it has lowest precedence. For example, expr ::= expr TIMES expr would have the same precedence as TIMES. When there is a shift/reduce conflict, the parser determines whether the terminal to be shifted has a higher precedence, or if the production to reduce by does. If the terminal has higher precedence, it it shifted, if the production has higher precedence, a reduce is performed. If they have equal precedence, associativity of the terminal determine what happens.
An associativity is assigned to each terminal used in the precedence/associativity declarations. The three associativities are left, right and nonassoc Associativities are also used to resolve shift/reduce conflicts, but only in the case of equal precedences. If the associativity of the terminal that can be shifted is left, then a reduce is performed. This means, if the input is a string of additions, like 3 + 4 + 5 + 6 + 7, the parser will always reduce them from left to right, in this case, starting with 3 + 4. If the associativity of the terminal is right, it is shifted onto the stack. hence, the reductions will take place from right to left. So, if PLUS were declared with associativity of right, the 6 + 7 would be reduced first in the above string. If a terminal is declared as nonassoc, then two consecutive occurrences of equal precedence non-associative terminals generates an error. This is useful for comparison operations. For example, if the input string is 6 == 7 == 8 == 9, the parser should generate an error. If '==' is declared as nonassoc then an error will be generated.
All terminals not used in the precedence/associativity declarations are treated as lowest precedence. If a shift/reduce error results, involving two such terminals, it cannot be resolved, as the above conflicts are, so it will be reported.
start with non-terminal;This indicates which non-terminal is the start or goal non-terminal for parsing. If a start non-terminal is not explicitly declared, then the non-terminal on the left hand side of the first production will be used. At the end of a successful parse, CUP returns an object of type java_cup.runtime.Symbol. This Symbol's value instance variable contains the final reduction result.
The grammar itself follows the optional start declaration. Each production in the grammar has a left hand side non-terminal followed by the symbol "::=", which is then followed by a series of zero or more actions, terminal, or non-terminal symbols, followed by an optional contextual precedence assignment, and terminated with a semicolon (;).
Each symbol on the right hand side can optionally be
labeled with a name. Label names appear after the symbol name separated by a
colon (:). Label names must be unique within the production, and can be used
within action code to refer to the value of the symbol. Along with the label,
two more variables are created, which are the label plus left and the
label plus right. These are int values that contain the right
and left locations of what the terminal or non-terminal covers in the input
file. These values must be properly initialized in the terminals by the lexer.
The left and right values then propagate to non-terminals to which productions
reduce.
If there are several productions for the same non-terminal they may be
declared together. In this case the productions start with the non-terminal and
"::=". This is followed by multiple right hand sides each separated by
a bar (|). The full set of productions is then terminated by a semicolon.
Actions appear in the right hand side as code strings (e.g., Java code inside
{: ... :} delimiters). These are executed by the parser at the
point when the portion of the production to the left of the action has been
recognized. (Note that the scanner will have returned the token one past the
point of the action since the parser needs this extra lookahead token for
recognition.)
Contextual precedence assignments follow all the symbols and
actions of the right hand side of the production whose precedence it is
assigning. Contextual precedence assignment allows a production to be assigned a
precedence not based on the last terminal in it. A good example is shown in the
above sample parser specification: In addition to the specification file, CUP's behavior can also be changed by
passing various options to it. Legal options are documented in
precedence left PLUS, MINUS;
precedence left TIMES, DIVIDE, MOD;
precedence left UMINUS;
expr ::= MINUS expr:e
{: RESULT = new Integer(0 - e.intValue()); :}
%prec UMINUS
Here, there production is declared as having the precedence of
UMINUS. Hence, the parser can give the MINUS sign two different precedences,
depending on whether it is a unary minus or a subtraction operation.
3. Running CUP
As mentioned above, CUP is written in Java. To invoke
it, one needs to use the Java interpreter to invoke the static method
java_cup.Main(), passing an array of strings containing options.
Assuming a Unix machine, the simplest way to do this is typically to invoke it
directly from the command line with a command such as: java java_cup.Main options < inputfile
Once
running, CUP expects to find a specification file on standard input and produces
two Java source files as output.
Main.java and include:
interface rather than
as a class.
