NAME

Resources - handling application defaults in Perl.

SYNOPSIS

use Resources;

$res = new Resources;
$res = new Resources "resfile";

DESCRIPTION

Resources are a way to specify information of interest to program or packages.

Applications use resource files to specify and document the values of quantities or attributes of interest.

Resources can be loaded from or saved to resource files. Methods are provided to search, modify and create resources.

Packages use resources to hardwire in their code the default values for their attributes, along with documentation for the attibutes themselves.

Packages inherit resources when subclassed, and the resource names are updated dynamically to reflect a class hierarchy.

Methods are provided for interactive resource inspection and editing.

1. Resource inheritance

Package attributes are inherited from base and member classes, their names are dynamically updated to reflect the inheritance, and values specified in derived/container classes override those inherited from base/member classes.

More precisely, there a few rules governing the inheritance of resource names and values, and they will be explained by way of examples.

As far as resource names, the rules are:

Base class

If Vehicle has a "speed" property, then it can use a resource named "vehicle.speed" to specify its default value.

Derived class

If Car is a Vehicle, then Car has a "car.speed" resource automagically defined by inheritance from the base class.

Container class

If Car has a member object called Tire, and Tire has a "tire.pressure" resource, then Car inherits a "car.tire.pressure" resource from the member class.

Application class

All resources of Car objects used by a program named "race" have the prefix "race." prepended to their names, e.g. "race.car.speed", "race.car.tire.pressure", etc.

With regard to assigning values to resources, the rules are:

Specification in a file

Resources specified in a resource file always override hardcoded resources (with the exception of "hidden" resources, see below).

Inheritance

Resources defined in a derived class (like Car) override those specified in a base class. Likewise, resources defined in a container class override those specified in the members.

In the above example, a default value for "car.speed" in Car overrides the value of "vehicle.speed" in any Car object, otherwise "car.speed" assumes the value of "vehicle.speed". Same for "car.tire.pressure".

2. Resource Files.

A resource specification in a (text) resource file is a line of the form:

sequence: value

There may be any number of whitespaces between the name and the colon character, and between the colon and the value.

sequence can have four forms:

(1) word

A word not containing whitespaces, colons (':'), dots ('.') or asterisks ('*'), nor starting with an underscore ('_').

Or, recursively:

(2) word.sequence   
(3) word*sequence   
(4) *sequence

The asterisks in a resource name act as wildcards, matching any sequence of characters.

For cases (3) or (4) the word must be or match the current application class, otherwise the resource specification is silently ignored (this means that an applications loads from a file only its own resources, and those whose application class is a wildcard).

No distinction is made between uppercase and lowercase letters.

value can be:

An unadorned word or a quoted sequence of whitespace-separated words. Both single (' ') and double quotes quotes (" ") are allowed, and they must be paired.

Any constant scalar constructor in Perl, including anon references to constant arrays or hashes.

The special words yes, true, no, false (case insensitive) are treated as boolean resources and converted 1 and 0, unless they are quoted.

Examples of valid resource specifications:

car*brand       : Ferrari    # A word.
car.price       : 200K       # Another word
car.name        : '312 BB'   # A quoted sentence
car*runs*alot   : yes        # A boolean, converted to 1.
car*noise*lotsa : 'yes'      # yes, taken verbatim
car.size        : [1, [2, 3]]           # An anon array.
car.lett        : {"P"=>1, "Q"=>[2, 3]} # An anon hash.

Examples of illegal resource names:

car pedal    # Whitespace in the name.
.carpedal    # Leading dot in name.
car._pedal   # Leading underscore in _dog.
carpedal*    # Trailing asterisk.
carpedal.    # Trailing dot.

A resource file may contain comments: anything from a hash ('#') character to the end of a line is ignored, unless the hash character appears inside a quoted value string.

Resource specifications may be split across successive lines, by terminating the split lines with a backslash, as per cpp(1).

3. The Resources hash

A non-my hash named %Resources can be used to specify the default values for the attributes of a package in its source code, along with documentation for the attributes themselves. The documentation itself is "dynamical" (as opposed to the static, pod-like variety) in that it follows a class hyerarchy and is suitable for interactive display and editing.

