/ 
Config-Model-1.228
River stage two • 11 direct dependents • 11 total dependents
/ Config::Model::ValueComputer

NAME

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

VERSION

version 1.228

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