=encoding utf8

=head1 NAME

Mail::Message::Head - the header of one message

=head1 INHERITANCE

 Mail::Message::Head
   is a Mail::Reporter

 Mail::Message::Head is extended by
   Mail::Message::Head::Complete
   Mail::Message::Head::Delayed
   Mail::Message::Head::Subset

=head1 SYNOPSIS

 my $head = Mail::Message::Head->new;
 $head->add('From: me@localhost');
 $head->add(From => 'me@localhost');
 $head->add(Mail::Message::Field->new(From => 'me'));
 my $subject = $head->get('subject');
 my @rec = $head->get('received');
 $head->delete('From');

=head1 DESCRIPTION

C<Mail::Message::Head> MIME headers are part of L<Mail::Message|Mail::Message> messages,
which are grouped in L<Mail::Box|Mail::Box> folders.

B<ATTENTION!!!> most functionality about e-mail headers is described
in L<Mail::Message::Head::Complete|Mail::Message::Head::Complete>, which is a matured header object.
Other kinds of headers will be translated to that type when time comes.

On this page, the general methods which are available on any header are
described.  Read about differences in the sub-class specific pages.

Extends L<"DESCRIPTION" in Mail::Reporter|Mail::Reporter/"DESCRIPTION">.
 
=head1 OVERLOADED

=over 4

=item overload: B<"">

(stringifaction) The header, when used as string, will format as if
L<Mail::Message::Head::Complete::string()|Mail::Message::Head::Complete/"Access to the header"> was called, so return a
nicely folder full header.  An exception is made for Carp, which will
get a simplified string to avoid unreadible messages from C<croak>
and C<confess>.

example: using a header object as string

 print $head;     # implicit stringification by print
 $head->print;    # the same

 print "$head";   # explicit stringication

=item overload: B<bool>

When the header does not contain any lines (which is illegal, according
to the RFCs), false is returned.  In all other cases, a true value is
produced.

=back

=head1 METHODS

Extends L<"METHODS" in Mail::Reporter|Mail::Reporter/"METHODS">.
 
=head2 Constructors

Extends L<"Constructors" in Mail::Reporter|Mail::Reporter/"Constructors">.
 
=over 4

=item Mail::Message::Head-E<gt>B<build>( [PAIR|$field]-LIST )

A fast way to construct a header with many lines.
The PAIRs are C<(name, content)> pairs of the header, but it is also possible
to pass L<Mail::Message::Field|Mail::Message::Field> objects.   A
L<Mail::Message::Head::Complete|Mail::Message::Head::Complete> header is created by simply calling
L<Mail::Message::Head::Complete::build()|Mail::Message::Head::Complete/"Constructors">, and then each field
is added.  Double field names are permitted.

example: 

 my $subject = Mail::Message::Field->new(Subject => 'xyz');

 my $head = Mail::Message::Head->build
  ( From     => 'me@example.com'
  , To       => 'you@anywhere.aq'
  , $subject
  , Received => 'one'
  , Received => 'two'
  );

 print ref $head;
  # -->  Mail::Message::Head::Complete

=item Mail::Message::Head-E<gt>B<new>(%options)

Create a new message header object.  The object will store all the
fields of a header.  When you get information from the header, it
will be returned to you as L<Mail::Message::Field|Mail::Message::Field> objects, although
the fields may be stored differently internally.

If you try to instantiate a L<Mail::Message::Head|Mail::Message::Head>, you will automatically
be upgraded to a L<Mail::Message::Head::Complete|Mail::Message::Head::Complete> --a full head.

 -Option    --Defined in     --Default
  field_type                   Mail::Message::Field::Fast
  log         Mail::Reporter   'WARNINGS'
  message                      undef
  modified                     <false>
  trace       Mail::Reporter   'WARNINGS'

=over 2

=item field_type => CLASS

The type of objects that all the fields will have.  This must be
an extension of L<Mail::Message::Field|Mail::Message::Field>.

=item log => LEVEL

=item message => MESSAGE

The MESSAGE where this header belongs to.  Usually, this is not known
at creation of the header, but sometimes it is.  If not, call the
message() method later to set it.

=item modified => BOOLEAN

=item trace => LEVEL

=back

=back

=head2 The header

=over 4

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

Headers may only be partially read, in which case they are called delayed.
This method returns true if some header information still needs to be
read. Returns false if all header data has been read.
Will never trigger completion.

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

Are there any fields defined in the current header?  Be warned that
the header will not be loaded for this: delayed headers will return
true in any case.

