Christopher Charles Cavnor > autoclass_v1_01 > Class::AutoClass

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DESCRIPTION ^

    1) get and set methods for simple attributes can be automatically
    generated

    2) argument lists are handled as described below

    3) the protocol for object creation and initialization is close to
    the 'textbook' approach generally suggested for object-oriented Perl
    (see below)

    4) object initialization is handled correctly in the presence of multiple inheritance

    @AUTO_ATTRIBUTES is a list of 'attribute' names: get and set methods
    are created for each attribute.  By default, the name of the method
    is identical to the attribute (but see $CASE below).  Values of
    attributes can be set via the 'new' constructor, %DEFAULTS, or the 
    'set' method as discussed below.

    @CLASS_ATTRIBUTES is a list of class attributes: get and set methods
    are created for each attribute. By default, the name of the method
    is identical to the attribute (but see $CASE below). Values of
    attributes can be set via the 'new' constructor, %DEFAULTS (initialized 
    at "declare time" (when the declare function is called) versus instance
    attributes, which are of course initialized at runtime), standard 
    class variable access syntax ($PackageName::AttributeName), or the 
    'set' method as discussed below. Normal inheritance rules apply to
    class attributes (but of course, instances of the same class share
        the same class variable).

    @OTHER_ATTRIBUTES is a list of attributes for which get and set
    methods are NOT generated, but whose values can be set via the 'new'
    constructor or the 'set' method as discussed below.

    %SYNONYMS is a hash that defines synonyms for attribues. Each entry
    is of the form 'new_attribute_name'=>'old_attribute_name'. get and
    set methods are generated for the new names; these methods simply
    call the method for the old name.
    
    %DEFAULTS is a hash that defines default values for attributes. Each 
    entry is of the form 'attribute_name'=>'default_value'. get and
    set methods are generated for each attributes.

    $CASE controls whether additional methods are generated with all
    upper or all lower case names.  It should be a string containing the
    strings 'upper' or 'lower' (case insenstive) if the desired case is
    desired.

    The declare function actually generates the method.
    This should be called once and no where else.

    AutoClass must be the first class in @ISA !! As usual, you create
    objects by calling 'new'. Since AutoClass is the first class in @ISA,
    it's 'new' method is the one that's called.  AutoClass's 'new'
examines the rest of @ISA and searches for a superclass that is
capable of creating the object.  If no such superclass is found,
AutoClass creates the object itself.  Once the object is created,
AutoClass arranges to have all subclasses run their initialization
methods (_init_self) in a top-down order.

Argument Processing

We support positional and keyword argument lists, but we strongly urge that each method pick one form or the other, as the combination is inherently ambiguous (see below).

Consider a method, foo, that takes two arguments, a first name and a last_name name. The positional form might be

  $object->foo('Nat', 'Goodman')

while the keyword form might be

  $object->foo(first_name=>'Nat', last_name=>'Goodman')

In keyword form, keywords are insensitive to case and leading dashes: the keywords

  first_name, -first_name, -FIRST_NAME, --FIRST_NAME, First_Name, -First_Name

are all equivalent. Internally, for those who care, our convention is to use uppercase, un-dashed keys for the attributes of an object.

We convert repeated keyword arguments into an ARRAY ref of the values. Thus:

  $object->foo(first_name=>'Nat', first_name=>'Nathan')

is equivalent to

  $object->foo(first_name=>['Nat', 'Nathan'])

Keyword arguments can be specified via ARRAY or HASH refs which are dereferenced back to their elements, e.g.,

  $object->foo([first_name=>'Nat', last_name=>'Goodman'])

  $object->foo({first_name=>'Nat', last_name=>'Goodman'})

are both equivalent to

  $object->foo(first_name=>'Nat', last_name=>'Goodman')

We can get away with this, because we encourage method writers to choose between positional and keyword argument lists. If a method uses positional arguments, it will interpret

  $object->foo($array)

as a call that is setting the first_name parameter to $array, while if it uses keyword arguments, it will dereference the array to a list of keyword, value pairs.

We also allow the argument list to be an object. This is often used in new to accomplish what a C++ programmer would call a cast. In simple cases, the object is just treated as a HASH ref and its attributes are passed to a the method as keyword, value pairs.

Why the Combination of Positional and Keyword Forms is Ambiguous

The keyword => value notation is just a Perl shorthand for stating two list members with the first one quoted. Thus,

  $object->foo(first_name=>'Nat', last_name=>'Goodman')

is completely equivalent to

  $object->foo('first_name', 'Nat', 'last_name', 'Goodman')

The ambiguity of allowing both positional and keyword forms should now be apparent. In this example,

  $object->foo('first_name', 'Nat')

there is s no way to tell whether the program is calling foo with the first_name parameter set to the value 'first_name' and the last_name parameter set to 'Nat', vs. calling foo with the first_name parameter set to 'Nat' and the last_name parameter left undefined.

