Reading dates about readr HOT 6 CLOSED

We need a way to specify this. I think read_csv could get some non standard evaluation to specify the classes. For this I was thinking something like this:

classes( "character", "integer", date( "%Y-%m-%d" ) )
classes( ymd() )

read_csv(file, classes( ... ) )

@hadley do you have syntax ideas.

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I don't think we need non-standard evaluation, just a succinct DSL for expressing column types. I'll have a think about it.

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[Moving discussion about specification into #14]

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We can probably again use boost spirit. See here: http://www.boost.org/doc/libs/1_47_0/libs/spirit/example/qi/parse_date.cpp

Some relevant extract:

    template <typename Iterator>
    bool parse_date(Iterator& first, Iterator last, boost::gregorian::date& d)
    {
        typedef boost::fusion::vector<int, int, int> date_parts;
        qi::rule<Iterator, date_parts(), qi::space_type> date =
            qi::int_ >> '-' >> qi::int_ >> '-' >> qi::int_;

        return phrase_parse(first, last, date, qi::space, d);
    }

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Got Date, borrowing some logic from fasttime.

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We can now read dates with "Date", and dates + times with "POSIXct". This is largely inspired from fasttime.

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