| List Parsers |

List Parsers are generated by the special predefined parser generator object `list_p`, which generates parsers recognizing list structures of the type

        item >> *(delimiter >> item) >> !end

where `item` is an expression, delimiter is a delimiter and end is an optional closing expression. As you can see, the `list_p` generated parser does not recognize empty lists, i.e. the parser must find at least one item in the input stream to return a successful match. If you wish to also match an empty list, you can make your list_p optional with operator! An example where this utility parser is helpful is parsing comma separated C/C++ strings, which can be easily formulated as:

        rule<> list_of_c_strings_rule
            =   list_p(confix_p('\"', *c_escape_char_p, '\"'), ',')
            ;

The `confix_p` and `c_escape_char_p` parser generators are described here and here.

The `list_p` parser generator object can be used to generate the following different types of List Parsers:

List Parsers
list_p | `list_p` used by itself parses comma separated lists without special item formatting, i.e. everything in between two commas is matched as an `item`, no `end` of list token is matched
list_p(delimiter) | generates a list parser, which recognizes lists with the given `delimiter` and matches everything in between them as an `item`, no `end` of list token is matched
list_p(item, delimiter) | generates a list parser, which recognizes lists with the given `delimiter` and matches items based on the given item parser, no `end` of list token is matched
list_p(item, delimiter, end) | generates a list parser, which recognizes lists with the given `delimiter` and matches items based on the given `item` parser and additionally recognizes an optional `end` expression

All of the parameters to list_p can be single characters, strings or, if more complex parsing logic is required, auxiliary parsers, each of which is automatically converted to the corresponding parser type needed for successful parsing.

If the `item` parser is an `action_parser_category` type (parser with an attached semantic action) we have to do something special. This happens, if the user wrote something like:

        list_p(item[func], delim)

where `item` is the parser matching one item of the list sequence and `func` is a functor to be called after matching one item. If we would do nothing, the resulting code would parse the sequence as follows:

        (item[func] - delim) >> *(delim >> (item[func] - delim))

what in most cases is not what the user expects. (If this is what you've expected, then please use one of the `list_p` generator functions `direct()`, which will inhibit refactoring of the `item` parser). To make the list parser behave as expected:

        (item - delim)[func] >> *(delim >> (item - delim)[func])

the actor attached to the item parser has to be re-attached to the `(item - delim)` parser construct, which will make the resulting list parser 'do the right thing'. This refactoring is done by the help of the Refactoring Parsers. Additionally special care must be taken, if the item parser is a `unary_parser_category` type parser as for instance:

        list_p(*anychar_p, ',')

which without any refactoring would result in

            (*anychar_p - ch_p(','))
        >> *( ch_p(',') >> (*anychar_p - ch_p(',')) )

and will not give the expected result (the first `*anychar_p` will eat up all the input up to the end of the input stream). So we have to refactor this into:

           *(anychar_p - ch_p(','))
        >> *( ch_p(',') >> *(anychar_p - ch_p(',')) )

what will give the correct result.

The case, where the item parser is a combination of the two mentioned problems (i.e. the item parser is a unary parser with an attached action), is handled accordingly too:

        list_p((*anychar_p)[func], ',')

will be parsed as expected:

            (*(anychar_p - ch_p(',')))[func]
        >> *( ch_p(',') >> (*(anychar_p - ch_p(',')))[func] )

The required refactoring is implemented with the help of the Refactoring Parsers.

Summary of List Parser refactorings
You write it as: | `It is refactored to:`
`list_p(item, delimiter)` | `(item - delimiter) >> *(delimiter >> (item - delimiter))`
`list_p(item[func], delimiter)` | `(item - delimiter)[func] >> *(delimiter >> (item - delimiter)[func])`
`list_p(*item, delimiter)` | `*(item - delimiter) >> *(delimiter >> *(item - delimiter))`
`list_p((*item)[func], delimiter)` | `(*(item - delimiter))[func] >> *(delimiter >> (*(item - delimiter))[func])`

Copyright © 2001-2002 Hartmut Kaiser
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