The Select Parser

Select parsers may be used to identify a single parser from a given list of parsers, which successfully recognizes the current input sequence. Example:

    rule<> rule_select = 
          , parser_b
            /* ... */
          , parser_n

The parsers (parser_a, parser_b etc.) are tried sequentially from left to right until a parser matches the current input sequence. If there is a matching parser found, the select_p parser returns the parser's position (zero based index). For instance, in the example above, 1 is returned if parser_b matches.

There are two predefined parsers of the select parser family: select_p and select_fail_p. These parsers differ in the way the no match case is handled (when none of the parsers match the current input sequence). While the select_p parser will return -1 if no matching parser is found, the select_fail_p parser will not match at all.

The following sample shows how the select parser may be used very conveniently in conjunction with a switch parser:

    int choice = -1;
    rule<> rule_select = 
            select_fail_p('a', 'b', 'c', 'd')[assign_a(choice)]
        >>  switch_p(var(choice)) 

This example shows a rule, which matches:

For other input sequences the give rule does not match at all.

The number of possible entries inside the select_p parser is limited by the Spirit compile time constant BOOST_SPIRIT_SELECT_LIMIT, which defaults to 3. This value should not be greater than the compile time constant given by PHOENIX_LIMIT (see phoenix).