Parsing Character Columns in Boost :: Spirit

Question

I am working on a Boost Spirit 2.0 parser for a small subset of Fortran 77. The problem I am facing is that Fortran 77 is column oriented and I could not find anything in the Spirit that could let its parsers know the columns . Is there any way to do this?

I don’t need to support Fortran’s full secret syntax, but it should be able to ignore lines that have a character in the first column (Fortran comments), and recognize lines with a character in the sixth column as continuation lines.

It seems that people working with batch files will at least have the same problem with the first column as I do. It seems that the spirit has an end-of-line parser, but not a start-line parser (and, strictly speaking, not a column parser (x)).

Answer 1

Well, since I now have the answer to this question, I think I should share it.

Fortran 77, like all other languages ​​that care about columns, is a line-oriented language. This means that your parser must track the EOL and actually use it when parsing.

Another important fact is that in my case it’s not important for me to parse the line numbers that Fortran can put in the early control columns. All I need to know is when he tells me to scan the rest of the line differently.

Given these two things, I could completely handle this problem with the help of the Spirit scroll analyzer. I wrote mine

• skip the whole line if the first (comment) column contains an alphabetic character.
• skip the entire line if there is nothing on it.
• ignore the previous EOL and all up to the fifth column if there is a character in the fifth column. (continuation of the line). This binds it to the previous line.
• skip all non-eol spaces (even spaces in Fortran don't matter. Yes, it's a weird language.)

Here is the code:

skip = // Full line comment (spirit::eol >> spirit::ascii::alpha >> *(spirit::ascii::char_ - spirit::eol)) [boost::bind (&fortran::parse_info::skipping_line, &pi)] | // remaining line comment (spirit::ascii::char_ ('!') >> *(spirit::ascii::char_ - spirit::eol) [boost::bind (&fortran::parse_info::skipping_line_comment, &pi)]) | // Continuation (spirit::eol >> spirit::ascii::blank >> spirit::qi::repeat(4)[spirit::ascii::char_ - spirit::eol] >> ".") [boost::bind (&fortran::parse_info::skipping_continue, &pi)] | // empty line (spirit::eol >> -(spirit::ascii::blank >> spirit::qi::repeat(0, 4)[spirit::ascii::char_ - spirit::eol] >> *(spirit::ascii::blank) ) >> &(spirit::eol | spirit::eoi)) [boost::bind (&fortran::parse_info::skipping_empty, &pi)] | // whitespace (this needs to be the last alternative). (spirit::ascii::space - spirit::eol) [boost::bind (&fortran::parse_info::skipping_space, &pi)] ; 

I would advise you not to blindly use this on your own for line-oriented Fortran, since I ignore line numbers, and different compilers have different rules for valid comment and continuation characters.