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 $b', 
                         variables => { '- sav', b => '- sbv' }
                       },
          },
       compute_with_replace 
       => {
            formula   => '$replace{$who} is the $replace{$what} of $replace{$country}',
            variables => {
                           who   => '! who' ,
                           what  => '! what' ,
                           country => '- country',
                         },
            replace   => { chief => 'president', 
                            America => 'USA'
                         },
    ]
      ]
 ) ;

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.

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)',

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_compute_override => 1 with the compute parameter:

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

AUTHOR

Dominique Dumont, (ddumont at cpan dot org)

SEE ALSO

Config::Model, Config::Model::Instance, Config::Model::Value

cpanm Config::Model

perl -MCPAN -e shell
install Config::Model