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In addition to standard headers, the Flex scanner includes the driver’s, then the parser’s to get the set of defined tokens.

%{ /* -*- C++ -*- */
# include <cerrno>
# include <climits>
# include <cstdlib>
# include <cstring> // strerror
# include <string>
# include "driver.hh"
# include "parser.hh"

Since our calculator has no #include-like feature, we don’t need yywrap. We don’t need the unput and input functions either, and we parse an actual file, this is not an interactive session with the user. Finally, we enable scanner tracing.

%option noyywrap nounput noinput batch debug

The following function will be handy to convert a string denoting a number into a NUMBER token.

  // A number symbol corresponding to the value in S.
  make_NUMBER (const std::string &s, const yy::parser::location_type& loc);

Abbreviations allow for more readable rules.

id    [a-zA-Z][a-zA-Z_0-9]*
int   [0-9]+
blank [ \t\r]

The following paragraph suffices to track locations accurately. Each time yylex is invoked, the begin position is moved onto the end position. Then when a pattern is matched, its width is added to the end column. When matching ends of lines, the end cursor is adjusted, and each time blanks are matched, the begin cursor is moved onto the end cursor to effectively ignore the blanks preceding tokens. Comments would be treated equally.

  // Code run each time a pattern is matched.
  # define YY_USER_ACTION  loc.columns (yyleng);
  // A handy shortcut to the location held by the driver.
  yy::location& loc = drv.location;
  // Code run each time yylex is called.
  loc.step ();
{blank}+   loc.step ();
\n+        loc.lines (yyleng); loc.step ();

The rules are simple. The driver is used to report errors.

"-"        return yy::parser::make_MINUS  (loc);
"+"        return yy::parser::make_PLUS   (loc);
"*"        return yy::parser::make_STAR   (loc);
"/"        return yy::parser::make_SLASH  (loc);
"("        return yy::parser::make_LPAREN (loc);
")"        return yy::parser::make_RPAREN (loc);
":="       return yy::parser::make_ASSIGN (loc);

{int}      return make_NUMBER (yytext, loc);
{id}       return yy::parser::make_IDENTIFIER (yytext, loc);
.          {
             throw yy::parser::syntax_error
               (loc, "invalid character: " + std::string(yytext));
<<EOF>>    return yy::parser::make_YYEOF (loc);

You should keep your rules simple, both in the parser and in the scanner. Throwing from the auxiliary functions is then very handy to report errors.

make_NUMBER (const std::string &s, const yy::parser::location_type& loc)
  errno = 0;
  long n = strtol (s.c_str(), NULL, 10);
  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
    throw yy::parser::syntax_error (loc, "integer is out of range: " + s);
  return yy::parser::make_NUMBER ((int) n, loc);

Finally, because the scanner-related driver’s member-functions depend on the scanner’s data, it is simpler to implement them in this file.

driver::scan_begin ()
  yy_flex_debug = trace_scanning;
  if (file.empty () || file == "-")
    yyin = stdin;
  else if (!(yyin = fopen (file.c_str (), "r")))
      std::cerr << "cannot open " << file << ": " << strerror (errno) << '\n';
      exit (EXIT_FAILURE);

driver::scan_end ()
  fclose (yyin);

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