The %Resources hash is just a hash of

$Name => [$Value, $Doc]

things. Each hash key $Name is a resource name in the above sequence form. Each hash value is a reference to an anon array [$Value, $Doc], with $Doc being an optional resource documentation.

The resource $Name cannot contain wildcard ('*') or colon (':') characters, nor start or end with a dot ('.'). Also, it must not be prefixed with the package name (since this is automatically prepended by the merge method, see below). Names starting with an underscore ('_') character are special in that they define "hidden" resources. These may not be specified in resource files, nor dynamically viewed/edited: they come handy to specify global parameters when you do not want to use global application-wide variables, and/or want to take advantage of the inheritance mechanism.

The resource $Value can be any constant scalar Perl constructor, including references to arrays and/or hashes of constants (or references thereof). Boolean values must be specified as 1 or 0.

The resource documentation is a just string of any length: it will be appropriately broken into lines for visualization purposes. It can also be missing, in which case an inherited documentation is used (if any exists, see the merge method below).

The content of a resource hash is registered in a global Resource object using the merge method.

Here is an example of deafults specification for a package.

     package Car;
     @ISA = qw( Vehicle );
     use vars qw( %Resources );

     %Resources = (
         brand    => ["FIAT", "The carmaker"],
         noise    => ["Ashtmatic", "Auditory feeling"],
         sucks    => [1, "Is it any good?"],
	 nuts     => [ { on => 2, off => [3, 5] }, "Spares"],
	 '_ghost' => [0, "Hidden. Mr. Invisible"] 
	 'tire.flat' => [0],
     );

The last line overrides a default in member class Tire. The corresponding doc string is supposedly in the source of that class. The last two hash keys are quoted because of the non alphanumeric characters in them.

4. Objects and resources

The recommended way to use resources with Perl objects is to pass a Resource object to the "new" method of a package. The method itself will merge the passed resources with the package defaults, and the passed resource will override the defaults where needed.

Resource inheritance via subclassing is then easily achieved via the merge method, as shown in the EXAMPLES section.

5. Methods in class Resources

5.1. Creation and initialization

new Resources ($resfile);

Creates a new resource database, initialized with the defaults for class Resources (see below for a list of them).

If a nonempty file name is specified in $resfile, it initializes the object with the content of the so named resource file. For safe (non overwriting) loading, see the load method below.

If the special file name "_RES_NODEFAULTS" is specified, the object is created completely empty, with not even the Resources class defaults in it.

Returns the new object, or undef in case of error.

load($resfile, $nonew);

Loads resources from a file named $resfile into a resource database.

The $nonew argument controls whether loading of non already defined resurces is allowed. If it is true, safe loading is performed: attempting to load non-wildcarded resource names that do not match those already present in the database causes an error. This can be useful if you want to make sure that only pre-defined resources (for which you presumably have hardwired defaults) are loaded. It can be a safety net against typos in a resource file.

Use is made of Safe::reval to parse values specified through Perl constructors (only constants, anon hashes and anon arrays are allowed).

Returns 1 if ok, 0 if error.

merge($class, @memberclasses);

Merges the %Resources hash of the package defining $class with those of its @memberclasses, writing the result in the resource database.

The merging reflects the resource inheritance explained above: the %Resources of all base classes and member classes of $class are inherited along the way. Eventually all these resources have their names prefixed with the name of the package in which $class is defined (lowercased and stripped of all foo::bar:: prefixes), and with the application class as well.

In the above example, the defaults of a Car object will be renamed, after merging as:

car.brand, car.noise, ..., 
car.tire.flat

and for a Civic object, where Civic is a (i.e. ISA) Car, they will be translated instead as

civic.brand, civic.noise, ..., 
civic.tire.flat

Finally, the application name ($0, a.k.a $PROGRAM_NAME in English) is prepended to all resource names, so, if the above Civic package is used by a Perl script named "ilove.pl", the final names after merging are

ilove.civic.brand, ilove.civic.noise, ..., 
ilove.civic.tire.flat

The new names are the ones to use when accessing these resources by name.

