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<html><head>
<title>CUP User's Manual</title></head><body>
<div class="container">
<hr>
<h1>CUP User's Manual</h1>
<h3><a href="http://www2.in.tum.de">Michael Petter, Technische Universität München</a><br />
<a href="http://www.cc.gatech.edu/gvu/people/Faculty/Scott.E.Hudson.html">
Scott E. Hudson</a><br>
<a href="http://www.cc.gatech.edu/gvu/gvutop.html">
Graphics Visualization and Usability Center</a><br>
<a href="http://www.gatech.edu/TechHome.html">
Georgia Institute of Technology</a></h3>
Modified by <a href="http://www.princeton.edu/%7Efrankf">Frank
Flannery</a>, <a href="http://www.pdos.lcs.mit.edu/%7Ecananian/">C. Scott Ananian</a>,
<a href="http://www.cs.princeton.edu/%7Edanwang">Dan Wang</a>, <a href="http://www2.cs.tum.edu/%7Epetter">Michael Petter</a> with advice from
<a href="http://www.cs.princeton.edu/%7Eappel">Andrew W. Appel</a><br>
<br>
Last updated 06/2014 (v0.11b)
<hr>
<h3>Table of Contents</h3>
<ol type="i">
<li> <a href="#about">About CUP Version 0.10</a>
</li><li> <a href="#about11">About CUP Version 0.11</a>
</li>
</ol>
<ol>
<li> <a href="#intro">Introduction and Example</a>
</li><li> <a href="#spec">Specification Syntax</a>
<ol><li><a href="#package_spec">Package and import specification</a>
</li><li> <a href="#code_part">User code componentes</a>
</li><li> <a href="#symbol_list">Symbol lists</a>
</li><li> <a href="#precedence">Precedence and associativity declarations</a>
</li><li> <a href="#production_list">The grammar</a></li></ol>
</li><li> Running CUP
<ol><li><a href="#running">Command line interface</a>
</li><li> <a href="#ant">CUP and ANT</a></li></ol>
</li><li> <a href="#parser">Customizing the Parser</a>
</li><li> <a href="#scanner">Scanner interface</a>
<ol><li> <a href="#basic-symbols">Basic Symbol management</a>
</li><li> <a href="#advanced-symbols">Advanced Symbol management</a>
</li><li> <a href="#symbolstream-caching">Symbol Stream caching</a></li></ol>
</li><li> <a href="#errors">Error Recovery</a>
<ol><li><a href="#error_sym">The error symbol</a>
</li><li> <a href="#continuation_symbols">Viable continuations</a></li></ol>
</li><li> <a href="#conclusion">Conclusion</a>
</li>
</ol>
<a href="#refs">References</a>
<ol type="A">
<li> <a href="#appendixa">Grammar for CUP Specification Files</a>
</li><li> <a href="#appendixb">A Very Simple Example Scanner</a>
</li><li> <a href="#changes">Incompatibilites between CUP 0.9 and CUP 0.10</a>
</li><li> <a href="#bugs">Bugs</a>
</li><li> <a href="#version">Change log</a>
</li>
</ol>
<a name="about"></a>
<h3>i. About CUP Version 0.10</h3>
Version
0.10 of CUP adds many new changes and features over the previous releases
of version 0.9. These changes attempt to make CUP more like its
predecessor, YACC. As a result, the old 0.9 parser specifications for CUP are
not compatible and a reading of <a href="#changes">appendix C</a> of the new
manual will be necessary to write new specifications. The new version,
however, gives the user more power and options, making parser specifications
easier to write.
<a name="about11"></a><h3>ii. About CUP Version 0.11</h3>
in version 0.11 the TUM team tries to continue the success story of CUP 0.10,
beginning with the introduction of generic data types for non-terminal symbols
as well as a modernisation of the user interface with a comfortable ANT plugin
structure.
<a name="intro">
<h3>1. Introduction and Example</h3></a>
This manual describes the basic operation and use of the
Java<a href="#trademark">(tm)</a>
Based Constructor of Useful Parsers (CUP for short).
CUP is a system for generating LALR parsers from simple specifications.
It serves the same role as the widely used program YACC
<a href="#YACCref">[1]</a> and in fact offers most of the features of YACC.