If a program wishes to permit both forms, we suggest that keywords be required to start with '-' (and that values do not start with '-'). Obviously, this is not fully general. We provide a method, _is_positional, that checks this convention. Subclasses are free to ignore this.

Protocol for Object Creation and Initializaton

We expect objects to be created by invoking new on its class. For example

  $object = new SomeClass(first=>'Nat', last=>'Goodman')

To correctly initialize objects that participate in multiple inheritance, we use a technqiue described in Chapter 10 of Paul Fenwick's excellent tutorial on Object Oriented Perl (see http://perltraining.com.au/notes/perloo.pdf). (We experimented with Damian Conway's interesting NEXT pseudo-pseudo-class discussed in Chapter 11 of Fenwick's tutorial available in CPAN at http://search.cpan.org/author/DCONWAY/NEXT-0.50/lib/NEXT.pm, but could not get it to traverse the inheritance structure in the correct, top-down order.)

    AutoClass class provides a 'new' method that expects a keyword argument
    list.  This method processes the argument list as discussed in
    L<Argument Processing>: it figures out the syntactic form (list of
                                                               keyword, value pairs, vs. ARRAY ref vs. HASH ref, etc.).  It then
    converts the argument list into a canonical form, which is a list of
    keyword, value pairs with all keywords uppercased and de-dashed.  Once
    the argument list is in this form, subsequent code treats it as a HASH
    ref.

AutoClass::new initializes the object's class structure from top to bottom, and is careful to initialize each class exactly once even in the presence of multiple inheritance. The net effect is that objects are initialized top-down as expected; a subclass object can assume that all superior classes are initialized by the time subclass initialization occurs.

AutoClass automatically initializes attributes and synonyms declared when the class is defined. If additional initialization is required, the class writer can provide an _init_self method. _init_self is called after all superclasses are initialized and after the automatic initialization for the class has been done.

AutoClass initializes attributes and synonyms by calling the set methods for these elements with the like-named parameter -- it does not simply slam the parameter into a slot in the object''s HASH. This allows the class writer implement non-standard initialization within the set method.

The main case where a subclass needs its own 'new' method is if it wishes to allow positional arguments. In this case, the subclass 'new' is responsible for is responsible for recognizing that positional arguments are being used (if the class permits keyword arguments also), and converting the positional arguments into keyword, value form. At this point, the method can simply call AutoClass::new with the converted argument list.

The subclass should not generally call SUPER::new as this would force redundant argument processing in any super-class that also has its own new. It would also force the super-class new to be smart enough to handle positional as well as keyword parameters, which as we've noted is inherently ambiguous.

    =head1 KNOWN BUGS AND CAVEATS

    This is still a work in progress.  

    =head2 Bugs, Caveats, and ToDos

    1) There is no way to manipulate the arguments that are sent to the
    real base class. There should be a way to specify a subroutine that
    reformats these if needed.

    2) DESTROY not handled

    3) Autogeneration of methods is hand crafted.  It may be better to
    use Class::MakeMethods or Damian Conway's Multimethod class for
  doing signature-based method dispatch
  
  4) Caveat: In order to specify that a class that uses AutoClass should return
  undef (versus an uninitialized (but blessed) object), one need to set:
  $self->{__NULLIFY__}=1;

AUTHOR - Nat Goodman ^

Email natg@shore.net

MAINTAINER - Christopher Cavnor ^

Email ccavnor@systemsbiology.net

COPYRIGHT ^

Copyright (c) 2003 Institute for Systems Biology (ISB). All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

APPENDIX ^

The rest of the documentation describes the methods. Note that internal methods are preceded with _

new

 Title   : new
 Usage   : $object=new Foo(first_name=>'Nat', last_name=>'Goodman')
           where Foo is a subclass of AutoClass
 Function: Create and initialize object
 Returns : New object of class $class
 Args    : Any arguments needed by subclasses
         -->> Arguments must be in keyword form.  See DESCRIPTION for more.
 Notes   : Tries to invoke superclass to actually create the object

_init

 Title   : _init
 Usage   : $self->_init($class,$args)
 Function: Initialize new object
 Returns : nothing useful
 Args    : $class -- lexical (static) class being initialized, not the
           actual (dynamic) class of $self
           $arg -- argument list in canonical keyword form
 Notes   : Adapted from Chapter 10 of Paul Fenwick''s excellent tutorial on 
           Object Oriented Perl (see http://perltraining.com.au/notes/perloo.pdf).

set

 Title   : set
 Usage   : $self->set(-first_name=>'Nat',-last_name=>'Goodman')
 Function: Set multiple attributes in existing object
 Args    : Parameter list in same format as for new
 Returns : nothing

set_attributes

 Title   : set_attributes
 Usage   : $self->set_attributes([qw(first_name last_name)],$args)
 Function: Set multiple attributes from a Class::AutoClass::Args object
           Any attribute value that is present in $args is set
 Args    : ARRAY ref of attributes
           Class::AutoClass::Args object
 Returns : nothing

get

 Title   : get
 Usage   : ($first,$last)=$self->get(qw(-first_name,-last_name))
 Function: Get values for multiple attributes
 Args    : Attribute names
 Returns : List of attribute values

