Geyacc Declarations

Geyacc Declarations

The geyacc declarations subsection of a geyacc grammar defines the symbols used in formulating the grammar.

All token type names (but not single-character literal tokens such as '+' and '*') must be declared. Nonterminal symbols must be declared if the Eiffel type used for the semantic value needs to be specified. The first rule in the file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly.

Tokens

The basic way to declare a token type name (terminal symbol) is as follows:

%token NAME

Geyacc will convert this into an integer constant feature in the parser, so that the routine read_token (if it is define in this class) can use the name NAME to stand for this token type's code. Any number of terminal symbols can be specified in the same %token declaration. Use spaces to separate the symbol names.

Alternatively, you can use %left, %right, or %nonassoc instead of %token, if you wish to specify precedence.

You can explicitly specify the numeric code for a token type by appending an integer value in the field immediately following the token name:

%token NUM 300

It is generally best, however, to let geyacc choose the numeric codes for all token types. Geyacc will automatically select codes that don't conflict with each other or with character codes.

When the type of semantic values needs to be specified (when it's different from the default detachable ANY), the syntax of %token may be extended to include the Eiffel type alternative (possibly an anchored type) delimited by angle-brackets:

%token <INTEGER> NUM       -- Define token NUM of type INTEGER
%token <STRING> NAME       -- Define token NAME of type STRING
%token <like token> DIGIT  -- Define token DIGIT of type like token

All tokens specified in the same %tokendeclaration will have the same value type.

A literal string token can be associated with a token type name by writing the literal string at the end of a %token declaration which declares the name. For example:

%token LE  "<="
%token <OPERATOR> ASSIGN 310 ":="

Once the literal string has been associated with the token name, you can use them interchangeably in the grammar rules. Note that literal strings may contain Unicode characters.

Operator Precedence

Use the %left, %right or %nonassoc declaration to declare a token and specify its precedence and associativity, all at once. These are called precedence declarations.

The syntax of a precedence declaration is the same as that of %token:

%left SYMBOLS ...

And indeed any of these declarations serves the purposes of %token. But in addition, they specify the associativity and relative precedence for all the SYMBOLS:

The associativity of an operator OP determines how repeated uses of the operator nest: whether X OP Y OP Z is parsed by grouping X with Y first or by grouping Y with Z first. %left specifies left-associativity (grouping X with Y first) and %right specifies right-associativity (grouping Y with Z first). %nonassoc specifies no associativity, which means that X OP Y OP Z is considered a syntax error.
The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity. When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first.

Nonterminal Symbols

You can declare the value type of each nonterminal symbol for which values are used (when it's different from the default detachable ANY). This is done with a %type declaration, like this:

%type <TYPE> NONTERMINAL ...

Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the name of the Eiffel type associated with this symbol. Anchored types such as:

%type <like foo> NONTERMINAL ...

where foo is a feature name, are also accepted. Any number of nonterminal symbols can be specified in the same %type declaration, if they have the same value type. Use spaces to separate the symbol names.

Suppressing Conflict Warnings

Geyacc normally warns if there are any conflicts in the grammar, but most real grammars have harmless shift/reduce conflicts which are resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress the warning about these conflicts unless the number of conflicts changes. You can do this with the %expect declaration. The declaration looks like this:

%expect N

Here N is a decimal integer. The declaration says there should be no warning if there are N shift/reduce conflicts and no reduce/reduce conflicts. The usual warning is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts. In general, using %expect involves these steps:

Compile your grammar without %expect. Use the -v option to get a verbose list of where the conflicts occur. Geyacc will also print the number of conflicts.
Check each of the conflicts to make sure that geyacc's default resolution is what you really want. If not, rewrite the grammar and go back to the beginning.
Add an %expect declaration, copying the number N from the number which geyacc printed.

Now geyacc will stop annoying you about the conflicts you have checked, but it will warn you again if changes in the grammar result in additional conflicts.

The Start-Symbol

Geyacc assumes by default that the start symbol for the grammar is the first nonterminal specified in the grammar specification section. The programmer may override this restriction with the %start declaration as follows:

%start SYMBOL

Geyacc Declaration Summary

Here is a summary of all geyacc declarations:

%token: Declare a terminal symbol (token type name) with no precedence or associativity specified.
%right: Declare a terminal symbol (token type name) that is right-associative.
%left: Declare a terminal symbol (token type name) that is left-associative.
%nonassoc: Declare a terminal symbol (token type name) that is nonassociative (using it in a way that would be associative is a syntax error).
%type: Declare the type of semantic values for a nonterminal symbol.
%start: Specify the grammar's start symbol.
%expect: Declare the expected number of shift-reduce conflicts.