=head1 NAME

Bio::Phylo::Manual - High-level user guide

=head1 INTRODUCTION

This is the manual for Bio::Phylo. Bio::Phylo is a perl5 package for
phylogenetic analysis. The stable URL for the
most recent distribution is L<http://search.cpan.org/dist/Bio-Phylo/>.

This manual is intended for readers who know how to program 
in perl and understand commonly-encountered concepts in phylogenetics. 
What is offered in this document is an overview of Bio::Phylo's
functionality, not a tutorial or a reference of all functions.

For exhaustive API documentation, consult the embedded L<perlpod> in
the classes of this release, for example by issuing C<perldoc Bio::Phylo::IO>
(or some other class name) in a terminal window.

Note that Bio::Phylo uses inheritance to a great extent,
such that any one object may inherit additional methods from a number
of superclasses. In such cases, this will be noted in the "SEE ALSO"
section at the bottom of that class's documentation. The Bio::Phylo documentation
system rewards the methodical reader who follows these document links.

For installation instructions, read the README file in the root directory 
of the distribution. 

=head1 INSTANT GRATIFICATION

The following sections will demonstrate some of the basic functionality,
with immediate, useful results.

=head2 One-liners

One-liners are perl statements that are executed directly on the command
line, using the C<-e '...statements...'> argument.
Often, you'll include the C<-MFoo::Bar> switch to include module
C<Foo::Bar> at runtime. (See L<perlrun> for more info on executing
the interpreter.) B<NOTE FOR WINDOWS USERS>: in the following examples,
switch the quotes around, i.e. use double quotes where single quotes are
used and vice versa.

=over

=item B<First steps>

=item Problem

No concept is valid in Perl if it cannot be expressed in a one-liner.
For the Bio::Phylo package, some operations can be performed using
a single expression from the command line. Here are some examples.

=item Solution 1: downloading and converting Tree of Life data

 perl -MBio::Phylo::IO=parse -e 'print parse->to_nexus' format tolweb as_project 1 url $URL

=item Discussion

Assuming that the environment variable $URL has been set to point to 
a node in the XML web service of the Tree of Life (L<http://tolweb.org>),
this command will download the output, parse it, and print the parsed output
as nexus. As an example, using this url:

 http://tolweb.org/onlinecontributors/app?service=external&page=xml/TreeStructureService&node_id=133799

Something like the following output would be produced:

 #NEXUS
 BEGIN TAXA;
 [! Taxa block written by Bio::Phylo::Taxa 0.31_1520 on Thu Nov 25 20:49:54 2010 ]	
         DIMENSIONS NTAX=3;
         TAXLABELS
 			'Bembidion alaskense'
 			'Bembidion argenteolum'
 			'Bembidion semenovi'
		 ;
 END;
 BEGIN TREES;
 [! Trees block written by Bio::Phylo::Forest 0.31_1520 on Thu Nov 25 20:49:54 2010 ]
 	TRANSLATE
 		1 'Bembidion alaskense',
 		2 'Bembidion argenteolum',
 		3 'Bembidion semenovi';
 	TREE Tree2 = [&R] (((2,3),1));
 END;

So what is happening here? Firstly, we provide the C<-MBio::Phylo::IO> switch,
to which we add C<=parse>, which means we import the C<parse> function from
L<Bio::Phylo::IO>. This function is supplied with named arguments, which can
also be provided on the command line, i.e. as part of the C<@ARGV> array.

Secondly, we use the C<-e> 'print parse->to_nexus' switch. Here we till perl
to execute the parse function, transform its return value to nexus, and print
that to STDOUT.

Following that, we provide the named command line arguments. C<format tolweb>
specifies that the input for the parse function is in the Tree of Life XML
format. C<as_project 1> specifies that the parse function should return its
contents as a newly created L<Bio::Phylo::Project> object. C<url $URL> 
specifies the data source to parse; in this case the data source lives at 
$URL. Other possible options for a data source are C<file> with a file name, 
C<string> with a string of phylogenetic data in some recognized format, or 
C<handle> with an open file handle.

(This example requires the otherwise optional modules L<LWP::UserAgent>
and L<XML::Twig> to be installed on your system.)

=item Solution 2: calculating tree balance

 perl -MBio::Phylo::IO=parse -e 'print \
 parse(-format=>"newick",-string=>"((A,B),C);")->first->calc_imbalance'

=item Discussion

The C<-MModule> switch is the equivalent of using 
C<use Module;> in a script. Here we use the L<Bio::Phylo::IO> module, which
is Bio::Phylo's entry point into file parsing and file writing.

