NAME

Config::Model::Value - Strongly typed configuration value

SYNOPSIS

my $model = Config::Model->new() ;
$model ->create_config_class 
 (
  name => "SomeClass",
  element => [
    country  => { type =>       'leaf',
                  value_type => 'enum',
                  choice =>      [qw/France US/]
                },
    president => { type =>        'leaf',
                   value_type => 'string',
                   warp => [ '- country', 
                            France => { default => 'Chirac' },
                            US     => { default => 'Bush' }]
                 },
    ]
 );

DESCRIPTION

This class provides a way to specify configuration value with the following properties:

• Strongly typed scalar: the value can either be an enumerated type, a boolean, a number, an integer or a string

• default parameter: a value can have a default value specified during the construction.

• built-in default parameter: specifies the default value that is built in the application to be configured. This built-in default value will not written in the configuration files. Only the fetch_standard method will return the built-in default value. This may be used for audit purpose.

• mandatory value: reading a mandatory value will raise an exception if the value is not specified and has no default value.

• dynamic change of property: A slave value can be registered to another master value so that the properties of the slave value can change according to the value of the master value. For instance, paper size value can be 'letter' for country 'US' and 'A4' for country 'France'.

• A reference to the Id of a hash of list element. In other word, the value is an enumerated type where the possible values (choice) is defined by the existing keys of a has element somewhere in the tree. See "Value Reference".

Default values

There are several kind of default values. They depend on where these values are defined (or found).

From the lowest default level to the "highest":

• built_in: The value is knows in the application, but is not written in the configuration file.

• default: The value is known by the model, but not by the application. This value must be written in the configuration file.

• computed: The value is computed from other configuration elements. This value must be written in the configuration file.

• preset: The value is not known by the model or by the application. But it can be found by an automatic program and stored while the configuration Config::Model::Instance is in Config::Model::Instance/"preset_start ()"

Then there is the value entered by the user. This will override all kind of "default" value.

The fetch_standard function will return the "highest" level of default value, but will not return a custom value, i.e. a value entered by the user.

Constructor

Value object should not be created directly.

Value model declaration

A leaf element must be declared with the following parameters:

value_type

Either boolean, enum, integer, number, uniline, string. Mandatory. See "Value types".

default

Specify the default value (optional)

built_in

Specify a built in default value (optional)

compute

Will compute a value according to a formula and other values. By default a computed value cannot be set. See Config::Model::ValueComputer for computed value declaration.

convert => [uc | lc ]

When stored, the value will be converted to uppercase (uc) or lowercase (lc).

min

Specify the minimum value (optional, only for integer, number)

max

Specify the maximum value (optional, only for integer, number)

mandatory

Set to 1 if the configuration value must be set by the configuration user (default: 0)

choice

Array ref of the possible value of an enum. Example :

choice => [ qw/foo bar/]

replace

Hash ref. Used for enum to substitute one value with another. This parameter must be used to enable user to upgrade a configuration with obsolete values. For instance, if the value foo is obsolete and replaced by foo_better, you will need to declare:

replace => { foo => 'foo_better' }

refer_to

Specify a path to an id element used as a reference. See "Value Reference" for details.

computed_refer_to

Specify a pathto an id element used as a computed reference. See "Value Reference" for details.

warp

See section below: "Warp: dynamic value configuration".

help

You may provide detailed description on possible values with a hash ref. Example:

help => { oui => "French for 'yes'", non => "French for 'no'"}

Value types

This modules can check several value types:

boolean

Accepts values 1 or 0, yes or no, true or false. The value read back is always 1 or 0.

enum

Enum choices must be specified by the choice parameter.

integer

Enable positive or negative integer

number

The value can be a decimal number

uniline

A one line string. I.e without "\n" in it.

string

Actually, no check is performed with this type.

reference

Like an enum where the possible values (aka choice) is defined by another location if the configuration tree. See "Value Reference".

Warp: dynamic value configuration

The Warp functionality enable a Value object to change its properties (i.e. default value or its type) dynamically according to the value of another Value object locate elsewhere in the configuration tree. (See Config::Model::WarpedThing for an explanation on warp mechanism).

For instance if you declare 2 Value element this way:

$model ->create_config_class (
  name => "TV_config_class",
  element => [
    country => {
      type => 'leaf',
      value_type => 'enum', 
      choice => [qw/US Europe Japan/]
    },
    tv_standard => {
      type => 'leaf',
      value_type => 'enum',
      choice => [qw/PAL NTSC SECAM/]  
      warp => { follow => '- country', # this points to the warp master
                rules => { US     => { default => 'NTSC'  },
                           France => { default => 'SECAM' },
                           Japan  => { default => 'NTSC'  },
                           Europe => { default => 'PAL'   },
                         }
              }
      ],
    },
  ]
 );

Setting country element to US will mean that tv_standard has a default value set to NTSC by the warp mechanism.

