x3/tutorial/warming_up.qbk

[/==============================================================================
    Copyright (C) 2001-2015 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 Warming up]

We'll start by showing examples of parser expressions to give you a feel on how
to build parsers from the simplest parser, building up as we go. When comparing
EBNF to __spirit__, the expressions may seem awkward at first. __spirit__ heavily
uses operator overloading to accomplish its magic.

[heading Trivial Example #1 Parsing a number]

Create a parser that will parse a floating-point number.

    double_

(You've got to admit, that's trivial!) The above code actually generates a
Spirit floating point parser (a built-in parser). Spirit has many pre-defined
parsers and consistent naming conventions help you keep from going insane!

[heading Trivial Example #2 Parsing two numbers]

Create a parser that will accept a line consisting of two floating-point numbers.

    double_ >> double_

Here you see the familiar floating-point numeric parser `double_` used twice,
once for each number. What's that `>>` operator doing in there? Well, they had
to be separated by something, and this was chosen as the "followed by" sequence
operator. The above program creates a parser from two simpler parsers, glueing
them together with the sequence operator. The result is a parser that is a
composition of smaller parsers. Whitespace between numbers can implicitly be
consumed depending on how the parser is invoked (see below).

[note When we combine parsers, we end up with a "bigger" parser, but
  it's still a parser. Parsers can get bigger and bigger, nesting more and more,
  but whenever you glue two parsers together, you end up with one bigger parser.
  This is an important concept.
]

[heading Trivial Example #3 Parsing zero or more numbers]

Create a parser that will accept zero or more floating-point numbers.

    *double_

This is like a regular-expression Kleene Star, though the syntax might look a
bit odd for a C++ programmer not used to seeing the `*` operator overloaded like
this. Actually, if you know regular expressions it may look odd too since the
star is before the expression it modifies. C'est la vie. Blame it on the fact
that we must work with the syntax rules of C++.

Any expression that evaluates to a parser may be used with the Kleene Star.
Keep in mind that C++ operator precedence rules may require you to put
expressions in parentheses for complex expressions. The Kleene Star
is also known as a Kleene Closure, but we call it the Star in most places.

[heading Trivial Example #4 Parsing a comma-delimited list of numbers]

This example will create a parser that accepts a comma-delimited list of
numbers.

    double_ >> *(char_(',') >> double_)

Notice `char_(',')`. It is a literal character parser that can recognize the
comma `','`. In this case, the Kleene Star is modifying a more complex parser,
namely, the one generated by the expression:

    (char_(',') >> double_)

Note that this is a case where the parentheses are necessary. The Kleene star
encloses the complete expression above.

[heading Let's Parse!]

We're done with defining the parser. So the next step is now invoking this
parser to do its work. There are a couple of ways to do this. For now, we will
use the `phrase_parse` function. One overload of this function accepts four
arguments:

# An iterator pointing to the start of the input
# An iterator pointing to one past the end of the input
# The parser object
# Another parser called the skip parser

In our example, we wish to skip spaces and tabs. Another parser named `space`
is included in Spirit's repertoire of predefined parsers. It is a very simple
parser that simply recognizes whitespace. We will use `space` as our skip
parser. The skip parser is the one responsible for skipping characters in
between parser elements such as the `double_` and `char_`.

Ok, so now let's parse!

    template <typename Iterator>
    bool parse_numbers(Iterator first, Iterator last)
    {
        using x3::double_;
        using x3::phrase_parse;
        using ascii::space;

        bool r = phrase_parse(
            first,                          //  Start Iterator
            last,                           //  End Iterator
            double_ >> *(',' >> double_),   //  The Parser
            space                           //  The Skip-Parser
        );
        if (first != last) // fail if we did not get a full match
            return false;
        return r;
    }
The parse function returns `true` or `false` depending on the result of
the parse. The first iterator is passed by reference. On a successful
parse, this iterator is repositioned to the rightmost position consumed
by the parser. If this becomes equal to `last`, then we have a full
match. If not, then we have a partial match. A partial match happens
when the parser is only able to parse a portion of the input.

Note that we inlined the parser directly in the call to parse. Upon calling
parse, the expression evaluates into a temporary, unnamed parser which is passed
into the parse() function, used, and then destroyed.

Here, we opted to make the parser generic by making it a template, parameterized
by the iterator type. By doing so, it can take in data coming from any STL
conforming sequence as long as the iterators conform to a forward iterator.

You can find the full cpp file here:
[@../../../example/x3/num_list/num_list1.cpp num_list1.cpp]

[note `char` and `wchar_t` operands

The careful reader may notice that the parser expression has `','` instead of
`char_(',')` as the previous examples did. This is ok due to C++ syntax rules of
conversion. There are `>>` operators that are overloaded to accept a `char` or
`wchar_t` argument on its left or right (but not both). An operator may be
overloaded if at least one of its parameters is a user-defined type. In this
case, the `double_` is the 2nd argument to `operator>>`, and so the proper
overload of `>>` is used, converting `','` into a character literal parser.

The problem with omitting the `char_` should be obvious: `'a' >> 'b'` is not a
spirit parser, it is a numeric expression, right-shifting the ASCII (or another
encoding) value of `'a'` by the ASCII value of `'b'`. However, both
`char_('a') >> 'b'` and `'a' >> char_('b')` are Spirit sequence parsers
for the letter `'a'` followed by `'b'`. You'll get used to it, sooner or later.
]

Finally, take note that we test for a full match (i.e. the parser fully parsed
the input) by checking if the first iterator, after parsing, is equal to the end
iterator. You may strike out this part if partial matches are to be allowed.

[endsect] [/ Warming up]