The C<-e> switch is used to evaluate the subsequent expression. We parse a 
string, C<((A,B),C);>, of format C<newick>. The parser returns a 
L<Bio::Phylo::Forest> object (i.e. a set of trees, in this case a set of 
one). From this set we retrieve the first (and only) tree, and calculate 
Colless' imbalance, which returns a number, which we print to standard out. 

This would print "1", because the tree is a ladder, and therefore completely
unbalanced. Note how this example uses the standard interface for 
L<Bio::Phylo::IO> as you would normally use it in code you write in a script
or a module. As the arguments to the C<parse> function can also be supplied in 
C<@ARGV> (useful for one-liners or other processes that launch 
shell commands) the example can be rewritten as:

 perl -MBio::Phylo::IO=parse -e 'print parse()->first->calc_imbalance' \
 format newick string "((A,B),C);"

In this alternative invocation, note how the arguments to the parse call are
now outside of the '...command...' quotes, making them "shell words",
which for various reasons may not be preceded by dashes.

=item B<Sets of trees>

=item Problem

You want a one-liner to iterate over a set of trees:

=item Solution

 perl -MBio::Phylo::IO=parse -lne 'print \
 parse(-format=>"newick",-string=>$_)->first->calc_i2' <file>

=item Discussion

The C<-n> switch wraps a "while(<>) { ... }" around the
program, so the trees from F<file> (that is, if they are one newick
tree description per line) are copied into C<$_> one tree at a
time. The C<-l> switch appends a line break to the printed output.

=item B<Stringifying trees>

=item Problem

You don't want a number printed to C<STDOUT>, you want a tree:

=item Solution

 perl -MBio::Phylo::IO=parse -e 'print \
 parse(-format=>"newick",-string=>"((A,B),C);")->first->to_newick'

=item Discussion

If you try to print a tree object, what's written is something like
C<Bio::Phylo::Forest::Tree=SCALAR(0x1a337dc)> (that is, the 
memory address of the object reference). This is probably not 
what you want, so the tree object has a C<to_newick> 
method that stringifies the tree to a newick string. Likewise, matrices,
taxa and tree blocks can write a NEXUS block using C<to_nexus>,
and all of them can also be written to NeXML (L<http://www.nexml.org>) using
C<to_xml> and to a JSON mapping thereof using C<to_json>.

=back

=head2 Input and output

The L<Bio::Phylo::IO> module is the unified front end for parsing and
unparsing phylogenetic data objects. It is a non-OO module that optionally
exports the C<parse> and C<unparse> subroutines into the caller's
namespace, using the C<use Bio::Phylo::IO qw(parse unparse);> directive.
Alternatively, you can call the subroutines as class methods. The C<parse>
and C<unparse> subroutines load and dispatch the appropriate sub-modules
at runtime, depending on the C<-format> argument.

=over

=item B<Parsing trees>

=item Problem

You want to create a L<Bio::Phylo::Forest::Tree> object from a newick string.

=item Solution

 use Bio::Phylo::IO;

 # get a newick string from some source
 my $tree_string = '(((A,B),C),D);';

 # Call class method parse from Bio::Phylo::IO
 my $tree = Bio::Phylo::IO->parse(
    -string => $tree_string,
    -format => 'newick'
 )->first;

 # note: newick parser returns 'Bio::Phylo::Forest'
 # Call ->first to retrieve the first tree of the forest.

 print ref $tree, "\n"; # prints 'Bio::Phylo::Forest::Tree'

=item Discussion

The L<Bio::Phylo::IO> module invokes format specific parser and unparser modules. 
It is Bio::Phylo's front door for data input and output from files, raw strings 
and file handles. 

In the solution the IO module calls the L<Bio::Phylo::Parsers::Newick> parser
which turns a tree description into a L<Bio::Phylo::Forest> object. (Several other 
parser and unparser modules live in the Bio::Phylo::Parsers::* and 
Bio::Phylo::Unparsers::* namespaces, respectively.)

The returned forest object subclasses L<Bio::Phylo::Listable>, as a forest models 
a list of trees that you can iterate over. By calling the C<->first> method, we 
get the first tree in the forest - a L<Bio::Phylo::Forest::Tree> object (in the 
example it's a very small forest, consisting of just this single tree).

=item B<Parsing tables>

=item Problem

You want to create a L<Bio::Phylo::Matrices::Matrix> object from a string.

