Config::Model::ValueComputer(3pm)

NAME
Config::Model::ValueComputer - Provides configuration value computation

SYNOPSIS
 my $model = Config::Model->new() ;

 $model ->create_config_class
  (
   name => "Master",
   element
   => [
       [qw/av bv/] => {type => 'leaf',
                       value_type => 'integer',
                      },
       compute_int
       => { type => 'leaf',
            value_type => 'integer',
            compute    => { formula   => '$a + $b',
                            variables => { a => '- av', b => '- bv'}
                          },
            min        => -4,
            max        => 4,
          },
       [qw/sav sbv/] => {type => 'leaf',
                         value_type => 'string',
                      },
       compute_string
       => { type => 'leaf',
            value_type => 'string',
            compute => { formula => 'meet $a and $b',
                         variables => { '- sav', b => '- sbv' }
                       },
          },
       compute_with_replace
       => { type => 'leaf',
            value_type => 'string',
            compute => {
               formula   => '$replace{$who} is the $replace{$what} of $replace{$country}',
               variables => {
                              who   => '! who' ,
                              what  => '! what' ,
                              country => '- country',
                             },
               replace   => { chief => 'president',
                              America => 'USA'
                            },
            },
       },

       url => { type => 'leaf', value_type => 'uniline'},
       extract_host_from_url
       => { type => 'leaf',
            value_type => 'uniline',
            compute    => { formula => '$old =~ m!http://([\w\.]+)!; $1 ;' ,
                            variables => { old => '- url' } ,
                            use_eval => 1 ,
                          },
          },
     ]
 ) ;

DESCRIPTION
This class provides a way to compute a configuration value. This computation uses a formula and some other configuration values from the configuration tree.

The computed value can be overridden, in other words, the computed value can be used as a defult value.

Computed value declaration
A computed value must be declared in a 'leaf' element. The leaf element must have a "compute" argument pointing to a hash ref.

This array ref contains:

•   A string formula that use variables and replace function.

•   A set of variable and their relative location in the tree (using the notation explained in grab() method

•   An optional set of replace rules.

•   An optional parameter to force a Perl eval of a string.

  Compute formula
The first element of the "compute" array ref must be a string that contains the computation algorithm (i.e. a formula for arithmetic computation for integer values or a string template for string values).

This string or formula should contain variables (like $foo or $bar). Note that these variables are not interpolated by perl.

For instance:

  'My cat has $nb legs'
  '$m * $c**2'

This string or formula may also contain:

•   The index value of the current object : &index or "&index()".

•   The index value of another object: "&index($other)"

•   The element name of the current object: &element or "&element()".

•   The element name of another object: "&element($other)"

For instance, you could have this template string:

   'my element is &element, my index is &index' .
    'upper element is &element($up), upper index is &index($up)',

If you need to perform more complex operations than substition, like extraction with regular expressions, you can force an eval done by Perl with "use_eval => 1". In this case, the result of the eval will be used as the computed value.

For instance:

  # extract host from url
  compute => { formula => '$old =~ m!http://[\w\.]+(?::\d+)?(/.*)!; $1 ;',
               variables => { old => '- url' } ,
               use_eval => 1 ,
             },

  # capitalize
  compute => { formula => 'uc($old)',
               variables => { old => '- small_caps' } ,
               use_eval => 1
             }

  Compute variables
The following arguments will be a set of "key => value" to define the variables used in the formula. The key is a variable name used in the computation string. The value is a string that will be used to get the correct Value object.

In this numeric example, "result" default value is "av + bv":

 element => [
  av => {
    type => 'leaf',
    value_type => 'integer'
  },
  bv => {
    type => 'leaf',
    value_type => 'integer'
  },
  result => {
    type => 'leaf',
    value_type => 'integer',
    compute => { formula => '$a + $b' ,
                 variables => { a => '- av', b => '- bv' },
               }
  }

In this string example, the default value of the "Comp" element is actually a string made of ""macro is "" and the value of the ""macro"" element of the object located 2 nodes above:

   comp => {
    type => 'leaf',
    value_type => 'string',
    compute => { formula => '"macro is $m"' ,
                 variables => { m => '- - macro' }
               }
   }

  Compute replace
Sometime, using the value of a tree leaf is not enough and you need to substitute a replacement for any value you can get. This replacement can be done using a hash like notation within the formula using the %replace hash.

For instance, if you want to display a summary of a config, you can do :

       compute_with_replace
       => {
            formula => '$replace{$who} is the $replace{$what} of $replace{$country}',
            variables => {
                           who   => '! who' ,
                           what  => '! what' ,
                           country => '- country',
                         },
            replace => {  chief => 'president',
                          America => 'USA'
                       },

  Complex formula
&index, &element, and replace can be combined. But the argument of &element or &index can only be a value object specification (I.e. something like '"- - foo"'), it cannot be a value replacement of another &element or &index.

I.e. "&element($foo)" is ok, but "&element(&index($foo))" is not allowed.

  computed variable
Compute variables can themselves be computed :

   compute => {
     formula => 'get_element is $replace{$s}, indirect value is \'$v\'',
     variables => { 's' => '! $where',
                     where => '! where_is_element',
                     v => '! $replace{$s}',
                  }
     replace   => { m_value_element => 'm_value',
                    compute_element => 'compute'
                  }
    }

Be sure not to specify a loop when doing recursive computation.

  compute override
In some case, a computed value must be interpreted as a default value and the user must be able to override this computed default value. In this case, you must use "allow_override => 1" with the compute parameter:

   computed_value_with_override => {
    type => 'leaf',
    value_type => 'string',
    compute => { formula => '"macro is $m"' ,
                 variables => { m => '- - macro' } ,
                 allow_override => 1,
               }
   }

AUTHOR
Dominique Dumont, (ddumont at cpan dot org)

SEE ALSO
Config::Model, Config::Model::Instance, Config::Model::Value