Config::Model::ValueComputer(3pm) User Contributed Perl Documentation Config::Model::ValueComputer(3pm)
NAME
Config::Model::ValueComputer - Provides configuration value computation
VERSION
version 2.021
SYNOPSIS
use Config::Model;
use Log::Log4perl qw(:easy);
Log::Log4perl->easy_init($WARN);
# define configuration tree object
my $model = Config::Model->new;
$model ->create_config_class (
name => "MyClass",
element => [
[qw/av bv/] => {type => 'leaf',
value_type => 'integer',
},
compute_int => {
type => 'leaf',
value_type => 'integer',
compute => { formula => '$a + $b',
variables => { a => '- av', b => '- bv'}
},
},
],
) ;
my $inst = $model->instance(root_class_name => 'MyClass' );
my $root = $inst->config_root ;
# put data
$root->load( step => 'av=33 bv=9' );
print "Computed value is ",$root->grab_value('compute_int'),"
";
# Computed value is 42
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 default 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:
o A string formula that use variables and replace function.
o A set of variable and their relative location in the tree (using the notation explained in grab() method
o An optional set of replace rules.
o 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 * $c**2'
This string or formula may also contain:
o The index value of the current object : &index or "&index()".
o The index value of a parent object: "&index(-)". Ancestor index value can be retrieved with "&index(-2)" or "&index(-3)".
o The element name of the current object: &element or "&element()".
o The element name of a parent object: "&element(-)". Likewise, ancestor element name can be retrieved with "&element(-2)" or
"&element(-3)".
=item*
The full location (path) of the current object: &location or "&location()".
For instance, you could have this template string:
'my element is &element, my index is &index' .
'upper element is &element(-), upper index is &index(-)',
If you need to perform more complex operations than substitution, like extraction with regular expressions, you can force an eval done by
Perl with "use_eval => 1". In this case, the result of the eval will be used as the computed value.
For instance:
# extract host from url
compute => { formula => '$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,
}
}
This computed default value will be written to the configuration file.
This default value may be already known by the application so the computed value should not be written to the configuration file. The
computed value is interesting because it cab be shown to the user. In this case, use the "use_as_upstream_default" parameter:
compute_known_upstream => {
type => 'leaf',
value_type => 'string',
compute => { formula => '"macro is $m"' ,
variables => { m => '- - macro' } ,
use_as_upstream_default => 1,
}
}
"use_as_upstream_default" implies "allow_override".
Undefined variables
You may need to compute value where one of the variables (i.e. other configuration parameter) is undefined. By default, any formula will
yield an undefined value if one variable is undefined.
You may change this behavior with "undef_is" parameter. Depending on your formula and whether "use_eval" is true or not, you may specify a
"fallback" value that will be used in your formula.
The most useful will probably be:
undef_is => "''", # for string values
undef_is => 0 , # for integers, boolean values
Example:
Source => {
value_type => 'string',
mandatory => 1,
migrate_from => {
use_eval => 1,
formula => '$old || $older ;',
undef_is => "''",
variables => {
older => '- Original-Source-Location',
old => '- Upstream-Source'
}
},
type => 'leaf',
},
[qw/Upstream-Source Original-Source-Location/] => {
value_type => 'string',
status => 'deprecated',
type => 'leaf'
}
Examples
String substitution
[qw/sav sbv/] => {
type => 'leaf',
value_type => 'string',
},
compute_string => {
type => 'leaf',
value_type => 'string',
compute => {
formula => 'meet $a and $b',
variables => { '- sav', b => '- sbv' }
},
},
Computation with on-the-fly replacement
compute_with_replace => {
type => 'leaf',
value_type => 'string',
compute => {
formula =>
'$replace{$who} is the $replace{$what} of $replace{$country}',
variables => {
who => '! who',
what => '! what',
country => '- country',
},
replace => {
chief => 'president',
America => 'USA'
},
},
},
Extract data from a value using a Perl regexp
Extract the host name from an URL:
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,
},
},
AUTHOR
Dominique Dumont, (ddumont at cpan dot org)
SEE ALSO
Config::Model, Config::Model::Instance, Config::Model::Value
perl v5.14.2 2012-11-09 Config::Model::ValueComputer(3pm)