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- // 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/config/warning_disable.hpp>
- #include <boost/spirit/include/qi.hpp>
- #include <boost/spirit/include/lex_lexertl.hpp>
- #include <boost/spirit/include/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;
- }
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