AUTO_ATTRIBUTES

 Title   : AUTO_ATTRIBUTES
 Usage   : @auto_attributes=AUTO_ATTRIBUTES('SubClass')
           @auto_attributes=$self->AUTO_ATTRIBUTES();
 Function: Get @AUTO_ATTRIBUTES for lexical class.
           @AUTO_ATTRIBUTES is defined by class writer. These are attributes for which get and set methods
           are automatically generated.  _init automatically
           initializes these attributes from like-named parameters in
           the argument list
 Args : class

OTHER_ATTRIBUTES

 Title   : OTHER_ATTRIBUTES
 Usage   : @other_attributes=OTHER_ATTRIBUTES('SubClass')
           @other_attributes=$self->OTHER_ATTRIBUTES();
 Function: Get @OTHER_ATTRIBUTES for lexical class.
           @OTHER_ATTRIBUTES is defined by class writer. These are attributes for which get and set methods
           are not automatically generated.  _init automatically
           initializes these attributes from like-named parameters in
           the argument list
 Args : class

SYNONYMS

 Title   : SYNONYMS
 Usage   : %synonyms=SYNONYMS('SubClass')
           %synonyms=$self->SYNONYMS();
 Function: Get %SYNONYMS for lexical class.
           %SYNONYMS is defined by class writer. These are alternate names for attributes generally
           defined in superclasses.  get and set methods are
           automatically generated.  _init automatically initializes
           these attributes from like-named parameters in the argument
           list
 Args : class

declare

 Title   : declare
 Usage   : @AUTO_ATTRIBUTES=qw(sex address dob);
           @OTHER_ATTRIBUTES=qw(age);
           %SYNONYMS=(name=>'id');
               AutoClass::declare(__PACKAGE__,'lower|upper');
 Function: Generate get and set methods for simple attributes and synonyms.
           Method names are identical to the attribute names including case
 Returns : nothing
 Args    : lexical class being created -- should always be __PACKAGE__
           ARRAY ref of attributes
           HASH ref of synonyms. Keys are new names, values are old
           code that indicates whether method should also be generated
            with all lower or upper case names

_enumerate

 Title   : _enumerate
 Usage   : _enumerate($class);
 Function: locates classes that have a callable constructor
 Args    : a class reference
 Returns : list of internal classes, a class with a callable constructor 

_fix_args

 Title   : _fix_args
 Usage   : $args=_fix_args(-name=>'Nat',-name=>Goodman,address=>'Seattle')
           $args=$self->_fix_args(@args)

 Function: Convert argument list into canonical form.  This is a HASH ref in 
           which keys are uppercase with no leading dash, and repeated
           keyword arguments are merged into an ARRAY ref.  In the
           example above, the argument list would be converted to this
           hash
              (NAME=>['Nat', 'Goodman'],ADDRESS=>'Seattle')
 Returns : argument list in canonical form
 Args    : argument list in any keyword form

_fix_keyword

 Title   : _fix_keyword
 Usage   : $keyword=_fix_keyword('-name')
           @keywords=_fix_keyword('-name','-address');
 Function: Convert a keyword or list of keywords into canonical form. This
           is uppercase with no leading dash.  In the example above,
           '-name' would be converted to 'NAME'. Non-scalars are left
           unchanged.
 Returns : keyword or list of keywords in canonical form 
 Args : keyword or list of keywords

_set_attributes

 Title   : _set_attributes
 Usage   :   my %synonyms=SYNONYMS($class);
             my $attributes=[AUTO_ATTRIBUTES($class),
                             OTHER_ATTRIBUTES($class),
                             keys %synonyms];
             $self->_set_attributes($attributes,$args);
 Function: Set a list of simple attributes from a canonical argument list
 Returns : nothing
 Args    : $attributes -- ARRAY ref of attributes to be set
           $args -- argument list in canonical keyword (hash) form 
 Notes   : The function calls the set method for each attribute passing 
           it the like-named parameter from the argument list

_is_positional

 Title   : _is_positional
 Usage  : if (_is_positional(@args)) {
             ($arg1,$arg2,$arg3)=@args; 
           }
 Function: Checks whether an argument list conforms to our convention 
           for positional arguments. The function returns true if 
           (1) the argument list has an odd number of elements, or
           (2) the first argument starts with a dash ('-').
           Obviously, this is not fully general.
 Returns : boolean
 Args    : argument list
 Notes   : As explained in DESCRIPTION, we recommend that methods not 
           support both positional and keyford argument lists, as this 
           is inherently ambiguous.
 BUGS    : NOT YET TESTED in this version

set_class_defaults

 Title   : set_class_defaults
 Usage   : $self->set_class_defaults($attributes,$class,$args);
 Function: Set default values for class argument
 Args    : reference to the class and a Class::AutoClass::Args object
           which contains the arguments to set
 Returns : nothing
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