phpman > perldoc > DateTime::Format::Builder::Tutorial(3pm)

NAME
    DateTime::Format::Builder::Tutorial - Quick class on using Builder

VERSION
    version 0.83

CREATING A CLASS
    As most people who are writing modules know, you start a package with a package declaration and
    some indication of module version:

        package DateTime::Format::ICal; our $VERSION = '0.04';

    After that, you call Builder with some options. There are only a few (detailed later). Right
    now, we're only interested in *parsers*.

        use DateTime::Format::Builder ( parsers => {...} );

    The *parsers* option takes a reference to a hash of method names and specifications:

        parsers => {
                parse_datetime => ... ,
                parse_datetime_with_timezone => ... ,
                ...
            }

    Builder will create methods in your class, each method being a parser that follows the given
    specifications. It is strongly recommended that one method is called *parse_datetime*, be it a
    Builder created method or one of your own.

    In addition to creating any of the parser methods it also creates a "new" method that can
    instantiate (or clone) objects of this class. This behaviour can be modified with the
    *constructor* option, but we don't need to know that yet.

    Each value corresponding to a method name in the parsers list is either a single specification,
    or a list of specifications. We'll start with the simple case.

        parse_briefdate => { params => [qw( year month day )], regex =>
            qr/^(\d\d\d\d)(\d\d)(\d\d)$/, },

    This will result in a method named *parse_briefdate* which will take strings in the form
    20040716 and return DateTime objects representing that date. A user of the class might write:

        use DateTime::Format::ICal;
        my $date = '19790716';
        my $dt   = DateTime::Format::ICal->parse_briefdate($date);
        print "My birth month is ", $dt->month_name, "\n";

    The "regex" is applied to the input string, and if it matches, then $1, $2, ... are mapped to
    the *params* given and handed to "DateTime->new". Essentially:

        my $rv = DateTime->new( year => $1, month => $2, day => $3 );

    There are more complicated things one can do within a single specification, but we'll cover
    those later.

    Often, you'll want a method to be able to take one string, and run it against multiple parser
    specifications. It would be very irritating if the user had to work out what format the datetime
    string was in and then which method was most appropriate.

    So, Builder lets you specify multiple specifications:

        parse_datetime => [
            {
                params => [qw( year month day hour minute second )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)(\d\d)$/,
            },
            {
                params => [qw( year month day hour minute )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)$/,
            },
            {
                params => [qw( year month day hour )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)$/,
            },
            {
                params => [qw( year month day )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)$/,
            },
        ],

    It's an arrayref of specifications. A parser will be created that will try each of these
    specifications sequentially, in the order you specified.

    There's a flaw with this though. In this example, we're building a parser for ICal datetimes.
    One can place a timezone id at the start of an ICal datetime. You might extract such an id with
    the following code:

        if ( $date =~ s/^TZID=([^:]+):// ) {
            $time_zone = $1;
        }

        # Z at end means UTC
        elsif ( $date =~ s/Z$// ) {
            $time_zone = 'UTC';
        }
        else {
            $time_zone = 'floating';
        }

    $date would end up without the id, and $time_zone would contain something appropriate to give to
    DateTime's *set_time_zone* method, or *time_zone* argument.

    But how to get this scrap of code into your parser? You might be tempted to call the parser
    something else and build a small wrapper. There's no need though because an option is provided
    for preprocessing dates:

        parse_datetime => [
            [ preprocess => \&_parse_tz ],    # Only changed line!
            {
                params => [qw( year month day hour minute second )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)(\d\d)$/,
            },
            {
                params => [qw( year month day hour minute )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)$/,
            },
            {
                params => [qw( year month day hour )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)$/,
            },
            {
                params => [qw( year month day )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)$/,
            },
        ],

    It will necessitate *_parse_tz* to be written, and that routine looks like this:

        sub _parse_tz {
            my %args = @_;
            my ( $date, $p ) = @args{qw( input parsed )};
            if ( $date =~ s/^TZID=([^:]+):// ) {
                $p->{time_zone} = $1;
            }

            # Z at end means UTC
            elsif ( $date =~ s/Z$// ) {
                $p->{time_zone} = 'UTC';
            }
            else {
                $p->{time_zone} = 'floating';
            }
            return $date;
        }

    On input it is given a hash containing two items: the input date and a hashref that will be used
    in the parsing. The return value from the routine is what the parser specifications will run
    against, and anything in the *parsed* hash ($p in the example) will be put in the call to
    "DateTime->new(...)".

