Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming yacc: Compiler-Compiler
A sample specification is given in a later section. The facilities in this subsection are
not triggered until they are used: in particular, the use of 96type will turn on these
mechanisms. When they are used, there is a fairly strict level of checking. For
example, use of $ or $$ to refer to something with no defined type is diagnosed. If these
facilities are not triggered, the yacc value stack is used to hold int's, as was true
historically.
A Small Desk Calcu lator
This example gives the complete yacc specification for a small desk calculator: the
desk calculator has 26 registers, labeled a through z, and accepts arithmetic expressions
made up of the operators +, -, *, /, % (mod operator), & (bitwise and), I (bitwise or), and
assignment. If an expression at the top level is an assignment, the value is not printed;
otherwise it is. As in C, an integer that begins with 0 (zero) is assumed to be octal;
otherwise, it is assumed to be decimal.
As an example of a yacc specification, the desk calculator does a reasonable job of
showing how precedences and ambiguities are used and demonstrating simple error
recovery. The major oversimplifications are that the lexical analysis phase is much
simpler than for most applications, and the output is produced immediately, line by line.
Note the way that decimal and octal integers are read in by the grammar rules; this job
is probably better done by the lexical analyzer.
I* desk calculator parser specification *I
%{
# include
# include
int regs[26];
int base;
% }
%start list
%token DIGIT LETTER
%left 'I'
%left '&'
%left I + I I-I
%left'*' 'I' '%'
%left UMINUS I* precedence for unary minus *I
%% I* beginning of rules section *I
list : I* empty *I
I list stat '\n'
I list error '\n'
{ yyerrok; }
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