Provided by: libdatetime-format-builder-perl_0.8300-1_all 
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@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@urth.org>
• Iain Truskett <spoon@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.