The resource values are inherited accoring to the rules previously indicated, hence with resource files having priority over hardcoded defaults, nnd derived or container classes over base or member classes.

Returns 1 if for success, otherwise 0.

5.2. Looking up resources

The values and documentation strings stored in a Resource object can be accessed by specifying their names in three basic ways:

directly ("byname" methods)

As in "my.nice.cosy.couch" .

by a pattern ("bypattern" methods)

As in "m??nice.*" .

hierarchically ("byclass" methods)

If class Nice is a Cosy, then asking for "couch" in package Cosy gets you the value/doc of "my.couch". If, instead, Nice has a Cosy member, that the method gets you "my.nice.cosy.couch". This behaviour is essential for the proper initialization of subclassed and member packages, as explained in detail below.

It is also possible to retrieve the whole content of a resource database ("names" and "each" methods)

Note that all the resource lookup methods return named (non "wildcarded") resources only. Wildcarded resources (i.e. those specified in resource files, and whose names contain one or more '*') are best thought as placeholders, to be used when the value of an actual named resource is set.

For example, a line in a resource file like

*background : yellow

fixes to yellow the color of all resources whose name ends with "background". However, your actual packages will never worry about unless they really need a background. In this case they either have a "background" resource in their defaults hash, or subclass a package that has one.

valbyname($name);

Retrieves the value of a named resource from a Resource database. The $name argument is a string containing a resource name with no wildcards.

Returns the undefined value if no such resource is defined.

docbyname($name);

Retrieves the documentation string of a named resource from a Resource database. The $name argument is a string containing a resource name with no wildcards.

Returns the undefined value if no such resource is defined.

bypattern($pattern);

Retrieves the full names, values and documentation strings of all the named (non wildcarded) resources whose name matches the given $pattern. The pattern itself is string containing a Perl regular expression, not enclosed in slashes.

Returns a new Resource object containing only the matching resources, or the undefined value if no matches are found.

valbypattern($pattern);

Retrieves the full names and values of all named (non wildcarded) resources whose name matches the given pattern.

Returns a new Resource object containing only names and values of the matching resources (i.e. with undefined doc strings), or the undefined value if no matches are found.

docbypattern($pattern);

Retrieves the full names and documentation strings of all named (non wildcarded) resources whose name matches the given pattern.

Returns a new Resource object containing only names and docs of the matching resources (i.e. with undefined resource values), or the undefined value if no matches are found.

byclass($object, $suffix);

To properly initialize the attributes of a package via resources we need a way to know whether the package defaults (contained in its %Resources hash) have been overridden by a derived or container class. For example, to set a field like $dog->{Weight} in a Dog object, we must know if this $dog is being subclassed by Poodle or Bulldog, or if it is a member of Family, since all these other classes might override whatever "weight" default is defined in the %Resources hash of Dog.pm.

This information must of course be gathered at runtime: if you tried to name explicitly a resource like "family.dog.weight" inside Dog.pm all the OOP crowd would start booing at you. Your object would not be reusable anymore, being explicitly tied to a particular container class. After all we do use objects mainly because we want to easily reuse code...

Enter the "by class" resource lookup methods: byclass, valbyclass and docbyclass.

Given an $object and a resource $suffix (i.e. a resource name stripped of all container and derived class prefixes), the byclass method returns a 3 element list containing the name/value/doc of that resource in $object. The returned name will be fully qualified with all derived/container classes, up to the application class.

For example, in a program called "bark", the statements

$dog = new Dog ($res); # $res is a Resources database
($name,$value,$doc) = $res->byclass($dog, "weight");

will set $name, $value and $doc equal to those of the "bark.poodle.weight" resource, if this Dog is subclassed by Poodle, and to those of "bark.family.dog.weight", if it is a member of Family instead.

The passed name suffix must not contain wildcards nor dots.

Be careful not to confuse the "byclass" with the "byname" and "bypattern" retrieval methods: they are used for two radically different goals. See the EXAMPLES section for more.