=item Solution

 use Bio::Phylo::IO;

 # parsing a table
 my $table_string = qq(A,1,2|B,1,2|C,2,2|D,2,1);
 my $matrix = Bio::Phylo::IO->parse(
    -string   => $table_string,
    -format   => 'table',     # See Bio::Phylo::Parsers::Table
    -type     => 'STANDARD',  # Data type
    -fieldsep => ',',         # field separator
    -linesep  => '|'          # line separator
 );

 print ref $matrix, "\n"; # prints 'Bio::Phylo::Matrices::Matrix'


=item Discussion

Here the L<Bio::Phylo::Parsers::Table> module parses a string
C<A,1,2|B,1,2|C,2,2|D,2,1>, where the C<|> is considered a record or
line separator, and the C<,> as a field separator. The default field and
line separators are the tabstop character "\t" and the line break "\n".

=item B<Parsing taxa>

=item Problem

You want to create a L<Bio::Phylo::Taxa> object from a string.

=item Solution

 use Bio::Phylo::IO;

 # parsing a list of taxa
 my $taxa_string = 'A:B:C:D';
 my $taxa = Bio::Phylo::IO->parse(
    -string   => $taxa_string,
    -format   => 'taxlist',
    -fieldsep => ':'
 );

 print ref $taxa, "\n"; # prints 'Bio::Phylo::Taxa'

=item Discussion

Here the L<Bio::Phylo::Parsers::Taxlist> module parses a string C<A:B:C:D>,
where the C<:> is considered a field separator. The parser returns a
L<Bio::Phylo::Taxa> object. Note that the same result can be obtained by building 
the taxa object from scratch (a more feasible proposition than building trees
or matrices from scratch):

 use Bio::Phylo::Factory; 
 
 # first instantiate the factory...
 my $factory = Bio::Phylo::Factory->new;
 
 # ...then use it to create other objects, such as taxa blocks
 my $taxa = $factory->create_taxa( -name => 'MyTaxa' );
 
 # or taxa, (with names A, B, C and D), and add them to the taxa block
 $taxa->insert( $factory->create_taxon( -name => $_ ) ) for qw(A B C D);
 
 # and write out as a nexus block
 print $taxa->to_nexus( -header => 1, -links => 1 );

This example uses the L<Bio::Phylo::Factory>, which is an object that can
create other objects. Here we have it create a L<Bio::Phylo::Taxa> block,
which we populate with four L<Bio::Phylo::Taxa::Taxon> objects. We then
write out the taxa block as nexus, complete with the #NEXUS header (this
is optional so that we can combine multiple blocks in the same file), and
a title, using the C<-links> switch. The latter is a facility that only 
seems to be used by Mesquite (L<http://mesquiteproject.org>) and Bio::Phylo.
It adds a "title" to the taxa block in the nexus output, and other blocks
(character state matrices and tree blocks) refer to this using a "links"
statement.  This is useful if you want to have multiple taxa blocks in the
same file and you want to distinguish them. Putting this all together, the
output is thus:

 #NEXUS
 BEGIN TAXA;
 [! Taxa block written by Bio::Phylo::Taxa 0.31_1520 on Thu Nov 25 21:31:58 2010 ]
 	TITLE MyTaxa;
         DIMENSIONS NTAX=4;
         TAXLABELS
 			A
 			B
 			C
 			D
         ;
 END;

=back

=head2 Iterating

The L<Bio::Phylo::Listable> module is the superclass of all container objects.
Container objects are objects that contain a set of objects of the same type.
For example, a L<Bio::Phylo::Forest::Tree> object is a container for
L<Bio::Phylo::Forest::Node> objects. Hence, the L<Bio::Phylo::Forest::Tree>
inherits from the L<Bio::Phylo::Listable> class. You can therefore iterate over
the nodes in a tree using the methods defined by L<Bio::Phylo::Listable>.

=over

=item B<Iterating over trees and nodes.>

=item Problem

You want to access trees and nodes contained in a L<Bio::Phylo::Forest>
object.

=item Solution

  use Bio::Phylo::IO qw(parse);

  my $string = '((A,B),(C,D));(((A,B),C)D);';
  my $forest = parse( -format => 'newick', -string => $string );

  print ref $forest; # prints 'Bio::Phylo::Forest'

  # access trees in $forest
  foreach my $tree ( @{ $forest->get_entities } ) {
      print ref $tree; # prints 'Bio::Phylo::Forest::Tree';

      # access nodes in $tree
      foreach my $node ( @{ $tree->get_entities } ) {
          print ref $node; # prints 'Bio::Phylo::Forest::Node';

      }
  }


=item Discussion

L<Bio::Phylo::Forest> and L<Bio::Phylo::Forest::Tree> are
nested subclasses of the iterator class L<Bio::Phylo::Listable>. Nested
iterator calls (such as C<->get_entities>) can be invoked on the
objects.