However, CUP is written in Java, uses specifications including embedded
Java code, and produces parsers which are implemented in Java.<p>
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
<a href="#dragonbook">[2</a>,<a href="#crafting">3]</a>) cover this material,
and discuss the YACC system (which is quite similar to this one) as a
specific example. </p><p>
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).</p><p>
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: </p><pre> 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
</pre>
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:
<pre><tt> expr_list, expr_part </tt> and <tt> expr </tt>.</pre>
For terminal names we might choose:
<pre><tt> SEMI, PLUS, MINUS, TIMES, DIVIDE, MOD, NUMBER, LPAREN,</tt>
and <tt>RPAREN</tt></pre>
The 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: <br>
<tt>3 + 4 * 6</tt><br>
The grammar can either evaluate the <tt>3 + 4</tt> and then multiply
seven by six, or it can evaluate <tt>4 * 6</tt> and then add three.
Older versions of CUP forced the user to write unambiguous grammars, but
now there is a construct allowing the user to specify precedences and
associativities for terminals. This means that the above ambiguous
grammar can be used, after specifying precedences and associativities.
There is more explanation later.
Based on these namings we can construct a small CUP specification
as follows:<br>
<hr>
<pre><tt>// 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
;
</tt></pre>
<hr><br>
We will consider each part of the specification syntax in detail later.
However, here we can quickly see that the specification contains four
main parts. The first part provides preliminary and miscellaneous declarations
to specify how the parser is to be generated, and supply parts of the
runtime code. In this case we indicate that the <tt>java_cup.runtime</tt>
classes should be imported, then supply a small bit of initialization code,
and some code for invoking the scanner to retrieve the next input token.
The second part of the specification declares terminals and non-terminals,
and associates object classes with each. In this case, the terminals
are declared as either with no type, or of type
<tt>Integer</tt>. The specified type of the
terminal or non-terminal is the type of the value of those terminals or
non-terminals. If no type is specified, the terminal or non-terminal
carries no value. Here, no type indicates that these
terminals and non-terminals hold no value.
The third part specifies the precedence and
associativity of terminals. The last precedence declaration give its
terminals the highest precedence. The final
part of the specification contains the grammar.<p>
To produce a parser from this specification we use the CUP generator.
If this specification were stored in a file <tt>parser.cup</tt>, then
(on a Unix system at least) we might invoke CUP using a command like:
</p><pre><tt> java -jar java-cup-11b.jar parser.cup</tt> </pre>
In this case, the system will produce two Java source files containing
parts of the generated parser: <tt>sym.java</tt> and <tt>parser.java</tt>.
As you might expect, these two files contain declarations for the classes
<tt>sym</tt> and <tt>parser</tt>. The <tt>sym</tt> 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
"<tt>return new Symbol(sym.SEMI);</tt>" ). The <tt>parser</tt> class
implements the parser itself.<p>
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 <i>code strings</i> which are surrounded by delimiters
of the form <tt>{:</tt> and <tt>:}</tt> (we can see examples of this in the
<tt>init with</tt> and <tt>scan with</tt> clauses above). In general, the
system records all characters within the delimiters, but does not try to check
that it contains valid Java code.</p><p>
A more complete CUP specification for our example system (with actions
embedded at various points in the grammar) is shown below:<br>
</p><hr>
<pre><tt>// 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; :}
;
</tt></pre>
<hr><br>
Here we can see several changes. Most importantly, code to be executed at
various points in the parse is included inside code strings delimited by
<tt>{:</tt> and <tt>:}</tt>. In addition, labels have been placed on various
symbols in the right hand side of productions. For example in:<br>
<pre> expr:e1 PLUS expr:e2
{: RESULT = new Integer(e1.intValue() + e2.intValue()); :}
</pre>
<a name="RES_part"></a>
the first non-terminal <tt>expr</tt> has been labeled with <tt>e1</tt>, and
the second with <tt>e2</tt>. The left hand side value
of each production is always implicitly labeled as <tt>RESULT</tt>.<p>
