Config::Model::ValueComputer(3pm)

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NAME

6       Config::Model::ValueComputer - Provides configuration value computation
7

VERSION

9       version 1.205
10

SYNOPSIS

12        my $model = Config::Model->new() ;
13
14        $model ->create_config_class
15         (
16          name => "Master",
17          element
18          => [
19              [qw/av bv/] => {type => 'leaf',
20                              value_type => 'integer',
21                             },
22              compute_int
23              => { type => 'leaf',
24                   value_type => 'integer',
25                   compute    => { formula   => '$a + $b',
26                                   variables => { a => '- av', b => '- bv'}
27                                 },
28                   min        => -4,
29                   max        => 4,
30                 },
31              [qw/sav sbv/] => {type => 'leaf',
32                                value_type => 'string',
33                             },
34              compute_string
35              => { type => 'leaf',
36                   value_type => 'string',
37                   compute => { formula => 'meet $a and $b',
38                                variables => { '- sav', b => '- sbv' }
39                              },
40                 },
41              compute_with_replace
42              => { type => 'leaf',
43                   value_type => 'string',
44                   compute => {
45                      formula   => '$replace{$who} is the $replace{$what} of $replace{$country}',
46                      variables => {
47                                     who   => '! who' ,
48                                     what  => '! what' ,
49                                     country => '- country',
50                                    },
51                      replace   => { chief => 'president',
52                                     America => 'USA'
53                                   },
54                   },
55              },
56
57              url => { type => 'leaf', value_type => 'uniline'},
58              extract_host_from_url
59              => { type => 'leaf',
60                   value_type => 'uniline',
61                   compute    => { formula => '$old =~ m!http://([\w\.]+)!; $1 ;' ,
62                                   variables => { old => '- url' } ,
63                                   use_eval => 1 ,
64                                 },
65                 },
66            ]
67        ) ;
68

DESCRIPTION

70       This class provides a way to compute a configuration value. This
71       computation uses a formula and some other configuration values from the
72       configuration tree.
73
74       The computed value can be overridden, in other words, the computed
75       value can be used as a defult value.
76

Computed value declaration

78       A computed value must be declared in a 'leaf' element. The leaf element
79       must have a "compute" argument pointing to a hash ref.
80
81       This array ref contains:
82
83       ·   A string formula that use variables and replace function.
84
85       ·   A set of variable and their relative location in the tree (using
86           the notation explained in grab() method
87
88       ·   An optional set of replace rules.
89
90       ·   An optional parameter to force a Perl eval of a string.
91
92   Compute formula
93       The first element of the "compute" array ref must be a string that
94       contains the computation algorithm (i.e. a formula for arithmetic
95       computation for integer values or a string template for string values).
96
97       This string or formula should contain variables (like $foo or $bar).
98       Note that these variables are not interpolated by perl.
99
100       For instance:
101
102         'My cat has $nb legs'
103         '$m * $c**2'
104
105       This string or formula may also contain:
106
107       ·   The index value of the current object : &index or "&index()".
108
109       ·   The index value of another object: "&index($other)"
110
111       ·   The element name of the current object: &element or "&element()".
112
113       ·   The element name of another object: "&element($other)"
114
115       For instance, you could have this template string:
116
117          'my element is &element, my index is &index' .
118           'upper element is &element($up), upper index is &index($up)',
119
120       If you need to perform more complex operations than substition, like
121       extraction with regular expressions, you can force an eval done by Perl
122       with "use_eval => 1". In this case, the result of the eval will be used
123       as the computed value.
124
125       For instance:
126
127         # extract host from url
128         compute => { formula => '$old =~ m!http://[\w\.]+(?::\d+)?(/.*)!; $1 ;',
129                      variables => { old => '- url' } ,
130                      use_eval => 1 ,
131                    },
132
133         # capitalize
134         compute => { formula => 'uc($old)',
135                      variables => { old => '- small_caps' } ,
136                      use_eval => 1
137                    }
138
139   Compute variables
140       The following arguments will be a set of "key => value" to define the
141       variables used in the formula. The key is a variable name used in the
142       computation string. The value is a string that will be used to get the
143       correct Value object.
144
145       In this numeric example, "result" default value is "av + bv":
146
147        element => [
148         av => {
149           type => 'leaf',
150           value_type => 'integer'
151         },
152         bv => {
153           type => 'leaf',
154           value_type => 'integer'
155         },
156         result => {
157           type => 'leaf',
158           value_type => 'integer',
159           compute => { formula => '$a + $b' ,
160                        variables => { a => '- av', b => '- bv' },
161                      }
162         }
163
164       In this string example, the default value of the "Comp" element is
165       actually a string made of ""macro is "" and the value of the ""macro""
166       element of the object located 2 nodes above:
167
168          comp => {
169           type => 'leaf',
170           value_type => 'string',
171           compute => { formula => '"macro is $m"' ,
172                        variables => { m => '- - macro' }
173                      }
174          }
175
176   Compute replace
177       Sometime, using the value of a tree leaf is not enough and you need to
178       substitute a replacement for any value you can get. This replacement
179       can be done using a hash like notation within the formula using the
180       %replace hash.
181
182       For instance, if you want to display a summary of a config, you can do
183       :
184
185              compute_with_replace
186              => {
187                   formula => '$replace{$who} is the $replace{$what} of $replace{$country}',
188                   variables => {
189                                  who   => '! who' ,
190                                  what  => '! what' ,
191                                  country => '- country',
192                                },
193                   replace => {  chief => 'president',
194                                 America => 'USA'
195                              },
196
197   Complex formula
198       &index, &element, and replace can be combined. But the argument of
199       &element or &index can only be a value object specification (I.e.
200       something like '"- - foo"'), it cannot be a value replacement of
201       another &element or &index.
202
203       I.e. "&element($foo)" is ok, but "&element(&index($foo))" is not
204       allowed.
205
206   computed variable
207       Compute variables can themselves be computed :
208
209          compute => {
210            formula => 'get_element is $replace{$s}, indirect value is \'$v\'',
211            variables => { 's' => '! $where',
212                            where => '! where_is_element',
213                            v => '! $replace{$s}',
214                         }
215            replace   => { m_value_element => 'm_value',
216                           compute_element => 'compute'
217                         }
218           }
219
220       Be sure not to specify a loop when doing recursive computation.
221
222   compute override
223       In some case, a computed value must be interpreted as a default value
224       and the user must be able to override this computed default value.  In
225       this case, you must use "allow_override => 1" with the compute
226       parameter:
227
228          computed_value_with_override => {
229           type => 'leaf',
230           value_type => 'string',
231           compute => { formula => '"macro is $m"' ,
232                        variables => { m => '- - macro' } ,
233                        allow_override => 1,
234                      }
235          }
236

AUTHOR

238       Dominique Dumont, (ddumont at cpan dot org)
239

NAME

VERSION

SYNOPSIS

DESCRIPTION

Computed value declaration

AUTHOR

SEE ALSO