libs/spirit/example/lex/example4.cpp
// Copyright (c) 2001-2010 Hartmut Kaiser
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
// This example shows how to create a simple lexer recognizing a couple of
// different tokens aimed at a simple language and how to use this lexer with
// a grammar. It shows how to associate attributes to tokens and how to access
// the token attributes from inside the grammar.
//
// We use explicit token attribute types, making the corresponding token instances
// carry convert the matched input into an instance of that type. The token
// attribute is exposed as the parser attribute if this token is used as a
// parser component somewhere in a grammar.
//
// Additionally, this example demonstrates, how to define a token set usable
// as the skip parser during parsing, allowing to define several tokens to be
// ignored.
//
// This example recognizes a very simple programming language having
// assignment statements and if and while control structures. Look at the file
// example4.input for an example.
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/lex_lexertl.hpp>
#include <boost/phoenix/operator.hpp>
#include <iostream>
#include <fstream>
#include <string>
#include "example.hpp"
using namespace boost::spirit;
using boost::phoenix::val;
///////////////////////////////////////////////////////////////////////////////
// Token definition
///////////////////////////////////////////////////////////////////////////////
template <typename Lexer>
struct example4_tokens : lex::lexer<Lexer>
{
example4_tokens()
{
// define the tokens to match
identifier = "[a-zA-Z_][a-zA-Z0-9_]*";
constant = "[0-9]+";
if_ = "if";
else_ = "else";
while_ = "while";
// associate the tokens and the token set with the lexer
this->self = lex::token_def<>('(') | ')' | '{' | '}' | '=' | ';' | constant;
this->self += if_ | else_ | while_ | identifier;
// define the whitespace to ignore (spaces, tabs, newlines and C-style
// comments)
this->self("WS")
= lex::token_def<>("[ \\t\\n]+")
| "\\/\\*[^*]*\\*+([^/*][^*]*\\*+)*\\/"
;
}
//[example4_token_def
// these tokens expose the iterator_range of the matched input sequence
lex::token_def<> if_, else_, while_;
// The following two tokens have an associated attribute type, 'identifier'
// carries a string (the identifier name) and 'constant' carries the
// matched integer value.
//
// Note: any token attribute type explicitly specified in a token_def<>
// declaration needs to be listed during token type definition as
// well (see the typedef for the token_type below).
//
// The conversion of the matched input to an instance of this type occurs
// once (on first access), which makes token attributes as efficient as
// possible. Moreover, token instances are constructed once by the lexer
// library. From this point on tokens are passed by reference only,
// avoiding them being copied around.
lex::token_def<std::string> identifier;
lex::token_def<unsigned int> constant;
//]
};
///////////////////////////////////////////////////////////////////////////////
// Grammar definition
///////////////////////////////////////////////////////////////////////////////
template <typename Iterator, typename Lexer>
struct example4_grammar
: qi::grammar<Iterator, qi::in_state_skipper<Lexer> >
{
template <typename TokenDef>
example4_grammar(TokenDef const& tok)
: example4_grammar::base_type(program)
{
using boost::spirit::_val;
program
= +block
;
block
= '{' >> *statement >> '}'
;
statement
= assignment
| if_stmt
| while_stmt
;
assignment
= (tok.identifier >> '=' >> expression >> ';')
[
std::cout << val("assignment statement to: ") << _1 << "\n"
]
;
if_stmt
= ( tok.if_ >> '(' >> expression >> ')' >> block
>> -(tok.else_ >> block)
)
[
std::cout << val("if expression: ") << _2 << "\n"
]
;
while_stmt
= (tok.while_ >> '(' >> expression >> ')' >> block)
[
std::cout << val("while expression: ") << _2 << "\n"
]
;
// since expression has a variant return type accommodating for
// std::string and unsigned integer, both possible values may be
// returned to the calling rule
expression
= tok.identifier [ _val = _1 ]
| tok.constant [ _val = _1 ]
;
}
typedef boost::variant<unsigned int, std::string> expression_type;
qi::rule<Iterator, qi::in_state_skipper<Lexer> > program, block, statement;
qi::rule<Iterator, qi::in_state_skipper<Lexer> > assignment, if_stmt;
qi::rule<Iterator, qi::in_state_skipper<Lexer> > while_stmt;
// the expression is the only rule having a return value
qi::rule<Iterator, expression_type(), qi::in_state_skipper<Lexer> > expression;
};
///////////////////////////////////////////////////////////////////////////////
int main()
{
// iterator type used to expose the underlying input stream
typedef std::string::iterator base_iterator_type;
//[example4_token
// This is the lexer token type to use. The second template parameter lists
// all attribute types used for token_def's during token definition (see
// calculator_tokens<> above). Here we use the predefined lexertl token
// type, but any compatible token type may be used instead.
//
// If you don't list any token attribute types in the following declaration
// (or just use the default token type: lexertl_token<base_iterator_type>)
// it will compile and work just fine, just a bit less efficient. This is
// because the token attribute will be generated from the matched input
// sequence every time it is requested. But as soon as you specify at
// least one token attribute type you'll have to list all attribute types
// used for token_def<> declarations in the token definition class above,
// otherwise compilation errors will occur.
typedef lex::lexertl::token<
base_iterator_type, boost::mpl::vector<unsigned int, std::string>
> token_type;
//]
// Here we use the lexertl based lexer engine.
typedef lex::lexertl::lexer<token_type> lexer_type;
// This is the token definition type (derived from the given lexer type).
typedef example4_tokens<lexer_type> example4_tokens;
// this is the iterator type exposed by the lexer
typedef example4_tokens::iterator_type iterator_type;
// this is the type of the grammar to parse
typedef example4_grammar<iterator_type, example4_tokens::lexer_def> example4_grammar;
// now we use the types defined above to create the lexer and grammar
// object instances needed to invoke the parsing process
example4_tokens tokens; // Our lexer
example4_grammar calc(tokens); // Our parser
std::string str (read_from_file("example4.input"));
// At this point we generate the iterator pair used to expose the
// tokenized input stream.
std::string::iterator it = str.begin();
iterator_type iter = tokens.begin(it, str.end());
iterator_type end = tokens.end();
// Parsing is done based on the token stream, not the character
// stream read from the input.
// Note how we use the lexer defined above as the skip parser. It must
// be explicitly wrapped inside a state directive, switching the lexer
// state for the duration of skipping whitespace.
bool r = qi::phrase_parse(iter, end, calc, qi::in_state("WS")[tokens.self]);
if (r && iter == end)
{
std::cout << "-------------------------\n";
std::cout << "Parsing succeeded\n";
std::cout << "-------------------------\n";
}
else
{
std::cout << "-------------------------\n";
std::cout << "Parsing failed\n";
std::cout << "-------------------------\n";
}
std::cout << "Bye... :-) \n\n";
return 0;
}