Each symbol appearing in a production is represented at runtime by an
object of type <tt>Symbol</tt> on the parse stack. The labels refer to
the instance variable <tt>value</tt> in those objects. In the
expression <tt>expr:e1 PLUS expr:e2</tt>, <tt>e1</tt> and <tt>e2</tt>
refer to objects of type Integer. These objects are in the value fields
of the objects of type <tt>Symbol</tt> representing those non-terminals
on the parse stack. <tt>RESULT</tt> is of type <tt>Integer</tt> as
well, since the resulting non-terminal <tt>expr</tt> was declared as of
type <tt>Integer</tt>. This object becomes the <tt>value</tt> instance
variable of a new <tt>Symbol</tt> object.</a></p><p>
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 <tt>left</tt> or <tt>right</tt>. for example, given
the right hand side of a production <tt>expr:e1 PLUS expr:e2</tt> the
user could not only access variables <tt>e1</tt> and <tt>e2</tt>, but
also <tt>e1left, e1right, e2left</tt> and <tt>e2right</tt>. these
variables are of type <tt>int</tt>.</p><p>
<a name="lex_part"></a>
The final step in creating a working parser is to create a <i>scanner</i> (also
known as a <i>lexical analyzer</i> or simply a <i>lexer</i>). 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
<tt>scanner.next_token()</tt>. The scanner should return objects of
type <tt>java_cup.runtime.Symbol</tt>. This type is very different than
older versions of CUP's <tt>java_cup.runtime.symbol</tt>. These Symbol
objects contains the instance variable <tt>value</tt> 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 <tt>terminal</tt> and <tt>non terminal</tt> declarations. In the
above example, if the lexer wished to pass a NUMBER token, it should
create a <tt>Symbol</tt> with the <tt>value</tt> instance variable
filled with an object of type <tt>Integer</tt>. <code>Symbol</code>
objects corresponding to terminals and non-terminals with no value
have a null value field.</a></p><p>
The code contained in the <tt>init with</tt> clause of the specification
will be executed before any tokens are requested. Each token will be
requested using whatever code is found in the <tt>scan with</tt> 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 <code>java_cup.runtime.Symbol</code> (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, <code>Symbol</code> reuse should be
detected if it occurs; the parser will throw an <code>Error</code>
telling you to fix your scanner.</a></p><p>
In the </a><a href="#spec">next section</a> a more detailed and formal
explanation of all parts of a CUP specification will be given.
<a href="#running">Section 3</a> describes options for running the
CUP system. <a href="#parser">Section 4</a> discusses the details
of how to customize a CUP parser, while <a href="#scanner">section 5</a>
discusses the scanner interface added in CUP 0.10j. <a href="#errors">Section
6</a> considers error recovery. Finally, <a href="#conclusion">Section 7</a>
provides a conclusion.
<a name="spec"></a>
<h3>2. Specification Syntax</h3>
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:
</p><ul>
<li> <a href="#package_spec">package and import specifications</a>,
</li><li> <a href="#parser_class_name">parser class name</a>,
</li><li> <a href="#code_part">user code components</a>,
</li><li> <a href="#symbol_list">symbol (terminal and non-terminal) lists</a>,
</li><li> <a href="#precedence">precedence declarations</a>, and
</li><li> <a href="#production_list">the grammar</a>.
</li></ul>
Each of these parts must appear in the order presented here. (A complete
grammar for the specification language is given in
<a href="#appendixa">Appendix A</a>.) The particulars of each part of
the specification are described in the subsections below.<p>
</p><h4><a name="package_spec"></a>2.1 Package and Import Specifications</h4>
A specification begins with optional <tt>package</tt> and <tt>import</tt>
declarations. These have the same syntax, and play the same
role, as the package and import declarations found in a normal Java program.
A package declaration is of the form:
<pre><tt> package <i>name</i>;</tt></pre>
where name <tt><i>name</i></tt> 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.<p>
After an optional <tt>package</tt> declaration, there can be zero or more
<tt>import</tt> declarations. As in a Java program these have the form:
</p><pre><tt> import <i>package_name.class_name</i>;</tt>
</pre>
or
<pre><tt> import <i>package_name</i>.*;</tt>
</pre>
The package declaration indicates what package the <tt>sym</tt> and
<tt>parser</tt> 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 <tt>parser</tt> class allowing various names from
that package to be used directly in user supplied action code.
