I am trying to parse a CSV and convert a column that holds the sex of a person into a Nullable{Bool}

My attempts failed:

sex_parser = CustomParser(Bool) do str, i, len, opts
    return (len == 0 ? Nullable{Bool}() : Nullable{Bool}(str[i] == 'M'), len == 0 ? i : i + 1)
end

I spent couple of hours reading the docs and the code, but I do not understand what the str is that the tryparsenext receives in a readcsv context, and whether my CustomParser must be aware of the delimiter symbol.

For example, consider a CSV with ';' delimiter:

123;F;6789;
124;M;6712;
125;;6716;

Is str a row and my custom parse receives pos=5, len=1 for the first row and pos=5, len=0 for the third row?
And after a successful parse in row one, do I return 6 or 7. The doc is saying "position the next token, if any, starts at", that sound like my parser needs to be aware of my delimiter!