This option
is typically used to work-around the java bytecode limitations on table
initialization code sizes. However, CUP 0.10h introduced a string-encoding for
the parser tables which is not subject to the standard method-size
limitations. Consequently, use of this option should no longer be required for
large grammars.
java_cup.runtime.Scanner. By default, the generated parser refers
to this interface, which means you cannot use these parsers with CUP runtimes
older than 0.10j. If your parser does not use the new scanner integration
features, then you may specify the -noscanner option to suppress
the java_cup.runtime.Scanner references and allow compatibility
with old runtimes. Not many people should have reason to do this.
-version flag will cause it to print
out the working version of CUP and halt. This allows automated CUP version
checking for Makefiles, install scripts and other applications which may
require it. 4. Customizing the Parser
Each generated parser consists of three
generated classes. The sym class (which can be renamed using the
-symbols option) simply contains a series of int constants,
one for each terminal. Non-terminals are also included if the -nonterms
option is given. The source file for the parser class (which can be
renamed using the -parser option) actually contains two class
definitions, the public parser class that implements the actual parser,
and another non-public class (called CUP$action) which encapsulates all
user actions contained in the grammar, as well as code from the action
code declaration. In addition to user supplied code, this class contains
one method: CUP$do_action which consists of a large switch statement
for selecting and executing various fragments of user supplied action code. In
general, all names beginning with the prefix of CUP$ are reserved for
internal uses by CUP generated code.
The parser class contains the actual generated parser. It is a subclass of java_cup.runtime.lr_parser which implements a general table driven framework for an LR parser. The generated parser class provides a series of tables for use by the general framework. Three tables are provided:
Beyond the parse tables, generated (or inherited) code provides a series of methods that can be used to customize the generated parser. Some of these methods are supplied by code found in part of the specification and can be customized directly in that fashion. The others are provided by the lr_parser base class and can be overridden with new versions (via the parser code declaration) to customize the system. Methods available for customization include:
getScanner().next_token().
In addition to the normal parser, the runtime system also provides a debugging version of the parser. This operates in exactly the same way as the normal parser, but prints debugging messages (by calling public void debug_message(String mess) whose default implementation prints a message to System.err).
Based on these routines, invocation of a CUP parser is typically done with code such as:
/* create a parsing object */
parser parser_obj = new parser();
/* open input files, etc. here */
Symbol parse_tree = null;
try {
if (do_debug_parse)
parse_tree = parser_obj.debug_parse();
else
parse_tree = parser_obj.parse();
} catch (Exception e) {
/* do cleanup here - - possibly rethrow e */
} finally {
/* do close out here */
}
To use the new code, your scanner should implement the
java_cup.runtime.Scanner interface, defined as:
package java_cup.runtime;
public interface Scanner {
public Symbol next_token() throws java.lang.Exception;
}
In addition to the methods described in section
4, the java_cup.runtime.lr_parser class has two new accessor
methods, setScanner() and getScanner(). The default
implementation of scan()
is:
public Symbol scan() throws java.lang.Exception {
return getScanner().next_token();
}
The generated parser also contains a constructor which takes a
Scanner and calls setScanner() with it. In most cases,
then, the init with and scan with directives may be
omitted. You can simply create the parser with a reference to the desired
scanner:
/* create a parsing object */
parser parser_obj = new parser(new my_scanner());
or set the scanner after the parser is created: /* create a parsing object */
parser parser_obj = new parser();
/* set the default scanner */
parser_obj.setScanner(new my_scanner());
Note that because the parser uses look-ahead, resetting the scanner in the
middle of a parse is not recommended. If you attempt to use the default
implementation of scan() without first calling
setScanner(), a NullPointerException will be thrown.
As an example of scanner integration, the following three lines in the lexer-generator input are all that is required to use a JLex scanner with CUP:
%implements java_cup.runtime.Scanner %function next_token %type java_cup.runtime.SymbolIt is anticipated that the JLex directive
%cup will
abbreviate the above three directive in the next version of JLex. Invoking the
parser with the JLex scanner is then simply: parser parser_obj = new parser( new Yylex( some_InputStream_or_Reader));
Note that you still have to handle EOF correctly; the JLex code to do so is something like:
%eofval{
return sym.EOF;
%eofval}
where sym is the name of the symbol class for your generated
parser.
The simple_calc example in the CUP distribution illustrates the use of the scanner integration features with a hand-coded scanner.