After the package and imports, a specification can include a parser
class name, as follows:
<pre><tt> class <i>name</i>;</tt>
</pre>
If a class name is supplied, then that is the name that will be used
for the generated parser code. As well, the generated symbols class
will have the given name followed by "<tt>Sym</tt>".
<p>If no class name is supplied, then the parser class is called
"<tt>parser</tt>", and the symbols class is called "<tt>sym</tt>".
<h4><a name="code_part"></a>2.2 User Code Components</h4>
Following these declarations are another series of optional declarations
that allow user code to be included
as part of the generated parser (see <a href="#parser">Section 4</a> for a
full description of how the parser uses this code). As a part of the parser
file, a separate non-public class to contain all embedded user actions is
produced. The first <tt>action code</tt> declaration section allows code to
be included in this class. Routines and variables for use by the code
embedded in the grammar would normally be placed in this section (a typical
example might be symbol table manipulation routines). This declaration takes
the form:
<pre><tt> action code {: ... :};</tt>
</pre>
where <tt>{: ... :}</tt> is a code string whose contents will be placed
directly within the <tt>action class</tt> class declaration.<p>
After the <tt>action code</tt> declaration is an optional
<tt>parser code</tt> 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 <tt>action code</tt> declaration and
takes the form:
</p><pre><tt> parser code {: ... :};</tt>
</pre>
Again, code from the code string is placed directly into the generated parser
class definition.<p>
Next in the specification is the optional <tt>init</tt> declaration
which has the form:
</p><pre><tt> init with {: ... :};</tt></pre>
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 <tt>void</tt> method inside the <tt>parser</tt>
class.<p>
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:
</p><pre><tt> scan with {: ... :};</tt></pre>
As with the <tt>init</tt> 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 <tt>java_cup.runtime.Symbol</tt>.
Consequently the code found in the <tt>scan with</tt> clause should
return such a value. See <a href="#scanner">section 5</a> for
information on the default behavior if the <code>scan with</code>
section is omitted.<p>
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.</p><p>
</p><h4><a name="symbol_list"></a>2.3 Symbol Lists</h4>
Following user supplied code comes the first required part of the
specification: the symbol lists. These declarations are responsible
for naming and supplying a type for each terminal and non-terminal
symbol that appears in the grammar. As indicated above, each terminal
and non-terminal symbol is represented at runtime with a <tt>Symbol</tt>
object. In
the case of terminals, these are returned by the scanner and placed on
the parse stack. The lexer should put the value of the terminal in the
<tt>value</tt> instance variable.
In the case of non-terminals these replace a series
of <tt>Symbol</tt> objects on the parse stack whenever the right hand side of
some production is recognized. In order to tell the parser which object
types should be used for which symbol, <tt>terminal</tt> and
<tt>non terminal</tt> declarations are used. These take the forms:
<pre><tt> terminal <i>classname</i> <i>name1, name2,</i> ...;</tt>
<tt> non terminal <i>classname</i> <i>name1, name2,</i> ...;</tt>
<tt> terminal <i>name1, name2,</i> ...;</tt>
</pre>
and
<pre><tt> non terminal <i>name1, name2,</i> ...;</tt>
</pre>
where <tt><i>classname</i></tt> can be a multiple part name separated with
"."s. The
<tt><i>classname</i></tt> 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 <tt><i>classname</i></tt>
can be of any type. If no <tt><i>classname</i></tt> 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 "<code>nonterminal</code>" (note, no
space) as well as the original "<code>non terminal</code>" spelling.<p>
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".</p><p>
</p><h4><a name="precedence"></a>2.4 Precedence and Associativity declarations</h4>
The third section, which is optional, specifies the precedences and
associativity of terminals. This is useful for parsing with ambiguous
grammars, as done in the example above. There are three type of
precedence/associativity declarations:
<pre><tt>
precedence left <i>terminal</i>[, <i>terminal</i>...];
precedence right <i>terminal</i>[, <i>terminal</i>...];
precedence nonassoc <i>terminal</i>[, <i>terminal</i>...];
</tt></pre>
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:
<pre><tt>
precedence left ADD, SUBTRACT;
precedence left TIMES, DIVIDE;
</tt></pre>
Precedence resolves shift reduce problems. For example, given the input
to the above example parser <tt>3 + 4 * 8</tt>, the parser doesn't know