Likewise, the warp mechanism enables you to dynamically change the possible values of an enum element:

state => {
     type => 'leaf',
     value_type => 'enum', # example is admittedly silly
     warp => [ follow => '- country',
               rules => { US     => { choice => ['Kansas', 'Texas'    ]},
                          Europe => { choice => ['France', 'Spain'    ]},
                          Japan  => { choice => ['Honshu', 'Hokkaido' ]}
                        }
     ]
}

Note that the state element is not available while country is undefined.

As syntactic sugar, similar rules can be grouped within an array ref instead of a hash ref. I.e., you can specify

rules => [ 
           [qw/UK Germany Italy/] => { default => 'PAL'  },
           US     => { default => 'NTSC'  },
         ]

instead of :

rules => { 
           UK      => { default => 'PAL'  },
           Germany => { default => 'PAL'  },
           Italy   => { default => 'PAL'  },
           US      => { default => 'NTSC'  },
         }

Cascaded warping

Warping value can be cascaded: A can be warped by B which can be warped by C. But this feature should be avoided since it can lead to a model very hard to debug. Bear in mind that:

• Warp loop are not detected and will end up in "deep recursion subroutine" failures.

• If you declare "diamond" shaped warp dependencies, the results will depend on the order of the warp algorithm and can be unpredictable.

• The keys declared in the warp rules (US, Europe and Japan in the example above) cannot be checked at start time against the warp master Value. So a wrong warp rule key will be silently ignored during start up and will fail at run time.

Value Reference

To set up an enumerated value where the possible choice depends on the key of a Config::Model::AnyId object, you must:

• Set value_type to reference.

• Specify the refer_to or computed_refer_to parameter. See refer_to parameter.

In this case, a IdElementReference object is created to handle the relation between this value object and the refered Id. See Config::Model::IdElementReference for details.

Introspection methods

The following methods returns the current value of the parameter of the value object (as declared in the model unless they were warped):

min

max

mandatory

choice

convert

value_type

default

built_in

index_value

element_name

name()

Returns the object name.

get_type

Returns leaf.

can_store()

Returns true if the value object can be assigned to. Return 0 for a read-only value (i.e. a computed value with no override allowed).

get_choice()

Query legal values (only for enum types). Return an array (possibly empty).

get_help ( [ on_value ] )

Returns the help strings passed to the constructor.

With on_value parameter, returns the help string dedicated to the passed value or undef.

Without parameter returns a hash ref that contains all the help strings.

check( value , [ 0 | 1 ] )

Check if the value is acceptable or not.

When the 2nd parameter is non null, check will not try to get extra informations from the tree. This is required in some cases to avoid loops in check, get_info, get_warp_info, re-check ...

In scalar context, return 0 or 1.

In array context, return an empty array when no error was found. In case of errors, returns an array of error strings that should be shown to the user.

Information management

store( value )

Store value in leaf element.

load_data( scalar_value )

Load scalar data. Data is simply forwarded to store.

fetch_custom

Returns the stored value if this value is different from a standard setting or built in seting. In other words, returns undef if the stored value is identical to the default value or the computed value or the built in value.

fetch_standard

Returns the standard value as defined by the configuration model. The standard value can be either a preset value, a computed value, a default value or a built-in default value.

fetch_no_check

Fetch value from leaf element without checking the value.

fetch( [ custom | preset | standard | default ] )

Check and fetch value from leaf element.

With a parameter, this method will return either:

custom

The value entered by the user (if different from built in, preset, computed or default value)

preset

The value entered in preset mode

standard

The preset or computed or default or built in value.

default

The default value (defined by the configuration model)

built_in

The built_in value. (defined by the configuration model)

non_built_in

The custom or preset or computed or default value. Will return undef if either of this value is identical to the built_in value. This feature is usefull to reduce data to write in configuration file.

user_value

Returns the value entered by the user. Does not use the default or computed value. Returns undef unless a value was actually stored.

fetch_preset

Returns the value entered in preset mode. Does not use the default or computed value. Returns undef unless a value was actually stored in preset mode.

EXCEPTION HANDLING

When an error is encountered, this module may throw the following exceptions:

Config::Model::Exception::Model
Config::Model::Exception::Formula
Config::Model::Exception::WrongValue
Config::Model::Exception::WarpError

See Config::Model::Exception for more details.

AUTHOR

Dominique Dumont, (ddumont at cpan dot org)

SEE ALSO

Config::Model, Config::Model::Node, Config::Model::AnyId, Config::Model::WarpedThing, Exception::Class

cpanm Config::Model

perl -MCPAN -e shell
install Config::Model