AYACC(local)AYACC(local)NAMEayacc - An Ada LALR(1) parser generator
SYNOPSISayacc {command line interface parameter associations}
DESCRIPTION
Ayacc provides Ada programmers with a convenient tool for the automatic
construction of parsers from a high level description of a context free
grammar. The input to Ayacc consists of a BNF-like specification of a
grammar accompanied by a set of Ada program statements to be executed
as each production is recognized. Ayacc outputs a set of Ada program
units that act as a parser for the specified grammar; these program
units may be interfaced to additional user-supplied routines to produce
a functional program.
Ayacc will produce a procedure called yyparse and three packages:
Tokens, Goto_Table, and Shift_Reduce_Table. All of these packages must
be visible to yyparse. Package Tokens contains the enumeration type
that is returned by the lexical analyzer. Packages Goto_table and
Shift_Reduce_Table contain the parsing tables used by yyparse.
The user must supply yyparse with a lexical analyzer and an error
reporting routine. The declarations of these routines should look like
the following:
function YYLEX return TOKENS.TOKEN;
procedure YYERROR(MESSAGE: in STRING);
The format of the ayacc input file must be as follows,
declarations section
%%
rules section
%%
user declarations section
The declarations section is used to specify the generated tokens pack‐
age. A token declaration consists of the keyword %token followed by a
list of token names that may optionally be separated by commas. Token
names must follow Ada enumeration type naming conventions. Ayacc pro‐
vides a means to associate an Ada data type with a grammar symbol.
This type must be called YYSType and must be declared in the tokens
declarations section and be surrounded by '{' '}'s . e.g.
{
subtype YYSType is Integer;
}
Since types declared in this section may require visibility to addi‐
tional packages, context clauses for the tokens package may be defined
by using the keywords %with and %use. These keywords must be located
before the YYStype declaration.
The rules section defines the grammar to be parsed. Each rules con‐
sists of a nonterminal symbol followed by a colon and a list of grammar
symbols terminated by a semicolon. For example, a rule corresponding
to a street address might be written as,
Address: Street City ',' State Zip;
A vertical bar may be used to combine rules with identical left hand
sides. Nonterminal names may be made up of alphanumeric characters as
well as periods and underscores. Ada reserved words are allowed.
Unlike, yacc all tokens and nonterminal names are case insensitive.
The start symbol of the grammar may be specified using the keyword
%start followed by the symbol. If the start symbol is not specified,
ayacc will use the left hand side of the first grammar rule.
Ayacc allows each grammar rule to have associated actions which are
executed whenever the rule is recognized by the parser. An action con‐
sists of Ada statements enclosed in braces and placed after the body of
a rule. Ayacc uses a pseudo-variable notation to denote the values as‐
sociate values with nonterminal and token symbols. The left hand side
of a rule may be set to a value by an assignment to the variable, $$.
For example, if YYSType is an integer, the action:
A : B C D {$$ := 1;}
sets the value of A to 1. Values of symbols on the right hand side of
the rule, may be accessed through the variables $1..$n ,where n refers
to the nth element of the right hand side. For example.
A : B '+' C {$$ := $1 + $3;}
sets A to the sum of the values of B and C.
The user declarations section is optional. By default, ayacc generates
a parameterless procedure, YYParse. If the user desires, the procedure
may be incorporated within a package provided in this section. The
user must use the key marker, ##, to indicate where the body of YYParse
is to be inserted. The user is responsible for providing with clauses
for the tokens, parse table, and Text_IO packages.
COMMAND LINE INTERFACE
Arguments are passed to ayacc via a command line interface. This com‐
mand line interface models the syntax and semantics of Ada procedure
calls, supporting both named and positional notation as well as default
initialization.
When the ayacc command is entered without arguments, the following
specification is displayed.
-- Ayacc: An Ada Parser Generator.
type Switch is (On, Off);
procedure Ayacc (File : in String;
C_Lex : in Switch := Off;
Debug : in Switch := Off;
Summary : in Switch := On;
Verbose : in Switch := Off;
Extension : in String := ".a");
-- File Specifies the Ayacc Input Source File.
-- C_Lex Specifies the Generation of a 'C' Lex
-- Interface.
-- Debug Specifies the Production of Debugging Output
-- By the Generated Parser.
-- Summary Specifies the Printing of Statistics About the
-- Generated Parser.
-- Verbose Specifies the Production of a Human Readable
-- Report of States in the Generated Parser.
-- Extension Specifies the File Extension to be Used for
-- Generated Ada Files.
The following examples are legal invocations of ayacc
ayacc ("file.y");
ayacc ("file.y, Verbose => On, Extension => ".ada");
For friendlier usage, some Ada rules have been relaxed,
1) Final semicolon on the procedure call is optional.
2) Outermost parentheses are optional.
3) Parentheses around aggregate parameters are optional
when the aggregate consists of only one component.
4) Quotes around string literals are optional.
making the following examples legal invocations as well.
ayacc file.y
ayacc file.y Verbose => On Extenstion => .ada
Note: Unix will interpret the => and parantheses prior to passing argu‐
ments to alex. As a result, they must be escaped on the command line
to prevent interpretation. i.e. => must be typed in as =\>.
FILES
file.y the input file to Ayacc
file.ada the generated parser
file.goto.ada package Goto_Table
file.shift_reduce.ada package Shift_Reduce_Table
file.tokens.ada package Tokens
file.verbose the verbose output
file.c_lex.ada package c_lexforinterfacingwithlex
file.h the C include file for interfacing with lex
BUGS
send questions and comment to ayacc-info@ics.uci.edu
send bug reports to ayacc-bugs@ics.uci.edu
SEE ALSO
Ayacc User's Manual
yacc(1), lex(1), alex(local)
3 June 1988 AYACC(local)
