Quickstart 2 - A better word counter using Spirit.Lex

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#### Quickstart 2 - A better word counter using Spirit.Lex

People knowing Flex will probably complain about the example from the section Lex Quickstart 1 - A word counter using Spirit.Lex as being overly complex and not being written to leverage the possibilities provided by this tool. In particular the previous example did not directly use the lexer actions to count the lines, words and characters. So the example provided in this step of the tutorial will show how to use semantic actions in Spirit.Lex. Even if it still will allow to count text elements only it introduces other new concepts and configuration options along the lines (for the full example code see here: word_count_lexer.cpp).

##### Prerequisites

In addition to the only required `#include` specific to Spirit.Lex this example needs to include a couple of header files from the Phoenix2 library. This example shows how to attach functors to token definitions, which could be done using any type of C++ technique resulting in a callable object. Using Phoenix2 for this task simplifies things and avoids adding dependencies to other libraries (Phoenix2 is already in use for Spirit anyway).

    #include <boost/spirit/include/support_argument.hpp>
    #include <boost/spirit/include/lex_lexer_lexertl.hpp>
    #include <boost/spirit/include/phoenix_operator.hpp>
    #include <boost/spirit/include/phoenix_statement.hpp>
    #include <boost/spirit/include/phoenix_algorithm.hpp>
    #include <boost/spirit/include/phoenix_core.hpp>

To make all the code below more readable we introduce the following namespaces.

    using namespace boost::spirit;
    using namespace boost::spirit::lex;

To give a preview at what to expect from this example, here is the flex program which has been used as the starting point. The useful code is directly included inside the actions associated with each of the token definitions.

    %{
        int c = 0, w = 0, l = 0;
    %}
    %%
    [^ \t\n]+  { ++w; c += yyleng; }
    \n         { ++c; ++l; }
    .          { ++c; }
    %%
    main()
    {
        yylex();
        printf("%d %d %d\n", l, w, c);
    }

##### Semantic Actions in Spirit.Lex

Spirit.Lex uses a very similar way of associating actions with the token definitions (which should look familiar to anybody knowlegdeable with Spirit as well): specifying the operations to execute inside of a pair of `[]` brackets. In order to be able to attach semantic actions to token definitions for each of them there is defined an instance of a `token_def<>`.

    template <typename Lexer>
    struct word_count_tokens : lexer_def<Lexer>
    {
        word_count_tokens()
          : c(0), w(0), l(0),
            word("[^ \t\n]+"), eol("\n"), any(".")  // define tokens
        {}

        template <typename Self>
        void def (Self& self)
        {
            using boost::phoenix::ref;
            using boost::phoenix::distance;
            using boost::spirit::arg_names::_1;

            // associate tokens with the lexer
            self =  word  [++ref(w), ref(c) += distance(_1)]
                |   eol   [++ref(c), ++ref(l)]
                |   any   [++ref(c)]
                ;
        }

        std::size_t c, w, l;
        token_def<> word, eol, any;
    };

The semantics of the shown code is as follows. The code inside the `[]` brackets will be executed whenever the corresponding token has been matched by the lexical analyzer. This is very similar to Flex, where the action code associated with a token definition gets executed after the recognition of a matching input sequence. The code above uses function objects constructed using Phoenix2, but it is possible to insert any C++ function or function object as long as it exposes the interface:

    void f (Range r, Idtype id, bool& matched, Context& ctx);

where:

`Range r`

  This is a `boost::iterator_range` holding two iterators pointing to the matched range in the underlying input sequence. The type of the held iterators is the same as specified while defining the type of the `lexertl_lexer<...>` (its first template parameter).

`Idtype id`

  This is the token id of the type `std::size_t` for the matched token.

`bool& matched`

  This boolean value is pre/initialized to `true`. If the functor sets it to `false` the lexer stops calling any semantic actions attached to this token and behaves as if the token have not been matched in the first place.

`Context& ctx`

  This is a reference to a lexer specific, unspecified type, providing the context for the current lexer state. It can be used to access different internal data items and is needed for lexer state control from inside a semantic action.

When using a C++ function as the semantic action the following prototypes are allowed as well:

    void f (Range r, Idtype id, bool& matched);
    void f (Range r, Idtype id);
    void f (Range r);

Even if it is possible to write your own function object implementations (i.e. using Boost.Lambda or Boost.Bind), the preferred way of defining lexer semantic actions is to use Phoenix2. In this case you can access the four parameters described in the table above by using the predefined Spirit placeholders: `_1` for the iterator range, `_2` for the token id, `_3` for the reference to the boolean value signaling the outcome of the semantic action, and `_4` for the reference to the internal lexer context.

##### Associating Token Definitions with the Lexer

If you compare the with the code from Lex Quickstart 1 - A word counter using Spirit.Lex with regard to the way how token definitions are associated with the lexer, you will notice a different syntax being used here. If in the previous example we have been using the `self.add()` style of the API, then here we directly assign the token definitions to `self`, combining the different token definitions using the `|` operator. Here is the code snippet again:

    self =  word  [++ref(w), ref(c) += distance(_1)]
        |   eol   [++ref(c), ++ref(l)]
        |   any   [++ref(c)]
        ;

This way we have a very powerful and natural way of building the lexical analyzer. If translated into English this may be read as: The lexical analyer will recognize ('`=`') tokens as defined by any of ('`|`') the token definitions `word`, `eol`, and `any`.

A second difference to the previous example is that we do not explicitly specify any token ids to use for the separate tokens. Using semantic actions to trigger some useful work free'd us from the need to define these. To ensure every token gets assigned a id the Spirit.Lex library internally assigns unique numbers to the token definitions, starting with the constant defined by `boost::spirit::lex::min_token_id`.