// gcom.gr
// parser for guarded command example language

verbatim {
  #include <stdlib.h>     // getenv, atoi
}

// the parser context class is useful in large examples;
// an empty one will suffice here
context_class GCom : public UserActions {
public:
  // empty for now
};

// pull in the tokens generated from lexer.h
terminals {
  include("tokens.tok")

  // declare token sval types
  token(int) TOK_LITERAL;
  token(char*) TOK_IDENTIFIER;

  precedence {
    left  20 "*";        // high precedence, left associative
    left  10 "+" "-";    // low precedence, left associative
  }
}

// now the grammar

nonterm(int) AExp {
  -> n:TOK_LITERAL         { return n; }
  -> a1:AExp "+" a2:AExp   { return a1 + a2; }
  -> a1:AExp "-" a2:AExp   { return a1 - a2; }
  -> a1:AExp "*" a2:AExp   { return a1 * a2; }
  -> "(" a:AExp ")"        { return a; }
  
  // interpret identifiers using environment variables; it's
  // a bit of a hack, and we'll do something better later
  -> x:TOK_IDENTIFIER {
       char const *envp = getenv(x);
       if (envp) {
         return atoi(envp);
       }
       else {
         return 0;      // not defined, call it 0
       }
     }
}