=item B<Iterating over taxa.>

=item Problem

You want to access the individual taxa in a L<Bio::Phylo::Taxa> object.

=item Solution

 use Bio::Phylo::IO qw(parse);

 my $string = 'A|B|C|D|E|F|G|H';
 my $taxa = parse(
     -string   => $string,
     -format   => 'taxlist',
     -fieldsep => '|'
 );
 print ref $taxa; # prints 'Bio::Phylo::Taxa';

 while ( my $taxon = $taxa->next ) {
     print ref $taxon; # prints 'Bio::Phylo::Taxa::Taxon'
 }

=item Discussion

A L<Bio::Phylo::Taxa> object is a subclass of the
L<Bio::Phylo::Listable> class. Hence, you could also call 
C<->get_entities> on the taxa object, which returns a 
reference to an array of taxon objects contained by the 
taxa object. Note however the shorthand:

 while ( my $taxon = $taxa->next ) { ... }

=item B<Iterating over datum objects.>

=item Problem

You want to access the datum objects contained by
a L<Bio::Phylo::Matrices::Matrix> object.

=item Solution

 use Bio::Phylo::IO;

 # parsing a table
 my $table_string = qq(A,1,2|B,1,2|C,2,2|D,2,1);
 my $matrix = Bio::Phylo::IO->parse(
    -string   => $table_string,
    -format   => 'table',     # See Bio::Phylo::Parsers::Table
    -type     => 'STANDARD',  # Data type
    -fieldsep => ',',         # field separator
    -linesep  => '|'          # line separator
 );

 print ref $matrix, "\n"; # prints 'Bio::Phylo::Matrices::Matrix'

 my $datum = $matrix->get_by_index( 0, -1 );
 print ref $datum; # NOTE: prints 'ARRAY'! 

=item Discussion

The L<Bio::Phylo::Matrices::Matrix> object subclasses the
L<Bio::Phylo::Listable> object. Hence, its iterator methods are applicable
here as well. In the above example, the get_by_index method is used. With
a single argument it returns a Bio::Phylo object. With multiple arguments
the semantics are nearly identical to array slicing (see L<perldata>), 
except that an array I<reference> is returned. Bio::Phylo generally passes
lists by reference (see L<perlref>).

=back

=head2 Simulating trees

The L<Bio::Phylo::Generator> module simulates trees under various
models of clade growth.

=over

=item B<Generating Yule trees.>

Here's how to generate a forest of ten trees with ten tips:

  use Bio::Phylo::Generator;
  my $gen = Bio::Phylo::Generator->new;
  my $trees = $gen->gen_rand_pure_birth(
      -trees => 10,
      -tips  => 10,
      -model => 'yule'
  );
  print ref $trees; # prints 'Bio::Phylo::Forest'

=item B<Expected versus randomly drawn waiting times.>

The generator object simulates trees under the Yule or the Hey model,
returning. The C<gen_rand_pure_birth> method call returns branch lengths
drawn from the appropriate distribution, while C<gen_exp_pure_birth>
returns the expected waiting times (e.g. 1/n where n=number of lineages for
the Yule model).

=back

=head2 Filtering

=over

=item B<Filtering objects by numerical value.>

To retrieve, for example, the nodes from a tree that are close to the
root, call:

 my @deep_nodes = @{ $tree->get_by_value(
    -value => 'calc_nodes_to_root',
    -le    => 2
 ) };

Which retrieves the nodes no more than 2 ancestors away from the root.
Any method that returns a numerical value can be specified with the
C<-value> flag. The C<-le> flag specifies that the returned
value is I<l>ess-than-or-I<e>qual to 2.

=item B<Filtering objects by regular expression.>

String values that are returned by objects can be filtered using a
compiled regular expression. For example:

 my @lemurs = @{ $tree->get_by_regular_expression(
      -value => 'get_name',
      -match => qr/[Ll]emur_.+$/
 ) };

Retrieves all nodes whose genus name matches Eulemur, Lemur or
Hapalemur.