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

Returns whether the header has been modified after being read.

example: 

 if($head->isModified) { ... }

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

Like L<Mail::Message::Head::Complete::names()|Mail::Message::Head::Complete/"Access to the header">, but only returns the known
header fields, which may be less than C<names> for header types which are
partial.  C<names()> will trigger completion, where C<knownNames()> does not.

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

Get (after setting) the message where this header belongs to.
This does not trigger completion.

=item $obj-E<gt>B<modified>( [BOOLEAN] )

Sets the modified flag to BOOLEAN.  Without value, the current setting is
returned, but in that case you can better use L<isModified()|Mail::Message::Head/"The header">.
Changing this flag will not trigger header completion.

example: 

 $head->modified(1);
 if($head->modified) { ... }
 if($head->isModified) { ... }

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

Retuns the fields ordered the way they were read or added.

=back

=head2 Access to the header

=over 4

=item $obj-E<gt>B<get>( $name, [$index] )

Get the data which is related to the field with the $name.  The case of the
characters in $name does not matter.

If there is only one data element defined for the $name, or if there is an
$index specified as the second argument, only the specified element will be
returned. If the field $name matches more than one header the return value
depends on the context. In LIST context, all values will be returned in
the order they are read. In SCALAR context, only the last value will be
returned.

example: 

 my $head = Mail::Message::Head->new;
 $head->add('Received: abc');
 $head->add('Received: xyz');
 $head->add('Subject: greetings');

 my @rec_list   = $head->get('Received');
 my $rec_scalar = $head->get('Received');
 print ",@rec_list,$rec_scalar,"     # ,abc xyz, xyz,
 print $head->get('Received', 0);    # abc
 my @sub_list   = $head->get('Subject');
 my $sub_scalar = $head->get('Subject');
 print ",@sub_list,$sub_scalar,"     # ,greetings, greetings,

=item $obj-E<gt>B<study>( $name, [$index] )

Like L<get()|Mail::Message::Head/"Access to the header">, but puts more effort in understanding the contents of the
field.  L<Mail::Message::Field::study()|Mail::Message::Field/"Access to the content"> will be called for the field
with the specified FIELDNAME, which returns L<Mail::Message::Field::Full|Mail::Message::Field::Full>
objects. In scalar context only the last field with that name is returned.
When an $index is specified, that element is returned.

=back

=head2 About the body

=over 4

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

Try to estimate the size of the body of this message, but without parsing
the header or body.  The result might be C<undef> or a few percent of
the real size.  It may even be very far of the real value, that's why
this is a guess.

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

Returns whether the body of the related message is a multipart body.
May trigger completion, when the C<Content-Type> field is not defined.

=back

=head2 Internals

=over 4

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

Add a field, like L<Mail::Message::Head::Complete::add()|Mail::Message::Head::Complete/"Access to the header"> does, but
avoid the loading of a possibly partial header.  This method does not
test the validity of the argument, nor flag the header as changed.
This does not trigger completion.

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

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

Returns the location of the header in the file, as a pair begin and end.  The
begin is the first byte of the header.  The end is the first byte after
the header.

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

Be sure that the header is loaded.  This returns the loaded header
object.

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

Move the registration of the header in the file.

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

Read the header information of one message into this header structure.  This
method is called by the folder object (some L<Mail::Box|Mail::Box> sub-class), which
passes the $parser as an argument.

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

Set a field, but avoid the loading of a possibly partial header as set()
does.  This method does not test the validity of the argument, nor flag the
header as changed.  This does not trigger completion.

=back

=head2 Error handling

Extends L<"Error handling" in Mail::Reporter|Mail::Reporter/"Error handling">.
 
=over 4

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

Inherited, see L<Mail::Reporter/"Error handling">

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

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<defaultTrace>( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )

=item Mail::Message::Head-E<gt>B<defaultTrace>( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )

Inherited, see L<Mail::Reporter/"Error handling">

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

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<log>( [$level, [$strings]] )

=item Mail::Message::Head-E<gt>B<log>( [$level, [$strings]] )

Inherited, see L<Mail::Reporter/"Error handling">

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

=item Mail::Message::Head-E<gt>B<logPriority>($level)

Inherited, see L<Mail::Reporter/"Error handling">

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

Inherited, see L<Mail::Reporter/"Error handling">

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

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<report>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<reportAll>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<trace>( [$level] )

Inherited, see L<Mail::Reporter/"Error handling">

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

Inherited, see L<Mail::Reporter/"Error handling">

=back

=head2 Cleanup

Extends L<"Cleanup" in Mail::Reporter|Mail::Reporter/"Cleanup">.
 
