Config::Model::Value(3pm)

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

SYNOPSIS
 my $model = Config::Model->new() ;
 $model ->create_config_class
  (
   name => "SomeClass",
   element => [
     country => {
       type =>       'leaf',
       value_type => 'enum',
       choice =>      [qw/France US/]
     },
     president => {
       type =>        'leaf',
       value_type => 'string',
       warp => {
         follow => { c => '- country'},
         rules  => {
           '$c eq "France"' => { default => 'Chirac' },
           '$c eq "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. This default value will be written in the target configuration file. ("default" parameter)

•   upstream default parameter: specifies a default value that will be used by the application when no information is provided in the configuration file. This upstream_default value will not written in the configuration files. Only the "fetch_standard" method will return the builtin value. This parameter was previously refered as "built_in" value. This may be used for audit purpose. ("upstream_default" parameter)

•   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":

•   "upstream_default": 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)

upstream_default
    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.

migrate_from
    This is a special parameter to cater for smooth configuration upgrade. This parameter can be used to copy the value of a deprecated parameter to its replacement. See "upgrade"" in " for details.

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/]

match
    Perl regular expression. The value will be match with the regex to assert its validity. Example "match => '^foo'" means that the parameter value must begin with "foo". Valid only for "string" or "uniline" values.

warn_if_match
    Perl regular expression. A warning will be issued when the value match the passed regular expression. Valid only for "string" or "uniline" values.

warn_unless_match
    Perl regular expression. A warning will be issued when the value does not match the passed regular expression. Valid only for "string" or "uniline" values.

warn
    String. Issue a warning to user with the specified string any time a value is set or read.

grammar
    Setup a Parse::RecDescent grammar to perform validation.

    If the grammar does not start with a "check" rule (i.e does not start with "check: "), the first line of the grammar will be modified to add "check" rule and set up this rules so the entire value must match the passed grammar.

    I.e. the grammar:

      token (oper token)(s?)
      oper: 'and' | 'or'
      token: 'Apache' | 'CC-BY' | 'Perl'

    will be changed to

      check: token (oper token)(s?) /^\Z/ {$return = 1;}
      oper: 'and' | 'or'
      token: 'Apache' | 'CC-BY' | 'Perl'

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:

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

  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
upstream_default
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.

  warning_msg
Returns warning concerning this value. Returns a list in list context and a string in scalar context.

  check_value ( value )
Check the consistency of the value. Does not check for undefined mandatory values.

"check_value" also accepts named parameters:

value
quiet
    When non null, check will not try to get extra information from the tree. This is required in some cases to avoid loops in check, get_info, get_warp_info, re-check ...

silent
    Don't display value warning on STDOUT. User is expected to retrieve them witj warning_msg.

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.

  check( value )
Like "check_value". Also ensure that mandatory value are defined

Information management

  store( value )
Can be called as "value => ..., check => yes|no|skip )"

Store value in leaf element. "check" parameter can be used to skip validation check.

  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( ... )
Check and fetch value from leaf element. The method can have one parameter (the fetch mode) or several pairs:

mode
    Whether to fetch default, custom, etc value. See below for details

check
    Whether to check if the value is valid or not before returning it. Default is 'yes'. Possible value are

    yes Perform check and raise an exception for bad values

    skip
        Perform check and return undef for bad values

    no  Do not check and return values even if bad

silent
    When set to 1, warning are not displayed on STDOUT. User is expected to read warnings with warning_msg.

According to the "mode" parameter, this method will return either:

empty mode parameter (default)
    Value entered by user or default value if the value is different from upstream_default

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)

upstream_default
    The upstream_default value. (defined by the configuration model)

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

allow_undef
    This mode will accept to return undef for mandatory values. Normally, trying to fetch an undefined mandatory value leads to an exception.

  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.

  get( path , [ custom | preset | standard | default ])
Get a value from a directory like path.

  set( path , value )
Set a value from a directory like path.

Upgrade
Upgrade is a special case when the configuration of an application has changed. Some parameters can be removed and replaced by another one. To avoid trouble on the application user side, Config::Model offers a possibility to handle the migration of configuration data through a special declaration in the configuration model.

This declaration must:

•   Declare the deprecated parameter with a "status" set to "deprecated"

•   Declare the new parameter with the intructions to load the semantic content from the deprecated parameter. These instructions are declared in the "migrate_from" parameters (which is similar to the "compute" parameter)

Here an example where a url parameter is changed to a set of 2 parameters (host and path):

       'old_url' => { type => 'leaf',
                      value_type => 'uniline',
                      status => 'deprecated',
                    },
       'host'
       => { type => 'leaf',
            value_type => 'uniline',
            # the formula must end with '$1' so the result of the capture is used
            # as the host value
            migrate_from => { formula => '$old =~ m!http://([\w\.]+)!; $1 ;' ,
                              variables => { old => '- old_url' } ,
                              use_eval => 1 ,
                            },
                        },
       'path' => { type => 'leaf',
                   value_type => 'uniline',
                   migrate_from => { formula => '$old =~ m!http://[\w\.]+(/.*)!; $1 ;',
                                     variables => { old => '- old_url' } ,
                                     use_eval => 1 ,
                                   },
                 },

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 Config::Model::ValueComputer,