=back

=head2 Drawing trees

You can create visualize tree objects using the
L<Bio::Phylo::Treedrawer> module:

  use Bio::Phylo::Treedrawers;
  use Bio::Phylo::IO;

  my $treedrawer = Bio::Phylo::Treedrawers->new(
     -width  => 400,
     -height => 600,
     -shape  => 'CURVY',
     -mode   => 'CLADO',
     -format => 'SVG'
  );

  my $tree = Bio::Phylo::IO->parse(
     -format => 'newick',
     -string => '((A,B),C);'
  )->first;

  $treedrawer->set_tree($tree);
  $treedrawer->set_padding(50);

  my $string = $treedrawer->draw;

Read the L<Bio::Phylo::Treedrawer> perldoc for more info.

=head2 Tips and tricks

=over

=item B<Generic metadata>

You can append generic key/value pairs to any object, by calling
$obj->set_generic( 'key' => 'value');. Subsequently calling
$obj->get_generic('key'); returns 'value'. This is a very useful
feature in many situations where you may want to attach, for example,
results from analyses by outside programs (e.g. likelihood scores)
to the tree objects they refer to. Likewise, multiple numbers (e.g.
bootstrap values, posteriors, bremer values) can be attached to the
same node in this way.

=back

=head1 OBJECT AND DATA MODEL

=head2 Perl objects

Object-oriented perl is a massive subject. To learn about the
basic syntax of OO-perl, the following perldocs might be of interest:

=over

=item L<perlboot>

Introduction to OO perl. Read at least this one if you have no
experience with OO perl.

=item L<perlobj>

Details about perl objects.

=item L<perltooc>

Class data.

=item L<perltoot>

Advanced objects: "Tom's object-oriented tutorial for perl"

=item L<perlbot>

The "Bag'o Object Tricks" (the BOT).

=back

=head2 The Bio::Phylo object model

The following sections discuss the nested objects that model
phylogenetic information and entities.

=over

=item The L<Bio::Phylo> root object.

The Bio::Phylo object is never used directly. However, all
other objects inherit from it, which means that all objects
have getters and setters for their name, description, score.
They can all return a globally unique ID, log messages, and
keep track of more administrative things such as the version
number of the release.

=item The Bio::Phylo::Forest::* namespace

According to Bio::Phylo, there is a Forest (which is
modelled by the Bio::Phylo::Forest object), which contains
Bio::Phylo::Forest::Tree objects, which contain
Bio::Phylo::Forest::Node objects.

=item The L<Bio::Phylo::Forest::Node> object

A node 'knows' a couple
of things: its name, its branch length (i.e. the length
of the branch connecting it and its parent), who its
parent is, its next sister (on its right), its previous
sister (on the left), its first daughter and its last
daughter. Also, a taxon can be specified that the node
refers to (this makes most sense when the node is terminal).
These properties can be retrieved and modified by methods
classified as ACCESSORS and MUTATORS.

From this set of properties follows a number of
things which must be either true or false. For example,
if a node has no children it is a terminal node. By asking
a node whether it "is_terminal", it replies either with
true (i.e. 1) or false (undef). Methods such as this
are classified as TESTS.

Likewise, based on the properties of an individual
node we can perform a query to retrieve nodes related
to it. For example, by asking the node to
"get_ancestors" it returns a list of its ancestors,
being all the nodes and the path from its parent to,
and including, the root. These methods are QUERIES.

Lastly, some CALCULATIONS can be performed by the
node. By asking the node to "calc_path_to_root" it
calculates the sum of the lengths of the branches
connecting it and the root. Of course, in order to make
all this possible, a node has to exist, so it needs to
be constructed. The CONSTRUCTOR is the Bio::Phylo::Node->new()
method.

Once a node has served its purpose it
can be destroyed. For this purpose there is a
DESTRUCTOR, which cleans up once we're done with the
node. However, in most cases you don't have to worry
about constructing and destroying nodes as this is handled
by Bio::Phylo and perl for you.

For a detailed description of all the node methods,
their arguments and return values, consult the node
documentation, which, after install, can be viewed by
issuing the "perldoc Bio::Phylo::Forest::Node" command.

=item The L<Bio::Phylo::Forest::Tree> object

A tree knows very
little. All it really holds is a set of nodes, which
are there because of TREE POPULATION, i.e. the process
of inserting nodes in the tree. The tree can be queried
in a number of ways, for example, we can ask the tree
to "get_entities", to which the tree replies with a list
of all the nodes it holds. Be advised that this doesn't
mean that the nodes are connected in a meaningful way,
if at all. The tree doesn't care, the nodes are
supposed to know who their parents, sisters, and
daughters are. But, we can still get, for example, all
the terminal nodes (i.e. the tips) in the tree by
retrieving all the nodes in the tree and asking each
one of them whether it "is_terminal", discarding the
ones that aren't.