whether to reduce <tt>3 + 4</tt> 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.<p>
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, <tt>expr ::= expr TIMES expr</tt> would have
the same precedence as <tt>TIMES</tt>. 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.</p><p>
An associativity is assigned to each terminal used in the
precedence/associativity declarations. The three associativities are
<tt>left, right</tt> and <tt>nonassoc</tt> 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 <tt>left</tt>, then a reduce is performed. This means, if the input
is a string of additions, like <tt>3 + 4 + 5 + 6 + 7</tt>, the parser
will <i>always</i> reduce them from left to right, in this case,
starting with <tt>3 + 4</tt>. If the associativity of the terminal is
<tt>right</tt>, it is shifted onto the stack. hence, the reductions
will take place from right to left. So, if PLUS were declared with
associativity of <tt>right</tt>, the <tt>6 + 7</tt> would be reduced
first in the above string. If a terminal is declared as
<tt>nonassoc</tt>, 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
<tt>6 == 7 == 8 == 9</tt>, the parser should generate an error. If '=='
is declared as <tt>nonassoc</tt> then an error will be generated. </p><p>
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.</p><p>
</p><h4><a name="production_list"></a>2.5 The Grammar</h4>
The final section of a CUP declaration provides the grammar. This
section optionally starts with a declaration of the form:
<pre><tt> start with <i>non-terminal</i>;</tt>
</pre>
This indicates which non-terminal is the <i>start</i> or <i>goal</i>
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 <tt>java_cup.runtime.Symbol</tt>. This
<tt>Symbol</tt>'s value instance variable contains the final reduction
result.<p>
The grammar itself follows the optional <tt>start</tt> declaration. Each
production in the grammar has a left hand side non-terminal followed by
the symbol "<tt>::=</tt>", 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 (;).</p>
<a name="label_part"></a>
<h5>2.5.1 Labels and locations for symbols</h5>
<p>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 <tt>left</tt> and
the label plus <tt>right</tt>. These are <tt>int</tt> 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.</p><p>
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 "<tt>::=</tt>". This is followed by multiple right hand sides each
separated by a bar (|). The full set of productions is then terminated by a
semicolon.</p>
<p>However, integers alone might not be good enough for locating symbols in
an input file. This is why CUP also supports more complex location information
in the form of <code>Location</code> objects. These objects need to be created
by the <a href="#advanced-symbols">scanner component</a>. They become available
within the actions if the parser generator gets the right commandline parameter.
They are accessible via the handles <code>??xleft</code> and <code>??xright</code>,
with <code>??</code> being the label for the symbol.
</p>
<p>
<a name="action_part"></a>
<h5>2.5.2 User Actions</h5>
Actions appear in the right hand side as code strings (e.g., Java code inside
<tt>{:</tt> ... <tt>:}</tt> 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
<i>lookahead</i> token for recognition.)</p>
<a name="cpp"></a>
<h5>2.5.3 Contextual precedence</h5>
<p>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:
</p><pre>
precedence left PLUS, MINUS;
precedence left TIMES, DIVIDE, MOD;
precedence left UMINUS;
expr ::= MINUS expr:e
{: RESULT = new Integer(0 - e.intValue()); :}
%prec UMINUS
</pre>
<p>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.
</p>
<a name="running"></a>
<h3>3. Running CUP</h3>
<h4>3.1 Command line interface</h4>
As mentioned above, CUP is written in Java. To invoke it, one needs
to use the Java interpreter to invoke the static method
<tt>java_cup.Main()</tt>, 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:
<pre><tt> java -jar java-cup-11b.jar <i>options</i> <i>inputfile</i></tt></pre>
Once running, CUP expects to find a specification file on standard input
and produces two Java source files as output. <p>
In addition to the specification file, CUP's behavior can also be changed
by passing various options to it. Legal options are documented in
<code>Main.java</code> and include:
</p><dl>
<dt><tt>-package</tt> <i>name</i>
</dt><dd>Specify that the <tt>parser</tt> and <tt>sym</tt> classes are to be
placed in the named package. By default, no package specification
is put in the generated code (hence the classes default to the special
"unnamed" package).
</dd><dt><tt>-parser</tt> <i>name</i>
</dt><dd>Output parser and action code into a file (and class) with the given
name instead of the default of "<tt>parser</tt>".
