Grammar File Format (.y)

The grammar specification file has three sections separated by %%:

DECLARATIONS
%%
RULES
%%
USER CODE

Declarations Section

Prologue Code

Code in %{ and %} is copied to the output:

%{
#include <stdio.h>
#include <math.h>
extern int yylex();
void yyerror(const char *s);
%}

Token Declarations

Declare terminal symbols with %token:

%token NUMBER
%token PLUS MINUS TIMES DIVIDE
%token LPAREN RPAREN

Tokens with types:

%token <ival> NUMBER
%token <str> IDENTIFIER

Union Declaration

Define the semantic value type:

%union {
    int ival;
    double dval;
    char *str;
}

Type Declarations

Assign types to non-terminals:

%type <dval> expr term factor

Precedence Declarations

Declare operator precedence and associativity:

%left PLUS MINUS
%left TIMES DIVIDE
%right POWER
%nonassoc UMINUS

• %left: Left-associative
• %right: Right-associative
• %nonassoc: Non-associative (error on chaining)

Later declarations have higher precedence.

Start Symbol

Specify the grammar's start symbol:

%start program

If omitted, the left-hand side of the first rule is used.

Rules Section

Production rules define the grammar:

%%

program:
    statement_list
    ;

statement_list:
    statement
    | statement_list statement
    ;

statement:
    expr SEMICOLON      { printf("Result: %d\n", $1); }
    | error SEMICOLON   { yyerrok; }
    ;

expr:
    expr PLUS expr      { $$ = $1 + $3; }
    | expr MINUS expr   { $$ = $1 - $3; }
    | NUMBER            { $$ = $1; }
    ;

%%

Rule Syntax

nonterminal:
    production1     { action1 }
    | production2   { action2 }
    ;

Empty Productions

Use /* empty */ or nothing:

optional_items:
    /* empty */
    | item_list
    ;

User Code Section

Auxiliary functions copied to the output:

%%

void yyerror(const char *s) {
    fprintf(stderr, "Error: %s\n", s);
}

int main() {
    return yyparse();
}