This example implements a simple calculator. The calculator has an unspecified number of integer variables and the common binary integer operators (namely, addition, subtraction, multiplication, division, and modulus), and unary integer operators (+ and -).
The language will contain the following concepts (tokens): VAR (a variable: the corresponding s attribute will contain its name); INT (an integer: the corresponding i attribute contains its value); and the operators (see below).
The lexical analyzer (vars.l) is very simple and limited to recognizing variable names, integers numbers, and the operators themselves. <text> %option noyywrap %{
%} %% [_[:alpha:]][_[:alnum:]]* yylval.s = new std::string(yytext); return VAR; digit:+ yylval.i = strtol(yytext, NULL, 10); return INT; [-+*/%=^:,] return *yytext; .|\n ; /* ignore all the rest */ %% </text>
The syntactic analyzer will be built to immediately compute the expressions in a syntax-directed fashion as they occur. It is, thus, important to use trees that build nodes as the expressions occur (left-recursive grammars). If the grammar were right-recursive, the last node would be the first to be built and the (syntax-directed) evaluation would be from the last expression to the first.
The first task is to define the grammar itself (without worrying with semantics). <text> </text>