    So, we now have a happily working ICal parser. It parses the assorted formats, and can also
    handle timezones. Is there anything else it needs to do? No. But we can make it work more
    efficiently.

    At present, the specifications are tested sequentially. However, each one applies to strings of
    particular lengths. Thus we could be efficient and have the parser only test the given strings
    against a parser that handles that string length. Again, Builder makes it easy:

        parse_datetime => [
            [ preprocess => \&_parse_tz ],
            {
                length => 15,    # We handle strings of exactly 15 chars
                params => [qw( year month day hour minute second )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)(\d\d)$/,
            },
            {
                length => 13,                                # exactly 13 chars...
                params => [qw( year month day hour minute )],
                regex => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)$/,
            },
            {
                length => 11,                                    # 11..
                params => [qw( year month day hour )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)$/,
            },
            {
                length => 8,                                     # yes.
                params => [qw( year month day )],
                regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)$/,
            },
        ],

    Now the created parser will create a parser that only runs specifications against appropriate
    strings.

    So our complete code looks like:

        package DateTime::Format::ICal;
        use strict;
        our $VERSION = '0.04';

        use DateTime::Format::Builder (
            parsers => {
                parse_datetime => [
                    [ preprocess => \&_parse_tz ],
                    {
                        length => 15,
                        params => [qw( year month day hour minute second )],
                        regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)(\d\d)$/,
                    },
                    {
                        length => 13,
                        params => [qw( year month day hour minute )],
                        regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)(\d\d)$/,
                    },
                    {
                        length => 11,
                        params => [qw( year month day hour )],
                        regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)T(\d\d)$/,
                    },
                    {
                        length => 8,
                        params => [qw( year month day )],
                        regex  => qr/^(\d\d\d\d)(\d\d)(\d\d)$/,
                    },
                ],
            },
        );

        sub _parse_tz {
            my %args = @_;
            my ( $date, $p ) = @args{qw( input parsed )};
            if ( $date =~ s/^TZID=([^:]+):// ) {
                $p->{time_zone} = $1;
            }

            # Z at end means UTC
            elsif ( $date =~ s/Z$// ) {
                $p->{time_zone} = 'UTC';
            }
            else {
                $p->{time_zone} = 'floating';
            }
            return $date;
        }

        1;

    And that's an ICal parser. The actual DateTime::Format::ICal module also includes formatting
    methods and parsing for durations, but Builder doesn't support those yet. A drop in replacement
    (at the time of writing the replacement) can be found in the examples directory of the Builder
    distribution, along with similar variants of other common modules.

SEE ALSO
    "datetime AT perl.org" mailing list.

    http://datetime.perl.org/

    perl, DateTime, DateTime::Format::Builder

SUPPORT
    Bugs may be submitted at <https://github.com/houseabsolute/DateTime-Format-Builder/issues>.

    I am also usually active on IRC as 'autarch' on "irc://irc.perl.org".

SOURCE
    The source code repository for DateTime-Format-Builder can be found at
    <https://github.com/houseabsolute/DateTime-Format-Builder>.

AUTHORS
    *   Dave Rolsky <autarch AT urth.org>

    *   Iain Truskett <spoon AT cpan.org>

COPYRIGHT AND LICENSE
    This software is Copyright (c) 2020 by Dave Rolsky.

    This is free software, licensed under:

      The Artistic License 2.0 (GPL Compatible)

    The full text of the license can be found in the LICENSE file included with this distribution.