=over 4

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

Inherited, see L<Mail::Reporter/"Cleanup">

=back

=head1 DETAILS

=head2 Ordered header fields

Many Perl implementations make a big mistake by disturbing the order
of header fields.  For some fields (especially the I<resent groups>,
see L<Mail::Message::Head::ResentGroup|Mail::Message::Head::ResentGroup>) the order shall be
maintained.

MailBox will keep the order of the fields as they were found in the
source.  When your add a new field, it will be added at the end.  If
your replace a field with a new value, it will stay in the original
order.

=head2 Head class implementation

The header of a MIME message object contains a set of lines, which are
called I<fields> (by default represented by L<Mail::Message::Field|Mail::Message::Field>
objects).  Dependent on the situation, the knowledge about the fields can
be in one of three situations, each represented by a sub-class of this
module:

=over 4

=item * L<Mail::Message::Head::Complete|Mail::Message::Head::Complete>

In this case, it is sure that all knowledge about the header is available.
When you L<get()|Mail::Message::Head/"Access to the header"> information from the header and it is not there, it will
never be there.

=item * L<Mail::Message::Head::Subset|Mail::Message::Head::Subset>

There is no certainty whether all header lines are known (probably not).  This
may be caused as result of reading a fast index file, as described in
L<Mail::Box::MH::Index|Mail::Box::MH::Index>.  The object is automatically transformed
into a L<Mail::Message::Head::Complete|Mail::Message::Head::Complete> when all header lines must be known.

=item * L<Mail::Message::Head::Partial|Mail::Message::Head::Partial>

A partial header is like a subset header: probably the header is incomplete.
The means that you are not sure whether a L<get()|Mail::Message::Head/"Access to the header"> for a field fails because
the field is not a part of the message or that it fails because it is not
yet known to the program.  Where the subset header knows where to get the
other fields, the partial header does not know it.  It cannot hide its
imperfection.

=item * L<Mail::Message::Head::Delayed|Mail::Message::Head::Delayed>

In this case, there is no single field known.  Access to this header will
always trigger the loading of the full header.

=back

=head2 Subsets of header fields

Message headers can be quite large, and therefore MailBox provides
simplified access to some subsets of information.  You can grab these
sets of fields together, create and delete them as group.

On the moment, the following sets are defined:

=over 4

=item * L<Mail::Message::Head::ResentGroup|Mail::Message::Head::ResentGroup>

A I<resent group> is a set of fields which is used to log one step
in the transmission of the message from the original sender to the
destination.

Each step adds a set of headers to indicate when the message was received
and how it was forwarded (without modification).  These fields are
best created using L<Mail::Message::bounce()|Mail::Message::Construct::Bounce/"Constructing a message">.

=item * L<Mail::Message::Head::ListGroup|Mail::Message::Head::ListGroup>

Fields which are used to administer and log mailing list activity.  Mailing
list software has to play trics with the original message to be able to
get the reply on that message back to the mailing list.  Usually a large
number of lines are added.

=item * L<Mail::Message::Head::SpamGroup|Mail::Message::Head::SpamGroup>

A set of fields which contains header fields which are produced by
spam detection software.  You may want to remove these fields when
you store a message for a longer period of time.

=back

=head1 DIAGNOSTICS

=over 4

=item Error: Package $package does not implement $method.

Fatal error: the specific package (or one of its superclasses) does not
implement this method where it should. This message means that some other
related classes do implement this method however the class at hand does
not.  Probably you should investigate this and probably inform the author
of the package.

=back

=head1 SEE ALSO

This module is part of Mail-Message distribution version 3.004,
built on December 21, 2017.

Do not forget to read
L<Mail::Box-Overview>,
L<Mail::Box-Cookbook>, and
L<Mail::Box-Index>.
Examples are included in the Mail-Box distribution, directories 'examples'
and 'scripts'.

Browseable manuals, papers, and other released material van be found
at  Website: F<http://perl.overmeer.net/mailbox/>

The central modules (in separate distributions) in the MailBox suite are:
L<Mail::Message>,
L<Mail::Box>,
L<Mail::Box::IMAP4>,
L<Mail::Box::POP3>,
L<Mail::Box::Parser::C>,
L<Mail::Box::Dbx> (unpublished),
L<Mail::Transport>,
L<Object::Realize::Later>, and
L<User::Identity>.

Please post questions or ideas to the author F<markov@cpan.org>.

=head1 LICENSE

Copyrights 2001-2017 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
See F<http://dev.perl.org/licenses/>