Returns the empty list if no resources are found for the given suffix, or if the suffix is incorrect.

namebyclass($obj, $suffix);

As the byclass method above, but returns just the resource name (i.e. the suffix with all the subclasses prepended).

valbyclass($obj, $suffix);

As the byclass method above, but returns just the resource value.

docbyclass($suffix);

As the byclass method above, but returns just the resource documentation.

each;

Returns the next name/[value,doc] pair of the named (non wildcarded) resources in a resource database, exactly as the each Perl routine.

names;

Returns a list of the names of all named (non-wildcarded) resources in a resource database, or undef if the databasee is empty.

5.3. Assigning and removing Resources

put($name, $value, $doc);

Writes the value and doc of a resource in the database. It is possible to specify an empty documentation string, but name and value must be defined.

Wildcards ('*' characters) are allowed in the $name, but the $doc is ignored in this case (documentation is intended for single resources, not for sets of them).

The value is written unchanged unless the resource database already contains a wildcarded resource whose name includes $name (foo*bar includes foo.bar, foo.baz.bar, etc.). In this case the value of the wildcarded resource overrides the passed $value.

Returns 1 if ok, 0 if error.

removebyname($name);

Removes the named (non wildcarded) resources from the database.

Returns 1 if OK, 0 if the resource is not found in the database.

removebypattern($name);

Removes from the database all resources (both named and wildcarded) whose name mathes $pattern. An exactly matching name must be specified for wildcarded resources (foo*bar to remove foo*bar).

Returns the number of removed resources.

5.6. Viewing and editing resources.

view;

Outputs the current content of a Resource object by piping to a pager program.

The environment variable $ENV{RESPAGER}, the resource "resources.pager" and the environment variable $ENV{PAGER} are looked up, in this very order, to find the pager program. Defaults to /bin/more if none of them is found.

The output format is the same of a resource file, with the resource names alphabetically ordered, and the resource documentation strings written as comments.

Returns 1 if ok, 0 if error.

edit($nonew);

Provides dynamical resource editing of a Resource object via an external editor program. Only resource names and values can be edited (anyway, what is the point of editing a resource comment on the fly?).

The environment variables $ENV{RESEDITOR}, the resource "resouces.editor", the environment variables $ENV{VISUAL} and $ENV{EDITOR} are looked up, in this very order, to find the editor program. Defaults to /bin/vi if none is found.

The editor buffer is initialized in the same format of a resource file, with the resource names alphabetically ordered, and the resource documentation strings written as comments. The temporary file specified by the "resources.tmpfil" resource is used to initialize the editor, or '/tmp/resedit<pid>' if that resource is undefined.

When the editor is exited (after saving the buffer) the method attempts to reload the edited resources. If an error is found the initial object is left unchanged, a warning with the first offending line in the file is printed, and the method returns with undef. Controlled resource loading is obtained by specifying a true value for the $nonew argument (see load).

If the loading is successful, a new (edited) resource object is returned, which can be assigned to the old one for replacement.

After a successful edit, the value of the resource "resources.updates" (which is always defined to 0 whenever a new resource is created) is increased by one. This is meant to notify program the and/or packages of the resource change, so they can proceed accordingly if they wish.

5.5. Miscellaneous methods

write($filename);

Outputs all resources of a resource database into a resource file (overwriting it).

The resource documentation strings are normally written as comments, so the file itself is immediately available for resource loading. However, if the boolean resource "resources.writepod" is true, then the (non wildcarded) resources are output in POD format for your documentational pleasure.

As usual in Perl, the filename can allo be of the form "|command", in which case the output is piped into "comma1nd".

For resources whose value is a reference to an anon array or hash, it produces the appropriate constant Perl contructor by reverse parsing. The parser itself is available as a separate method named _parse (see package source for documentation).

Returns 1 if ok, 0 if error.

5. Resources of Resources

As you may have guessed at this point, the default configuration of this package itself is defined by resources. The resource class is, of course, "resources" (meaning that all the names have a leading "resources.").

To prevent chaos, however, these resources cannot be subclassed. This should not be a problem in normal applications, since the Resource package itself is not meant to be subclassed, but to help building a hierarchy of classes instead.