</dd><dt><tt>-symbols</tt> <i>name</i>
</dt><dd>Output the symbol constant code into a class with the given
name instead of the default of "<tt>sym</tt>".
</dd><dt><tt>-interface</tt>
</dt><dd>Outputs the symbol constant code as an <code>interface</code>
rather than as a <code>class</code>.
</dd><dt><tt>-nonterms</tt>
</dt><dd>Place constants for non-terminals into the symbol constant class.
The parser does not need these symbol constants, so they are not normally
output. However, it can be very helpful to refer to these constants
when debugging a generated parser.
</dd><dt><tt>-expect</tt> <i>number</i>
</dt><dd>During parser construction the system may detect that an ambiguous
situation would occur at runtime. This is called a <i>conflict</i>.
In general, the parser may be unable to decide whether to <i>shift</i>
(read another symbol) or <i>reduce</i> (replace the recognized right
hand side of a production with its left hand side). This is called a
<i>shift/reduce conflict</i>. Similarly, the parser may not be able
to decide between reduction with two different productions. This is
called a <i>reduce/reduce conflict</i>. Normally, if one or more of
these conflicts occur, parser generation is aborted. However, in
certain carefully considered cases it may be advantageous to
arbitrarily break such a conflict. In this case CUP uses YACC
convention and resolves shift/reduce conflicts by shifting, and
reduce/reduce conflicts using the "highest priority" production (the
one declared first in the specification). In order to enable automatic
breaking of conflicts the <tt>-expect</tt> option must be given
indicating exactly how many conflicts are expected. Conflicts
resolved by precedences and associativities are not reported.
</dd><dt><tt>-compact_red</tt>
</dt><dd>Including this option enables a table compaction optimization involving
reductions. In particular, it allows the most common reduce entry in
each row of the parse action table to be used as the default for that
row. This typically saves considerable room in the tables, which can
grow to be very large. This optimization has the effect of replacing
all error entries in a row with the default reduce entry. While this
may sound dangerous, if not down right incorrect, it turns out that this
does not affect the correctness of the parser. In particular, some
changes of this type are inherent in LALR parsers (when compared to
canonical LR parsers), and the resulting parsers will still never
read past the first token at which the error could be detected.
The parser can, however, make extra erroneous reduces before detecting
the error, so this can degrade the parser's ability to do
<a href="#errors">error recovery</a>.
(Refer to reference [2] pp. 244-247 or reference [3] pp. 190-194 for a
complete explanation of this compaction technique.) <br><br>
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.
</dd><dt><tt>-nowarn</tt>
</dt><dd>This options causes all warning messages (as opposed to error messages)
produced by the system to be suppressed.
</dd><dt><tt>-nosummary</tt>
</dt><dd>Normally, the system prints a summary listing such things as the
number of terminals, non-terminals, parse states, etc. at the end of
its run. This option suppresses that summary.
</dd><dt><tt>-progress</tt>
</dt><dd>This option causes the system to print short messages indicating its
progress through various parts of the parser generation process.
</dd><dt><tt>-dump_grammar</tt>
</dt><dt><tt>-dump_states</tt>
</dt><dt><tt>-dump_tables</tt>
</dt><dt><tt>-dump</tt>
</dt><dd> These options cause the system to produce a human readable dump of
the grammar, the constructed parse states (often needed to resolve
parse conflicts), and the parse tables (rarely needed), respectively.
The <tt>-dump</tt> option can be used to produce all of these dumps.
</dd><dt><tt>-time</tt>
</dt><dd>This option adds detailed timing statistics to the normal summary of
results. This is normally of great interest only to maintainers of
the system itself.
</dd><dt><tt>-debug</tt>
</dt><dd>This option produces voluminous internal debugging information about
the system as it runs. This is normally of interest only to maintainers
of the system itself.
</dd><dt><tt>-nopositions</tt>
</dt><dd>This option keeps CUP from generating code to propagate the left
and right hand values of terminals to non-terminals, and then from
non-terminals to other terminals. If the left and right values aren't
going to be used by the parser, then it will save some runtime
computation to not generate these position propagations. This option
also keeps the left and right label variables from being generated, so
any reference to these will cause an error.