Based on the set of nodes the tree holds it can
perform calculations, such as "calc_tree_length", which
simply means that the tree iterates over all its nodes,
summing their branch lengths, and returning the total.

The tree object also has a constructor and a
destructor, but normally you don't have to worry about
that. All the tree methods can be viewed by issuing the
"perldoc Bio::Phylo::Forest::Tree" command.

=item The L<Bio::Phylo::Forest> object

The object containing all others is the Forest object. It
serves merely as a container to hold multiple trees, which
are inserted in the Forest object using the "insert()" method,
and retrieved using the "get_entities" method. More information
can be found in the Bio::Phylo::Forest perldoc page.

=item The Bio::Phylo::Matrices::* namespace

Objects in the Bio::Phylo::Matrices namespace are used to handle
comparative data, as single observations, and in larger container
objects.

=item The L<Bio::Phylo::Matrices::Datum> object

The datum object holds observations of a predefined type,
such as molecular data, or continuous character states. The
Datum object can be linked to a taxon object, to specify which OTU
the observation refers to.

=item The L<Bio::Phylo::Matrices::Matrix> object

The matrix object is used to aggregate datum objects into a larger,
iterator object, which can be accessed using the methods of the
Bio::Phylo::Listable class.

=item The L<Bio::Phylo::Matrices> object

The top level opject in the Bio::Phylo::Matrices namespace is used
to contain multiple matrix or alignment objects, again implementing an
iterator interface.

=item The Bio::Phylo::Taxa::* namespace

Sets of taxa are modelled by the Bio::Phylo::Taxa object. It is
a container that holds Bio::Phylo::Taxa::Taxon objects. The taxon
objects at present provide no other functionality than to serve
as a means of crossreferencing nodes in trees, and datum or sequence
objects. This, however, is a very important feature. In order to 
be able to write, for example, files formatted for Mark Pagel's
Discrete, Continuous and Multistate programs a taxa object, a 
matrix and a tree object must be crossreferenced.

=item The L<Bio::Phylo::Taxa> object

The taxa object is analogous to a taxa block as implemented by
Mesquite (L<http://mesquiteproject.org>). Multiple matrix objects
and forests can be linked to a single taxa object, using 
C<$taxa->set_matrix( $matrix )>. Conversely,
the relationship from matrix to taxa and from forest to taxa is a 
one-to-one relationship.

=item The L<Bio::Phylo::Taxa::Taxon> object

Just as forests can be linked to taxa objects, so too can 
indidividual node and datum objects be linked to individual taxon
objects. Again, the taxon can hold references to multiple nodes
or multiple datum objects, but conversely there is a one-to-one
relationship. There is a constraint on these relationships:
a node can only refer to a taxon that belongs to a taxa object
that the forest object that contains the node references:

    

       YES!
  ______________    
 |FOREST        |  The taxon and node objects can
 |  __________  |  link to each other, because
 | |TREE      | |  their containers do also.
 | |  ______  | |  
 | | |NODE  | | |  
 | | |______| | |  
 | |_____^____| |                 
 |_______|______|              NO!       
      ^  |               ______________  
  ____|__|__            |FOREST 'B'    |  The taxon object 
 |TAXA   |  |           |  __________  |  cannot reference
 |  _____|  |           | |TREE      | |  forest 'A' while
 | |TAXON | |           | |  ______  | |  its container 
 | |______| |           | | |NODE  | | |  references forest
 |__________|           | | |______| | |  'B'. 
                        | |__________| |  
                        |______________|    ______________   
                             ^             |FOREST 'A'    |   
                         ____|_____        |  __________  |  
                        |TAXA      |       | |TREE      | |  
                        |  ______  |       | |  ______  | |  
                        | |TAXON |------------>|NODE  | | |  
                        | |______| |       | | |______| | |  
                        |__________|       | |__________| |  
                                           |______________|        
       


Trying to set the links in the example on the right will result in
errors: C<"Attempt to link X to taxon from wrong block">. 
So what happens if a taxon already links to a node in forest 
'A', and you link its enclosing taxa block to forest 'B'? The 
links at the taxon and node level will be removed, and the 
link between forest and taxa object will be enforced, yielding 
the warning C<"Reset X references from node objects to taxa 
outside taxa block">.

