Parametric Parsers

| Parametric Parsers |

We already have a hint of the dynamic nature of the Spirit framework. This capability is fundamental to Spirit. Dynamic parsing is a very powerful concept. We shall take this concept further through run-time parametric parsers. We are able to handle parsing tasks that are impossible to do with any EBNF syntax alone.

## A Little Secret

A little critter called `boost::ref` lurking in the boost distribution is quite powerful beast when used with Spirit's primitive parsers. We are used to seeing the Spirit primitive parsers created with string or character literals such as:

        ch_p('A')
        range_p('A', 'Z')
        str_p("Hello World")

str_p has a second form that accepts two iterators over the string:

        char const* first = "My oh my";
        char const* last = first + std::strlen(first);

        str_p(first, last)

What is not obvious is that we can use `boost::ref` as well:

        char ch = 'A';
        char from = 'A';
        char to = 'Z';

        ch_p(boost::ref(ch))
        range_p(boost::ref(from), boost::ref(to))

When `boost::ref` is used, the actual parameters to `ch_p` and `range_p` are held by reference. This means that we can change the values of `ch`, `from` and `to` anytime and the corresponding `ch_p` and `range_p` parser will follow their dynamic values. Of course, since they are held by reference, you must make sure that the referenced object is not destructed while parsing.

What about `str_p`?

While the first form of `str_p` (the single argument form) is reserved for null terminated string constants, the second form (the two argument first/last iterator form) may be used:

        char const* first = "My oh my";
        char const* last = first + std::strlen(first);

        str_p(boost::ref(first), boost::ref(last))

Hey, don't forget `chseq_p`. All these apply to this seldom used primitive as well.

## Functional Parametric Primitives

        #include <boost/spirit/attribute/parametric.hpp>

Taking this further, Spirit includes functional versions of the primitives. Rather than taking in characters, strings or references to characters and strings (using boost::ref), the functional versions take in functions or functors.

### f_chlit and f_ch_p

The functional version of `chlit`. This parser takes in a function or functor (function object). The function is expected to have an interface compatible with:

        CharT func()

where CharT is the character type (e.g. `char`, `int`, `wchar_t`).

The functor is expected to have an interface compatible with:

        struct functor
        {
            CharT operator()() const;
        };

where CharT is the character type (e.g. `char`, `int`, `wchar_t`).

Here's a contrived example:

        struct X
        {
            char operator()() const
            {
                return 'X';
            }
        };

Now we can use X to create our f_chlit parser:

        f_ch_p(X())

### f_range and f_range_p

The functional version of `range`. This parser takes in a function or functor compatible with the interfaces above. The difference is that `f_range` (and `f_range_p`) expects two functors. One for the start and one for the end of the range.

### f_chseq and f_chseq_p

The functional version of `chseq`. This parser takes in two functions or functors. One for the begin iterator and one for the end iterator. The function is expected to have an interface compatible with:

        IteratorT func()

where `IteratorT` is the iterator type (e.g. `char const*`, `wchar_t const*`).

The functor is expected to have an interface compatible with:

        struct functor
        {
            IteratorT operator()() const;
        };

where `IteratorT` is the iterator type (e.g. `char const*`, `wchar_t const*`).

### f_strlit and f_str_p

The functional version of `strlit`. This parser takes in two functions or functors compatible with the interfaces that `f_chseq` expects.

Copyright © 1998-2003 Joel de Guzman
Use, modification and distribution is subject to 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)