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Filename : Field.pod
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=encoding utf8

=head1 NAME

Mail::Field - base-class for manipulation of mail header fields

=head1 INHERITANCE

 Mail::Field is extended by
   Mail::Field::AddrList
   Mail::Field::Date
   Mail::Field::Generic

=head1 SYNOPSIS

 use Mail::Field;
    
 my $field = Mail::Field->new('Subject', 'some subject text');
 my $field = Mail::Field->new(Subject => 'some subject text');
 print $field->tag,": ",$field->stringify,"\n";

 my $field = Mail::Field->subject('some subject text');

=head1 DESCRIPTION

C<Mail::Field> creates and manipulates fields in MIME headers, collected
within a L<Mail::Header|Mail::Header> object.  Different field types have their
own sub-class (extension), defining additional useful accessors to the
field content.

People are invited to merge their implementation to special fields into
MailTools, to maintain a consistent set of packages and documentation.

=head1 METHODS

=head2 Constructors

Mail::Field (and it's sub-classes) define several methods which return
new objects. These can all be categorized as constructor.

=over 4

=item Mail::Field-E<gt>B<combine>($fields)

Take a LIST of C<Mail::Field> objects (which should all be of the same
sub-class) and create a new object in that same class.

=item Mail::Field-E<gt>B<extract>( $tag, $head [, $index ] )

Takes as arguments the tag name, a C<Mail::Head> object
and optionally an index.

If the index argument is given then C<extract> will retrieve the given tag
from the C<Mail::Head> object and create a new C<Mail::Field> based object.
I<undef> will be returned in the field does not exist.

If the index argument is not given the result depends on the context
in which C<extract> is called. If called in a scalar context the result
will be as if C<extract> was called with an index value of zero. If called
in an array context then all tags will be retrieved and a list of
C<Mail::Field> objects will be returned.

=item Mail::Field-E<gt>B<new>( $tag [, STRING | %options] )

Create an object in the class which defines the field specified by
the $tag argument.

=back

=head2 "Fake" constructors

=over 4

=item $obj-E<gt>B<create>(%options)

This constructor is used internally with preprocessed field information.
When called on an existing object, its original content will get
replaced.

=item $obj-E<gt>B<parse>()

Parse a field line.

=back

=head2 Accessors

=over 4

=item $obj-E<gt>B<set>(%options)

Change the settings (the content, but then smart) of this field.

=item $obj-E<gt>B<stringify>()

Returns the field as a string.

=item $obj-E<gt>B<tag>()

=item Mail::Field-E<gt>B<tag>()

Return the tag (in the correct case) for this item.  Well, actually any
casing is OK, because the field tags are treated case-insensitive; however
people have some preferences.

=back

=head2 Smart accessors

=over 4

=item $obj-E<gt>B<text>( [STRING] )

Without arguments, the field is returned as L<stringify()|Mail::Field/"Accessors"> does.  Otherwise,
the STRING is parsed with L<parse()|Mail::Field/""Fake" constructors"> to replace the object's content.

It is more clear to call either L<stringify()|Mail::Field/"Accessors"> or L<parse()|Mail::Field/""Fake" constructors"> directly, because
this method does not add additional processing.

=back

=head1 DETAILS

=head2 SUB-CLASS PACKAGE NAMES

All sub-classes should be called Mail::Field::I<name> where I<name> is
derived from the tag using these rules.

=over 4

=item *

Consider a tag as being made up of elements separated by '-'

=item *

Convert all characters to lowercase except the first in each element, which
should be uppercase.

=item *

I<name> is then created from these elements by using the first
N characters from each element.

=item *

N is calculated by using the formula :-

    int((7 + #elements) / #elements)

=item *

I<name> is then limited to a maximum of 8 characters, keeping the first 8
characters.

=back

For an example of this take a look at the definition of the 
C<_header_pkg_name()> subroutine in C<Mail::Field>

=head1 DIAGNOSTICS

=over 4

=item Error: Undefined subroutine <method> called

Mail::Field objects use autoloading to compile new functionality.
Apparently, the method called is not implemented for the specific
class of the field object.

=back

=head1 SEE ALSO

This module is part of the MailTools distribution,
F<http://perl.overmeer.net/mailtools/>.

=head1 AUTHORS

The MailTools bundle was developed by Graham Barr.  Later, Mark
Overmeer took over maintenance without commitment to further development.

Mail::Cap by Gisle Aas E<lt>aas@oslonett.noE<gt>.
Mail::Field::AddrList by Peter Orbaek E<lt>poe@cit.dkE<gt>.
Mail::Mailer and Mail::Send by Tim Bunce E<lt>Tim.Bunce@ig.co.ukE<gt>.
For other contributors see ChangeLog.

=head1 LICENSE

Copyrights 1995-2000 Graham Barr E<lt>gbarr@pobox.comE<gt> and
2001-2017 Mark Overmeer E<lt>perl@overmeer.netE<gt>.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
See F<http://www.perl.com/perl/misc/Artistic.html>