=back

=head2 Encapsulation

Unlike most other implementations of tree structures (or any
other perl objects) the Bio::Phylo objects are truly encapsulated:
Most perl objects are hash references, so in most cases you can
do C<$obj->{'key'} = 'value'>. Not so for Bio::Phylo. The objects
are implemented as 'InsideOut' objects. How they work exactly
is outside of the scope of this document, but the upshot as that
the state of an object can only be changed through its methods. 
This is a feature that helps keep the code base maintainable as
this project grows. Also, the way it is implemented is more 
memory-efficient and faster than the standard approach. The 
encapsulation forces users of this module to use the documented
interfaces of the objects. This, however, is a good thing: as long
as the interfaces stay the same, any code using Bio::Phylo will
continue to work, regardless of the implementation under the
surface.

=head2 'Is-a' relationships: Inheritance

The objects in Bio::Phylo are related in various ways. Some objects
inherit from superclasses. Hence the object I<is a> special
case of the superclass. This has important implications for the API:
the documentation for each class only lists the methods defined locally
in that class, not the methods of the superclasses. Therefore, many
objects can do much more than would seem from their local POD. Always
inspect the "SEE ALSO" section of any class's documentation to see if
there are superclasses where more functionality might be defined.

=head2 'Has-a' relationships

Some objects contain other objects. For example, a
L<Bio::Phylo::Forest::Tree> contains L<Bio::Phylo::Forest::Node>
objects, a matrix object holds datum objects, and so on.
The container objects all behave like L<Bio::Phylo::Listable>
objects: you can iterate over them (also recursively).
The contains / container relationships implemented by
Bio::Phylo are shown below:

=head1 CONTAINERS

      ______________     ________________
     |FOREST        |   |MATRICES        |
     |  __________  |   |  __________    |
     | |TREE      | |   | |MATRIX    |   |
     | |  ______  | |   | |  ______  |   |
     | | |NODE  | | |   | | |DATUM | |   |
     | | |______| | |   | | |______| |   |
     | |__________| |   | |__________|   |
     |______________|   |________________|
                        
      __________        
     |TAXA      |      
     |  ______  |     
     | |TAXON | |     
     | |______| |    
     |__________| 


=head1 ARGUMENT FORMATS

=head2 Named arguments when number of arguments >= 2.

When the number of arguments to a method call exceeds 1, named
arguments are used. The order in which the arguments are specified
doesn't matter, but the arguments must be all lower case and preceded
by a dash:

  use Bio::Phylo::Forest::Tree;

  my $node = Bio::Phylo::Forest::Tree->new(
      -name  => 'PHYLIP_1',
      -score => 123,
  );


=head2 Type checking

Argument type is always checked. Numbers are checked for being
numbers, names are checked for being sane strings, without '():;,'.
Objects are checked for type. Internally, Bio::Phylo never checks
type based on class name, for example using
C<$obj->isa('Some::Class')>. Instead, object identity is validated
using a system of constants defined in L<Bio::Phylo::Util::CONSTANT>.
If Bio::Phylo needs to test validate object type, it'll do something
like:

 use Bio::Phylo::Util::CONSTANT qw(:objecttypes);
 use Bio::Phylo::Forest::Node;
 my $node = Bio::Phylo::Forest::Node->new;
 print "It's a node!" if $node->_type == _NODE_;

Hence, Bio::Phylo uses a form of "duck typing" ("if it walks like a
duck, and quacks like a duck, it probably is a duck"), as opposed
to one that is based on inheritance from a java-like interface, as
is the convention in bioperl. Both systems have their advantages and
drawbacks, but luckily they can coexist side by without problems.

As a new feature, a utility function is provided that does this type
checking for you, returning true or throwing an exception (see below),
so that the following will either succeed or die (so you might want
to put it inside an eval{} block):

 if ( looks_like_object( $node, _NODE_ ) ) {
      # do something
 }

=head2 Constructor arguments

All mutators (i.e. setters, methods called set_*) for a class and its
superclasses can be accessed from the constructor. E.g. because the
Bio::Phylo superclass of object Bio::Phylo::Forest::Node has a
"set_name" method, you can pass the following to the constructor:

 use Bio::Phylo::Forest::Node;
 my $node = Bio::Phylo::Forest::Node->new( -name => "node1" );

The arguments will be passed up the inheritance tree, and will 
eventually be turned into method calls by the root class.

=head1 RETURN VALUES AND EXCEPTIONS

=head2 Retun values

Apart from scalar variables, all other return values are passed by
reference, either as a reference to an object or to an array.

=over

=item Lists returned as array references

Multiple return values are never returned as a list, always as an
array reference:

 my $nodes = $tree->get_entities;
 print ref $nodes;

 #prints ARRAY.