</dd><dt><tt>-locations</tt>
</dt><dd>This option makes CUP generate xleft/xright handles for
accessing Location objects for symbol start/end inside actions.
</dd><dt><tt>-xmlactions</tt>
</dt><dd>This option makes CUP generate generic actions that produce
XMLElement-objects for any symbol, that is labeled by the CUP spec
author. These objects directly mirror the according parse subtree. For this
option, You will want to remove types from any symbol as well as
ignore the RESULT-handle in any action, should you still want
to perform any.
</dd><dt><tt>-genericlabels</tt>
</dt><dd>This option goes one step further then -xmlactions by producing
the full parse tree as XMLElement-tree.
</dd><dt><tt>-noscanner</tt>
</dt><dd>CUP 0.10j introduced <a href="#scanner">improved scanner
integration</a> and a new interface,
<code>java_cup.runtime.Scanner</code>. 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 <code>-noscanner</code> option to suppress the
<code>java_cup.runtime.Scanner</code> references and allow
compatibility with old runtimes. Not many people should have reason
to do this.
</dd><dt><tt>-version</tt>
</dt><dd>Invoking CUP with the <code>-version</code> 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.
</dd></dl>
<a name="ant"></a>
<h4>3.2 Integrating CUP into an <a href="http://ant.apache.org/">ANT</a>
script</h4>
<p>To use cup in an ANT script, You have to add the following task definition to
Your <tt>build.xml</tt> file:</p>
<pre><taskdef name="cup"
classname="java_cup.anttask.CUPTask"
classpathref="cupclasspath"
/>
</pre>
<p>Now, You are ready to use Your new <cup/> task to generate own parsers
from within ANT. Such a generation statement could look like:</p>
<pre><target name="cup">
<cup srcfile="path/to/cupfile/Parser.cup"
destdir="path/to/javafiles"
interface="true"
/>
</target>
</pre>
<p>You can specify all commandline flags from chapter <bf>3.1</bf> as boolean
parameters to Your cuptask to achieve a similar behaviour (as done with
<tt>-interface</tt> in this little example).</p>
<a name="parser"></a>
<h3>4. Customizing the Parser</h3>
Each generated parser consists of three generated classes. The
<tt>sym</tt> class (which can be renamed using the <tt>-symbols</tt>
option) simply contains a series of <tt>int</tt> constants,
one for each terminal. Non-terminals are also included if the <tt>-nonterms</tt>
option is given. The source file for the <tt>parser</tt> class (which can
be renamed using the <tt>-parser</tt> option) actually contains two
class definitions, the public <tt>parser</tt> class that implements the
actual parser, and another non-public class (called <tt>CUP$action</tt>) which
encapsulates all user actions contained in the grammar, as well as code from
the <tt>action code</tt> declaration. In addition to user supplied code, this
class contains one method: <tt>CUP$do_action</tt> which consists of a large
switch statement for selecting and executing various fragments of user
supplied action code. In general, all names beginning with the prefix of
<tt>CUP$</tt> are reserved for internal uses by CUP generated code. <p>
The <tt>parser</tt> class contains the actual generated parser. It is
a subclass of <tt>java_cup.runtime.lr_parser</tt> which implements a
general table driven framework for an LR parser. The generated <tt>parser</tt>
class provides a series of tables for use by the general framework.
Three tables are provided:
</p><dl compact="compact">
<dt>the production table
</dt><dd>provides the symbol number of the left hand side non-terminal, along with
the length of the right hand side, for each production in the grammar,
</dd><dt>the action table
</dt><dd>indicates what action (shift, reduce, or error) is to be taken on each
lookahead symbol when encountered in each state, and
</dd><dt>the reduce-goto table
</dt><dd>indicates which state to shift to after reduces (under each non-terminal
from each state).
</dd></dl>
(Note that the action and reduce-goto tables are not stored as simple arrays,
but use a compacted "list" structure to save a significant amount of space.
See comments the runtime system source code for details.)<p>
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
<tt>lr_parser</tt> base class and can be overridden with new versions (via
the <tt>parser code</tt> declaration) to customize the system. Methods
available for customization include:
</p><dl compact="compact">
<dt><tt>public void user_init()</tt>
</dt><dd>This method is called by the parser prior to asking for the first token
from the scanner. The body of this method contains the code from the
<tt>init with</tt> clause of the the specification.
