.SH
Section 1: Basic Specifications
.PP
As we noted above, names refer to either tokens or nonterminal symbols.
Yacc requires those names which will be
used as token names to be declared as such.
In addition, for reasons which will be discussed in Section 3, it is usually desirable
to include the lexical analyzer as part of the specification file;
it may be useful to include other programs as well.
Thus, every specification file consists of three sections:
the
.ul
declarations,
.ul
(grammar) rules,
and
.ul
programs.
The sections are separated by double percent ``%%'' marks.
(The per-cent ``%'' is generally used in Yacc specifications as an escape character.)
.PP
In other words, a full specification file looks like
.DS
declarations
%%
rules
%%
programs
.DE
.PP
The declaration section may be empty.
Moreover, if the programs section is omitted, the second %% mark may be omitted also;
thus, the smallest legal Yacc specification is
.DS
%%
rules
.DE
.PP
Blanks, tabs, and newlines are ignored except
that they may not appear in names or multi-character reserved symbols.
Comments may appear wherever a name or operator is legal; they are enclosed
in /* . . . */, as in C and PL/I.
.PP
The rules section is made up of one or more grammar rules.
A grammar rule has the form:
.DS
A  :  BODY  ;
.DE
A represents a nonterminal name, and BODY represents a sequence of zero or more names and literals.
Notice that the colon and the semicolon are Yacc punctuation.
.PP
Names may be of arbitrary length, and may be made up of letters, dot ``.'', underscore ``\_'', and
non-initial digits.
Notice that Yacc considers that upper and lower case letters are distinct.
The names used in the body of a grammar rule may represent tokens or nonterminal symbols.
.PP
A literal consists of a character enclosed in single quotes ``\'''.
As in C, the backslash ``\e'' is an escape character within literals, and all the C escapes
are recognized.
Thus
.DS
\'\en\'	represents newline
\'\er\'	represents return
\'\e\'\'	represents single quote ``\'''
\'\e\e\'	represents backslash ``\e''
\'\et\'	represents tab
\'\eb\'	represents backspace
\'\exxx\' represents ``xxx'' in octal
.DE
For a number of technical reasons, the nul character (\'\e0\' or 000) should never
be used in grammar rules.
.PP
If there are several grammar rules with the same left hand side, the vertical bar ``|''
can be used to avoid rewriting the left hand side.
In addition,
the semicolon at the end of a rule can be dropped before a vertical bar.
Thus the grammar rules
.DS
A : B C D   ;
A : E F   ;
A : G   ;
.DE
can be given to Yacc as
.DS
A :	B C D |
	E F |
	G ;
.DE
It is not necessary that all grammar rules with the same left side appear together in the grammar rules section,
although it makes the input much more readable, and easy to change.
.PP
If a nonterminal symbol matches the empty string, this can be indicated in the obvious way:
.DS
empty :   ;
.DE
.PP
As we mentioned above, names which represent
tokens must be declared as such.
The simplest way of doing this is to write
.DS
%token   name1 name2 . . .
.DE
in the declarations section.
(See Sections 3 and 4 for much more discussion).
Every name not defined in the declarations section is assumed to represent a nonterminal symbol.
If, by the end of the rules section, some nonterminal symbol has not appeared on the left
of any rule, then an error message is produced and Yacc halts.
.PP
The left hand side of the
.I
first
.R
grammar rule in the grammar rules section has special importance; it is taken to be the
controlling nonterminal symbol for the entire input process;
in technical language it is called the
.I
start symbol.
.R
In effect, the parser is designed to recognize the start symbol; thus,
this symbol generally represents the largest,
most general structure described by the grammar rules.
.PP
The end of the input is signaled by a special token, called the
.ul
endmarker.
If the tokens up to, but not including, the endmarker form a structure
which matches the start symbol, the parser subroutine returns to its caller
when the endmarker is seen; we say that it
.ul
accepts
the input.
If the endmarker is seen in any other context, it is an error.
.PP
It is the job of the user supplied lexical analyzer
to return the endmarker when appropriate; see section 3, below.
Frequently, the endmarker token represents some reasonably obvious 
I/O status, such as ``end-of-file'' or ``end-of-record''.