6. Error Recovery
A final important aspect of building parsers with
CUP is support for syntactic error recovery. CUP uses the same error recovery
mechanisms as YACC. In particular, it supports a special error symbol (denoted
simply as error). This symbol plays the role of a special non-terminal
which, instead of being defined by productions, instead matches an erroneous
input sequence.
The error symbol only comes into play if a syntax error is detected. If a syntax error is detected then the parser tries to replace some portion of the input token stream with error and then continue parsing. For example, we might have productions such as:
stmt ::= expr SEMI | while_stmt SEMI | if_stmt SEMI | ... | error SEMI ;This indicates that if none of the normal productions for stmt can be matched by the input, then a syntax error should be declared, and recovery should be made by skipping erroneous tokens (equivalent to matching and replacing them with error) up to a point at which the parse can be continued with a semicolon (and additional context that legally follows a statement). An error is considered to be recovered from if and only if a sufficient number of tokens past the error symbol can be successfully parsed. (The number of tokens required is determined by the error_sync_size() method of the parser and defaults to 3).
Specifically, the parser first looks for the closest state to the top of the
parse stack that has an outgoing transition under error. This generally
corresponds to working from productions that represent more detailed constructs
(such as a specific kind of statement) up to productions that represent more
general or enclosing constructs (such as the general production for all
statements or a production representing a whole section of declarations) until
we get to a place where an error recovery production has been provided for. Once
the parser is placed into a configuration that has an immediate error recovery
(by popping the stack to the first such state), the parser begins skipping
tokens to find a point at which the parse can be continued. After discarding
each token, the parser attempts to parse ahead in the input (without executing
any embedded semantic actions). If the parser can successfully parse past the
required number of tokens, then the input is backed up to the point of recovery
and the parse is resumed normally (executing all actions). If the parse cannot
be continued far enough, then another token is discarded and the parser again
tries to parse ahead. If the end of input is reached without making a successful
recovery (or there was no suitable error recovery state found on the parse stack
to begin with) then error recovery fails.
7. Conclusion
This manual has briefly described the CUP LALR parser
generation system. CUP is designed to fill the same role as the well known YACC
parser generator system, but is written in and operates entirely with Java code
rather than C or C++. Additional details on the operation of the system can be
found in the parser generator and runtime source code. See the CUP home page
below for access to the API documentation for the system and its runtime.
This document covers version 0.10j of the system. Check the CUP home page: http://www.cs.princeton.edu/~appel/modern/java/CUP/ for the latest release information, instructions for downloading the system, and additional news about CUP. Bug reports and other comments for the developers should be sent to the CUP maintainer, C. Scott Ananian, at cananian@alumni.princeton.edu
CUP was originally written by Scott Hudson, in August of 1995.
It was extended to support precedence by Frank Flannery, in July of 1996.
On-going improvements have been done by C. Scott Ananian, the CUP maintainer, from December of 1997 to the present.
java_cup_spec ::= package_spec import_list code_parts
symbol_list precedence_list start_spec
production_list
package_spec ::= PACKAGE multipart_id SEMI | empty
import_list ::= import_list import_spec | empty
import_spec ::= IMPORT import_id SEMI
code_part ::= action_code_part | parser_code_part |
init_code | scan_code
code_parts ::= code_parts code_part | empty
action_code_part ::= ACTION CODE CODE_STRING opt_semi
parser_code_part ::= PARSER CODE CODE_STRING opt_semi
init_code ::= INIT WITH CODE_STRING opt_semi
scan_code ::= SCAN WITH CODE_STRING opt_semi
symbol_list ::= symbol_list symbol | symbol
symbol ::= TERMINAL type_id declares_term |
NON TERMINAL type_id declares_non_term |
NONTERMINAL type_id declares_non_term |
TERMINAL declares_term |
NON TERMINAL declares_non_term |
NONTERMIANL declared_non_term
term_name_list ::= term_name_list COMMA new_term_id | new_term_id
non_term_name_list ::= non_term_name_list COMMA new_non_term_id |
new_non_term_id
declares_term ::= term_name_list SEMI
declares_non_term ::= non_term_name_list SEMI