The currently recognized resources, and their default values, are:

resources.appclass : "$PROGRAM_NAME"

The application name of this Resource object.

resources.editor : /bin/vi

Resource editor.

resources.mergeclass : true

Boolean. True to merge with class inheritance.

resources.pager : /bin/more

Resource pager.

resources.separator : ':'

Pattern separating names from values in resource files.

resources.tmpfil : ''

Editor temporary file.

resources.updates : 0

Number of resource updates.

resources.verbosity : 1

True to print warnings.

resources.viewcols : 78

Width of view/edit window.

resources.writepod : false

Boolean. True if the write method should output in POD format.

EXAMPLES

Here is a more complex example of hyerarchical inheritance of resources. Package HotDog is subclassed from Junk and Food. The subclass has defaults for two resources defined by the base classes ("food.edible" and "junk.germs"), and their values will override the base class defaults.

Remember that after merging all resources names are prefixed with the current class name.

   use Resources;
   package Food;
   %FoodDefaults = ( edible => "Sure" );
   
   sub new {
      my ($pack, $res) = @_;
      $res = new Resources unless $resources;
      $res->merge(\%FoodDefaults) || die ("can't merge defaults");
       
      my $food= bless {};
      $food->{Edible} = $res->valbyclass("edible");
      # Use valbyclass so a subclass like HotDog can change this by its
      # defaults.   
   }
 
   # A Food method to say if it can be eaten.
   sub eat { 
      my $food=shift; 
      return $food->{Edible}; 
   }

   package HotDog;
   use Junk;
   use Food;
   use Carp;
   @ISA = qw( Junk Food );

   %HotDogDefaults = 
      (
       taste         => "Yuck!",
       meatcolor     => "Cadaveric",
       stinks        => 1,
       'food.edible' => "Perhaps", # Quotes needed in these 
       'junk.germs'  => "Amoeba"   #    names because of the dots
      );

   sub new {
      my ($package, $res) = @_;
      
      $res = new Resources unless $resources;
      $res->merge(\%HotDogDefaults) || die ("can't merge defaults");
	       
      my $food   = new Food ($res); # Override Food's edible.
      my $junk   = new Junk ($res); # Override Junks's germs.
      my $hd = bless { %{food}, %{junk} }; # Merge and subclass.

      $hd->{Meatcolor} = $res->valbyclass("meatcolor");
      # Same reason as above

      return $hd;
   }

   # A HotDog method to get a taste via resources
   sub taste { 
      my ($hd, $res) = @_; 
      print $res->valbyname("hotdog.taste");
   }

   package main;
   # Whatever
   #
   $res = new Resources("AppDefltFile") || die;
   $snack = new HotDog($res);  
   $gnam = $snack->eat();  # hotdog.food.edible overridees food.edible, 
                           #   so here $gnam equals "Perhaps"

   $slurp = $snack->taste($res) # $slurp equals "Yuck!", unless 
                                # this resource too was overridden 
                                # by a subclass of HotDog , or
                                # differently specified in 
                                # "AppDefltFile"

Note the different use of valbyclass and valbyname to access resources:

valbyclass

Is used for the resources like "edible" in Food, i.e. resources which are defined by the current class in its defaults. This because a subclass of it (e.g. HotDog) might have overridden them (by having an entry for "food.edible" in its defaults.

Generally speaking, you should not use valbyclass outside the new (creation and initialization) method of a class.

valbyname

Is used after the initialization, when all subclassed resources have been resolved nicely. In any case, its arguments is the complete resource name, with all its dots.

SEE ALSO

Safe(3).

BUGS

The underlying idea is to use a centralized resource database for the whole application. This ensures uniformity of behaviour across kin objects, but allow special characterizations only at the cost of subclassing.

AUTHOR

	Francesco Callari <franco@cim.mcgill.ca> 
	Artifical Perception Laboratory,
	Center for Intelligent Machines, 
	McGill University.

        WWW: http://www.cim.mcgill.ca/~franco/Home.html

COPYRIGHT

Copyright 1996 Francesco Callari, McGill University

Permission to use, copy, modify, and distribute this software and its documentation for any purpose without fee is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of McGill not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. McGill makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

MCGILL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL MCGILL BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

cpanm Resources

perl -MCPAN -e shell
install Resources