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

yacc: Compiler-Compiler XENIX Programming
This way of handling multiple types is very instructive, but not very general. If there
were many kinds of expression types, instead of just two, the number of rules needed
would increase dramatically, and the conflicts even more dramatically. Thus, while this
example is instructive, it is better practice in a more normal programm ing language
environment to keep the type information as part of the value, and not as part of the
grammar.
Finally, a word about the lexical analysis. The only unusual feature is the treat ment of
floating-point constants. The C library routine atof is used to do the actual conversion
from a character string to a double precision value. If the lexical analyzer detects an
error, it responds by returning a token that is illegal in the grammar, provoking a syntax
error in the parser, and thence error recovery.
10-34
I* advanced desk calculator parser specification *I
% {
# include
# include
typedef struct interval {
double lo, hi;
} INTERVAL;
INTERVAL vmul(), vd iv();
double atof();
double dreg[ 26 ];
INTERVAL vreg[ 26 ];
% }
%start lines
%union {
int ivai;
double dval;
INTERVAL vval ;
}
%token DREG VREG I* indices into dreg, vreg arrays *I
%token CONST I* floating-point constant *I
%type dexp I* expression *I
%type vexp I* interval expression *I