precedence_list ::= precedence_l | empty
precedence_l ::= precedence_l preced + preced;
preced ::= PRECEDENCE LEFT terminal_list SEMI
| PRECEDENCE RIGHT terminal_list SEMI
| PRECEDENCE NONASSOC terminal_list SEMI
terminal_list ::= terminal_list COMMA terminal_id | terminal_id
start_spec ::= START WITH nt_id SEMI | empty
production_list ::= production_list production | production
production ::= nt_id COLON_COLON_EQUALS rhs_list SEMI
rhs_list ::= rhs_list BAR rhs | rhs
rhs ::= prod_part_list PERCENT_PREC term_id |
prod_part_list
prod_part_list ::= prod_part_list prod_part | empty
prod_part ::= symbol_id opt_label | CODE_STRING
opt_label ::= COLON label_id | empty
multipart_id ::= multipart_id DOT ID | ID
import_id ::= multipart_id DOT STAR | multipart_id
type_id ::= multipart_id
terminal_id ::= term_id
term_id ::= symbol_id
new_term_id ::= ID
new_non_term_id ::= ID
nt_id ::= ID
symbol_id ::= ID
label_id ::= ID
opt_semi ::= SEMI | empty
// Simple Example Scanner Class
import java_cup.runtime.*;
import sym;
public class scanner {
/* single lookahead character */
protected static int next_char;
/* advance input by one character */
protected static void advance()
throws java.io.IOException
{ next_char = System.in.read(); }
/* initialize the scanner */
public static void init()
throws java.io.IOException
{ advance(); }
/* recognize and return the next complete token */
public static Symbol next_token()
throws java.io.IOException
{
for (;;)
switch (next_char)
{
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
/* parse a decimal integer */
int i_val = 0;
do {
i_val = i_val * 10 + (next_char - '0');
advance();
} while (next_char >= '0' && next_char <= '9');
return new Symbol(sym.NUMBER, new Integer(i_val));
case ';': advance(); return new Symbol(sym.SEMI);
case '+': advance(); return new Symbol(sym.PLUS);
case '-': advance(); return new Symbol(sym.MINUS);
case '*': advance(); return new Symbol(sym.TIMES);
case '/': advance(); return new Symbol(sym.DIVIDE);
case '%': advance(); return new Symbol(sym.MOD);
case '(': advance(); return new Symbol(sym.LPAREN);
case ')': advance(); return new Symbol(sym.RPAREN);
case -1: return new Symbol(sym.EOF);
default:
/* in this simple scanner we just ignore everything else */
advance();
break;
}
}
};
For more information, refer to the manual on scanners.
terminal classname terminal [, terminal ...];still works. The classname, however indicates the type of the value of the terminal or non-terminal, and does not indicate the type of object placed on the parse stack. A declaration, such as:
terminal terminal [, terminal ...];indicates the terminals in the list hold no value.
For more information, refer to the manual on declarations.
For more information, refer to the manual on labels.
For more information, refer to the manual on RESULT.
For more information, refer to the manual on positions.
precedence {left| right | nonassoc} terminal[, terminal ...];
...
The terminals are assigned a precedence, where terminals on the same line
have equal precedences, and the precedence declarations farther down the list of
precedence declarations have higher precedence. left, right and
nonassoc specify the associativity of these terminals. left
associativity corresponds to a reduce on conflict, right to a shift on conflict,
and nonassoc to an error on conflict. Hence, ambiguous grammars may now be used.
For more information, refer to the manual on precedence.
lhs ::= {right hand side list of terminals, non-terminals and actions}
%prec {terminal};
this production would then have a precedence equal to the terminal
specified after the %prec. Hence, shift/reduce conflicts can be
contextually resolved. Note that the %prec terminal part comes
after all actions strings. It does not come before the last action string.
For more information, refer to the manual on contextual precedence. These changes implemented by:
This is because the parsing tables (and parsing engine) are in one object (belonging to class parser or whatever name is specified by the -parser directive), and the semantic actions are in another object (of class CUP$actions).
However, there is a way to do it, though it's a bit inelegant. The action object has a private final field named parser that points to the parsing object. Thus, methods and instance variables of the parser can be accessed within semantic actions as:
parser.report_error(message,info);
x = parser.mydata;
Perhaps this will not be necessary in a future release, and that such methods and variables as report_error and mydata will be available directly from the semantic actions; we will achieve this by combining the "parser" object and the "actions" object together.
For a list of any other currently known bugs in CUP, see http://www.cs.princeton.edu/~appel/modern/java/CUP/bugs.html.
Appendix E: Change log
java_cup.runtime.Scanner
interface.