%token <string> NUM
%token PLUS MINUS TIMES DIV
%token LPAREN RPAREN
%token EOL
%left PLUS MINUS        /* lowest precedence */
%left TIMES DIV         /* medium precedence */
%nonassoc UMINUS        /* highest precedence */
%start line             /* the entry point */
%type <string> line
%%
  line:
  expr EOL                  { $1 }
  ;
  expr:
    NUM                     { $1 }
  | LPAREN expr RPAREN      { $2 }
  | expr PLUS expr          { Printf.sprintf"(add %s %s)" $1 $3 }
  | expr MINUS expr         { Printf.sprintf"(sub %s %s)" $1 $3 }
  | expr TIMES expr         { Printf.sprintf"(mul %s %s)" $1 $3 }
  | expr DIV expr           { Printf.sprintf"(div %s %s)" $1 $3 }
  | MINUS expr %prec UMINUS { Printf.sprintf"-%s" $2 }
  ;