[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    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)
===============================================================================/]

[section Sum - adding numbers]

Here's a parser that sums a comma-separated list of numbers.

[import ../../example/qi/sum.cpp]

Ok we've glossed over some details in our previous examples. First, our
includes:

[tutorial_adder_includes]

Then some using directives:

[tutorial_adder_using]

[table
  [[Namespace]                   [Description]]
  [[boost::phoenix]              [All of phoenix]]
  [[boost::spirit]               [All of spirit]]
  [[boost::spirit::qi]           [All of spirit.qi]]
  [[boost::spirit::ascii]        [ASCII version of `char_` and all char related parsers. Other
                                 encodings are also provided (e.g. also an ISO8859.1)]]
  [[boost::spirit::arg_names]    [Special phoenix placeholders for spirit]]
]

[note If you feel uneasy with using whole namespaces, feel free to qualify your
code, use namespace aliases, etc. For the purpose of this tutorial, we will be
presenting unqualified names for both Spirit and __phoenix__. No worries, we
will always present the full working code, so you won't get lost. In fact, all
examples in this tutorial have a corresponding cpp file that QuickBook (the
documentation tool we are using) imports in here as code snippets.]

Now the actual parser:

[tutorial_adder]

The full cpp file for this example can be found here: [@../../example/qi/sum.cpp]

This is almost like our original numbers list example. We're incrementally
building on top of our examples. This time though, like in the complex number
example, we'll be adding the smarts. There's an accumulator (`double& n`) that
adds the numbers parsed. On a successful parse, this number is the sum of all
the parsed numbers.

The first `double_` parser attaches this action:

    ref(n) = _1

This assigns the parsed result (actually, the attribute of `double_`) to `n`.
`ref(n)` tells __phoenix__ that `n` is a mutable reference. `_1` is a
__phoenix__ placeholder for the parsed result attribute.

The second `double_` parser attaches this action:

    ref(n) += _1

So, subsequent numbers add into `n`.

That wasn't too bad, was it :-) ?

[endsect]