doc/qi/mini_xml.qbk

[/==============================================================================
    Copyright (C) 2001-2011 Joel de Guzman
    Copyright (C) 2001-2011 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)
===============================================================================/]

[section Mini XML - ASTs!]

Stop and think about it... We've come very close to generating an AST
(abstract syntax tree) in our last example. We parsed a single structure and
generated an in-memory representation of it in the form of a struct: the
`struct employee`. If we changed the implementation to parse one or more
employees, the result would be a `std::vector<employee>`. We can go on and add
more hierarchy: teams, departments, corporations. Then we'll have an AST
representation of it all.

In this example (actually two examples), we'll now explore how to create
ASTs. We will parse a minimalistic XML-like language and compile the results
into our data structures in the form of a tree.

Along the way, we'll see new features:

* Inherited attributes
* Variant attributes
* Local Variables
* Not Predicate
* Lazy Lit

The full cpp files for these examples can be found here:
[@../../example/qi/mini_xml1.cpp] and here: [@../../example/qi/mini_xml2.cpp]

There are a couple of sample toy-xml files in the mini_xml_samples subdirectory:
[@../../example/qi/mini_xml_samples/1.toyxml],
[@../../example/qi/mini_xml_samples/2.toyxml], and
[@../../example/qi/mini_xml_samples/3.toyxml] for testing purposes.
The example [@../../example/qi/mini_xml_samples/4.toyxml] has an error in it.

[import ../../example/qi/mini_xml1.cpp]
[import ../../example/qi/mini_xml2.cpp]

[heading First Cut]

Without further delay, here's the first version of the XML grammar:

[tutorial_xml1_grammar]

Going bottom up, let's examine the `text` rule:

    rule<Iterator, std::string(), space_type> text;

and its definition:

    text = lexeme[+(char_ - '<')        [_val += _1]];

The semantic action collects the chars and appends them (via +=) to the
`std::string` attribute of the rule (represented by the placeholder `_val`).

[heading Alternates]

    rule<Iterator, mini_xml_node(), space_type> node;

and its definition:

    node = (xml | text)                 [_val = _1];

We'll see a `mini_xml_node` structure later. Looking at the rule
definition, we see some alternation going on here. An xml `node` is
either an `xml` OR `text`. Hmmm... hold on to that thought...

    rule<Iterator, std::string(), space_type> start_tag;

Again, with an attribute of `std::string`. Then, it's definition:

    start_tag =
            '<'
        >>  !char_('/')
        >>  lexeme[+(char_ - '>')       [_val += _1]]
        >>  '>'
    ;

[heading Not Predicate]

`start_tag` is similar to the `text` rule apart from the added `'<'` and `'>'`.
But wait, to make sure that the `start_tag` does not parse `end_tag`s too, we
add: `!char_('/')`. This is a "Not Predicate":

    !p

It will try the parser, `p`. If it is successful, fail; otherwise, pass. In
other words, it negates the result of `p`. Like the `eps`, it does not consume
any input though. It will always rewind the iterator position to where it
was upon entry. So, the expression:

    !char_('/')

basically says: we should not have a `'/'` at this point.

[heading Inherited Attribute]

The `end_tag`:

    rule<Iterator, void(std::string), space_type> end_tag;

Ohh! Now we see an inherited attribute there: `std::string`. The `end_tag` does
not have a synthesized attribute. Let's see its definition:

    end_tag =
            "</"
        >>  lit(_r1)
        >>  '>'
    ;

`_r1` is yet another __phoenix__ placeholder for the first inherited attribute
(we have only one, use `_r2`, `_r3`, etc. if you have more).

[heading A Lazy Lit]

Check out how we used `lit` here, this time, not with a literal string, but with
the value of the first inherited attribute, which is specified as `std::string` in
our rule declaration.

Finally, our `xml` rule:

    rule<Iterator, mini_xml(), space_type> xml;

`mini_xml` is our attribute here. We'll see later what it is. Let's see its
definition:

    xml =
            start_tag                   [at_c<0>(_val) = _1]
        >>  *node                       [push_back(at_c<1>(_val), _1)]
        >>  end_tag(at_c<0>(_val))
    ;

Those who know __fusion__ now will notice `at_c<0>` and `at_c<1>`. This gives us
a hint that `mini_xml` is a sort of a tuple - a fusion sequence. `at_c<N>` here
is a lazy version of the tuple accessors, provided by __phoenix__.

[heading How it all works]

So, what's happening?

# Upon parsing `start_tag`, the parsed start-tag string is placed in
  `at_c<0>(_val)`.

# Then we parse zero or more `node`s. At each step, we `push_back` the result
  into `at_c<1>(_val)`.

# Finally, we parse the `end_tag` giving it an inherited attribute: `at_c<0>(_val)`.
  This is the string we obtained from the `start_tag`. Investigate `end_tag` above.
  It will fail to parse if it gets something different from what we got from the
  `start_tag`. This ensures that our tags are balanced.

To give the last item some more light, what happens is this:

    end_tag(at_c<0>(_val))

calls:

    end_tag =
            "</"
        >>  lit(_r1)
        >>  '>'
    ;

passing in `at_c<0>(_val)`, the string from start tag. This is referred to in the
`end_tag` body as `_r1`.

[heading The Structures]

Let's see our structures. It will definitely be hierarchical: xml is
hierarchical. It will also be recursive: xml is recursive.

[tutorial_xml1_structures]

[heading Of Alternates and Variants]

So that's what a `mini_xml_node` looks like. We had a hint that it is either
a `string` or a `mini_xml`. For this, we use __boost_variant__. `boost::recursive_wrapper`
wraps `mini_xml`, making it a recursive data structure.

Yep, you got that right: the attribute of an alternate:

    a | b

is a

  boost::variant<A, B>

where `A` is the attribute of `a` and `B` is the attribute of `b`.

[heading Adapting structs again]

`mini_xml` is no brainier. It is a plain ol' struct. But as we've seen in our
employee example, we can adapt that to be a __fusion__ sequence:

[tutorial_xml1_adapt_structures]

[heading One More Take]

Here's another version. The AST structure remains the same, but this time,
you'll see that we make use of auto-rules making the grammar
semantic-action-less. Here it is:

[tutorial_xml2_grammar]

This one shouldn't be any more difficult to understand after going through the
first xml parser example. The rules are almost the same, except that, we got rid
of semantic actions and used auto-rules (see the employee example if you missed
that). There is some new stuff though. It's all in the `xml` rule:

[heading Local Variables]

    rule<Iterator, mini_xml(), locals<std::string>, space_type> xml;

Wow, we have four template parameters now. What's that `locals` guy doing there?
Well, it declares that the rule `xml` will have one local variable: a `string`.
Let's see how this is used in action:

    xml %=
            start_tag[_a = _1]
        >>  *node
        >>  end_tag(_a)
    ;

# Upon parsing `start_tag`, the parsed start-tag string is placed in
  the local variable specified by (yet another) __phoenix__ placeholder:
  `_a`. We have only one local variable. If we had more, these are designated
  by `_b`..`_z`.

# Then we parse zero or more `node`s.

# Finally, we parse the `end_tag` giving it an inherited attribute: `_a`, our
  local variable.

There are no actions involved in stuffing data into our `xml` attribute. It's
all taken care of thanks to the auto-rule.

[endsect]