</dd><dt><a name="scan_method"></a><tt>public java_cup.runtime.Symbol scan()</tt>
</dt><dd>This method encapsulates the scanner and is called each time a new
terminal is needed by the parser. The body of this method is
supplied by the <tt>scan with</tt> clause of the specification, if
present; otherwise it returns <code>getScanner().next_token()</code>.
</dd><dt><tt>public java_cup.runtime.Scanner getScanner()</tt>
</dt><dd>Returns the default scanner. See <a href="#scanner">section 5</a>.
</dd><dt><tt>public void setScanner(java_cup.runtime.Scanner s)</tt>
</dt><dd>Sets the default scanner. See <a href="#scanner">section 5</a>.
</dd><dt><tt> public void report_error(String message, Object info)</tt>
</dt><dd>This method should be called whenever an error message is to be issued. In
the default implementation of this method, the first parameter provides
the text of a message which is printed on <tt>System.err</tt>
and the second parameter is simply ignored. It is very typical to
override this method in order to provide a more sophisticated error
reporting mechanism.
</dd><dt><tt>public void report_fatal_error(String message, Object info)</tt>
</dt><dd>This method should be called whenever a non-recoverable error occurs. It
responds by calling <tt>report_error()</tt>, then aborts parsing
by calling the parser method <tt>done_parsing()</tt>, and finally
throws an exception. (In general <tt>done_parsing()</tt> should be called
at any point that parsing needs to be terminated early).
</dd><dt><tt>public List<Integer> expected_token_ids()</tt>
</dt><dd>This method informs during parsing and especially when handling
parsing problems and invalid input about valid continuations for parsing.
It returns a list of integer constants which correspond to the constants
in the generated <tt>sym.java</tt>. You can retranslate the ids to their
string representations, which where used in the grammar specification to
declare (non)terminal names, with <tt>symbol_name_from_id(id)</tt>
This can be typically used to implement syntax completion or more detailed
error messages.
</dd><dt><tt>public String symbol_name_from_id(int id)</tt>
</dt><dd>This method translates numerical constants, representing symbol ids
back to their textual representation, declared in the <tt>.cup</tt>
parser specification file.
</dd><dt><tt>public void syntax_error(Symbol cur_token)</tt>
</dt><dd>This method is called by the parser as soon as a syntax error is detected
(but before error recovery is attempted). In the default implementation it
calls: <tt>report_error("Syntax error", null);</tt>.
</dd><dt><tt>public void unrecovered_syntax_error(Symbol cur_token)</tt>
</dt><dd>This method is called by the parser if it is unable to recover from a
syntax error. In the default implementation it calls:
<tt>report_fatal_error("Couldn't repair and continue parse", null);</tt>.
</dd><dt><tt> protected int error_sync_size()</tt>
</dt><dd>This method is called by the parser to determine how many tokens it must
successfully parse in order to consider an error recovery successful.
The default implementation returns 3. Values below 2 are not recommended.
See the section on <a href="#errors">error recovery</a> for details.
</dd></dl>
Parsing itself is performed by the method <tt>public Symbol parse()</tt>.
This method starts by getting references to each of the parse tables,
then initializes a <tt>CUP$action</tt> object (by calling
<tt>protected void init_actions()</tt>). Next it calls <tt>user_init()</tt>,
then fetches the first lookahead token with a call to <tt>scan()</tt>.
Finally, it begins parsing. Parsing continues until <tt>done_parsing()</tt>
is called (this is done automatically, for example, when the parser
accepts). It then returns a <tt>Symbol</tt> with the <tt>value</tt>
instance variable containing the RESULT of the start production, or
<tt>null</tt>, if there is no value.<p>
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
<tt>public void debug_message(String mess)</tt> whose default implementation
prints a message to <tt>System.err</tt>).</p><p>
Based on these routines, invocation of a CUP parser is typically done
with code such as:
</p><pre> /* 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 */
}
</pre>
<a name="scanner"></a>
<h3>5. Scanner Interface</h3>
<a name="basic-symbols" ></a>
<h4>5.1 Basic Symbol management</h4>