Sequential Or Parser (a || b)

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##### Sequential Or Parser (`a || b`)

###### Description

The sequential-or operator, `a || b`, matches `a` or `b` or `a` followed by `b`. That is, if both `a` and `b` match, it must be in sequence; this is equivalent to `a >> -b | b`:

    a || b || ...

###### Header

    // forwards to <boost/spirit/home/qi/operator/sequential_or.hpp>
    #include <boost/spirit/include/qi_sequential_or.hpp>

Also, see Include Structure.

###### Model of

> `NaryParser`

Notation

`a`, `b`

  A `Parser`

###### Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in `NaryParser`.

Expression | Semantics
--- | ---
`a || b` | Match `a` or `b` in sequence. equivalent to `a >> -b | b`

###### Attributes

See Compound Attribute Notation.

Expression | Attribute
--- | ---

`a || b`

    a: A, b: B --> (a || b): tuple<optional<A>, optional<B> >
    a: A, b: Unused --> (a || b): optional<A>
    a: Unused, b: B --> (a || b): optional<B>
    a: Unused, b: Unused --> (a || b): Unused

    a: A, b: A --> (a || b): vector<optional<A> >

| Note

The sequential-or parser behaves attribute-wise very similar to the plain sequence parser (`a >> b`) in the sense that it exposes the attributes of its elements separately. For instance, if you attach a semantic action to the whole sequential-or:

    (int_ || int_)[print_pair(_1, _2)]

the function object `print_pair` would be invoked with the attribute of the first `int_` (`boost::optional<int>`) as its first parameter and the attribute of the second `int_` (`boost::optional<int>` as well) as its second parameter.

###### Complexity

> The overall complexity of the sequential-or parser is defined by the sum of the complexities of its elements. The complexity of the sequential-or itself is O(N), where N is the number of elements in the sequence.

###### Example

| Note
The test harness for the example(s) below is presented in the Basics Examples section.

Some using declarations:

    using boost::spirit::qi::int_;

Correctly parsing a number with optional fractional digits:

    test_parser("123.456", int_ || ('.' >> int_));  // full
    test_parser("123", int_ || ('.' >> int_));      // just the whole number
    test_parser(".456", int_ || ('.' >> int_));     // just the fraction

A naive but incorrect solution would try to do this using optionals (e.g.):

    int_ >> -('.' >> int_)  // will not match ".456"
    -int_ >> ('.' >> int_)  // will not match "123"
    -int_ >> -('.' >> int_) // will match empty strings! Ooops.