To receive nodes in C<@nodes>, dereference the returned array
reference (for clarity, all array dereferencing in this 
document is indicated by using braces in addition to this sigil):

 my @nodes = @{ $tree->get_entities };

=item Returns self on mutators

Mutator method calls always return the modified object, and so they
can be chained:

 $node->set_name('Homo_sapiens')->set_branch_length(0.2343);


=item False but defined return values

When a value requested through an Accessor hasn't been set, the return
value is C<undef>. Here you should take care how you test. For example:

 if ( ! $node->get_parent ) {
 	$root = $node;
 }

This works as expected - object references are always "true", so if
C<get_parent> returns "false", C<$node> has no parent - hence it must 
be the root. However:

 if ( ! $node->get_branch_length ) {

 	# is there really no branch length?
 	if ( defined $node->get_branch_length ) {

 		# perhaps there is, but of length 0.
 	}
 }

...warrants caution. Zero is evaluated as false-but-defined.

=back

=head2 Exceptions

The Bio::Phylo modules throw exceptions that subclass L<Exception::Class>.
Exceptions are thrown when something I<exceptional> has happened. Not when the
value requested through an accessor method is undefined. If a node has no
parent, C<undef> is returned. Usually, you will encounter exceptions in
response to invalid input.

=over

=item Trying/Catching exceptions

If some method call returns an exception, wrap the call inside an C<eval>
block. The error now becomes non-fatal:

 # try something:
 eval { $node->set_branch_length('a bad value'); };

 # handle exception, if any
 if ($@) {
    # do something, e.g.:
    print $@->trace->as_string; # <- $@ is an object!
 }

=item Stack traces

If an exception of a particular type is caught, you can print a stack trace
and find out what might have gone wrong starting from your script drilling
into the module code.

 # exception caught.
 if ( UNIVERSAL::isa( $@, 'Bio::Phylo::Util::Exceptions::BadNumber' ) ) {

    # prints stack trace in addition to error
    warn $@->error, "\n, $@->trace->as_string, "\n";

    # further metadata from exception object
    warn join ' ',  $@->euid, $@->egid, $@->uid, $@->gid, $@->pid, $@->time;
    exit;
 }

As a new feature (from v.0.17 onwards) exceptions have become more descriptive,
with a generic explanation of what the thrown exception class typically means
added to the error message, and stack traces are printed out by default.

=item Exception types

Several exception classes are defined. The type of the thrown exception should
give you a hint as to what might be wrong. The types are specified in the
L<Bio::Phylo::Util::Exceptions> perldoc.

=back

=head1 TO DO

Below is a list of things that hopefully will be implemented in future 
versions of Bio::Phylo.

=over

=item More DNA sequence methods

Such as $seq->complement;. This would imply larger constant translation 
tables, including various tables for mtDNA and so on. Will probably be 
implemented, must likely using BioPerl tools.

=item Databases

Implement/improve access to TreeBASE, TolWeb and other databases. This could
probably be done best using PhyloWS.

=item Tests

Test coverage is reasonable, but some of the newer features need to be exercised
more.

=item Interoperability with BioPerl

The eventual aim of the Bio::Phylo project is to glue together the phylogenetics 
aspects of BioPerl (L<http://www.bioperl.org>), L<Bio::NEXUS>.

=back

=head1 FORUM

CPAN hosts a discussion forum for Bio::Phylo. If you have trouble
using this module the discussion forum is a good place to start
posting questions (NOT bug reports, see below):
L<http://www.cpanforum.com/dist/Bio-Phylo>

=head1 BUGS

Please report any bugs or feature requests to C<< bug-bio-phylo@rt.cpan.org >>,
or through the web interface at
L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Bio-Phylo>. I will be notified,
and then you'll automatically be notified of progress on your bug as I make
changes.

=head1 AUTHORS

Rutger Vos, Aki Mimoto, Klaas Hartmann, Jason Caravas, Mark Jensen
and Chase Miller

=over

=item email: C<< rutgeraldo@gmail.com >>

=item web page: L<http://rutgervos.blogspot.com/>

=back

=head1 CITATION

If you use Bio::Phylo in published research, please cite it:

B<Rutger A Vos>, B<Jason Caravas>, B<Klaas Hartmann>, B<Mark A Jensen>
and B<Chase Miller>, 2011. Bio::Phylo - phyloinformatic analysis using Perl.
I<BMC Bioinformatics> B<12>:63.
L<http://dx.doi.org/10.1186/1471-2105-12-63>

=head1 COPYRIGHT & LICENSE

Copyright 2005-2010 Rutger A. Vos, All Rights Reserved. This program is free
software; you can redistribute it and/or modify it under the same terms as Perl
itself.

=cut
