| Confix Parsers |

Confix Parsers

Confix Parsers recognize a (possibly nested) sequence out of three independent elements: an opening, an expression and a closing. This utility parser may be used to parse structures where the opening is possibly contained in the expression and the whole sequence is only matched after seeing the closing part that matches the first opening subsequence. A simple example is a nested PASCAL comment:

        { This is a { nested } PASCAL-comment }

which could be parsed through the following rule definition:``

        rule<> pascal_comment_rule
            =   confix_nest_p('{', *anychar_p, '}')
            ;

If a non-nested Confix Parser is needed, the `confix_p` parser generator should be used for generating the required Confix Parser. If the expression part may contain a valid confix sequence (the confix sequence is allowed to be nested), the `confix_nest_p` parser generator template should be used to generate the parser. The three parameters to `confix_p` and `confix_nest_p` can be single characters (as above), 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.

The generated parser is equivalent to the following rule:

        open >> (expr - close) >> close

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

        confix_p(open, expr[func], close)

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

        open >> (expr[func] - close) >> close

which in most cases is not what the user expects. (If this is what you've expected, then please use the `confix_p` or `confix_nest_p` generator function `direct()`, which will inhibit the parser refactoring). To make the confix parser behave as expected:

        open >> (expr - close)[func] >> close

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

        confix_p(open, *anychar_p, close)

which without any refactoring would result in

        open >> (*anychar_p - close) >> close

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

        open >> *(anychar_p - close) >> close

what will give the correct result.

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

        confix_p(start, (*anychar_p)[func], close)

will be parsed as expected:

        open >> (*(anychar_p - end))[func] >> close

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

Summary of Confix Parser refactorings
You write it as: | `It is refactored to:`
`confix_p(open, expr, close)` | `start >> (expr - close) >> close`
`confix_p(open, expr[func], close)` | `start >> (expr - close)[func] >> close`
`confix_p(open, *expr, close)` | `start >> *(expr - close) >> close`
`confix_p(open, (*expr)[func], close)` | `start >> (*(expr - close))[func] >> close`

Comment Parsers

The Comment Parser generator templates `comment_p` and `comment_nest_p` are helpers for generating a correct Confix Parser from auxiliary parameters, which is able to parse comment constructs as follows:

        StartCommentToken >> Comment text >> EndCommentToken

There are the following types supported as parameters: parsers, single characters and strings (see as_parser). There are two diffenerent predefined comment parser generators: `comment_p` and `comment_nest_p`, which may be used for creating special comment parsers in two different ways. If these are used with one parameter, a comment starting with the given first parser parameter up to the end of the line is matched. So for instance the following parser matches C++ style comments:

        comment_p("//")

If these are used with two parameters, a comment starting with the first parser parameter up to the second parser parameter is matched. For instance a C style comment parser could be constrcuted as:

        comment_p("/*", "*/")

The `comment_p` parser generator allows to generate parsers for matching non-nested comments (as for C/C++ comments). Sometimes it is necessary to parse nested comments as for instance allowed in Pascal. Such nested comments are parseable through parsers generated by the `comment_nest_p` generator template functor. The following example shows a parser, which can be used for parsing the two different (nestable) Pascal comment styles:

        rule<> pascal_comment
            =   comment_nest_p("(*", "*)")
            |   comment_nest_p('{', '}')
            ;

Please note, that a comment is parsed implicitly as if the whole `comment_p(...)` statement were embedded into a `lexeme_d[]` directive, i.e. during parsing of a comment no token skipping will occur, even if you've defined a skip parser for your whole parsing process.

Copyright © 2001-2002 Hartmut Kaiser
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