constant::more - Constants and Enumerations. Assign constant values from the command line
Can use as a direct alternative to use constant:
use constant::more PI => 4 * atan2(1, 1);
use constant:more DEBUG => 0;
print "Pi equals ", PI, "...\n" if DEBUG;
use constant::more {
SEC => 0,
MIN => 1,
HOUR => 2,
MDAY => 3,
MON => 4,
YEAR => 5,
WDAY => 6,
YDAY => 7,
ISDST => 8,
};Can use as a alternative to use enum:
use constant::more qw(Sun=0 Mon Tue Wed Thu Fri Sat);
# Sun == 0, Mon == 1, etc
use constant::More qw(Forty=40 FortyOne Five=5 Six Seven);
# Yes, you can change the start indexes at any time as in CParse command line arguments and/or environment variables to assign to constants:
# ###
# example.pl
use constant::more {
FEATURE_A_ENABLED=>{ #Name of the constant
val=>0, #default value
opt=>"feature1", #Getopt::Long option specification
env=>"MY_APP_FEATURE_A" #Environment variable copy value from
},
FEATURE_B_CONFIG=>{
val=>"disabled",
opt=>"feature2=s", #Getopt::Long format
}
};
if(FEATURE_A_ENABLED){
#Do interesting things here
print "Feature a is enabled
}
print "Feature b config is: ".FEATURE_B_CONFIG."\n";
__END__
#######
# From command line
perl example.pl --feature1 --feature2=active
# ####
# output
Feature a is enabled
Feature b config is: active
On the surface this module performs similar tasks as use constant and use enum. Digging a little deeper, it has the ability to assign values to
constants from the command line arguments or environment variables.
These constants are only defined/set if they don't exist already, making configuring and overriding constants defined in sub modules possible. A module can specify a default value which is used if the constant hasn't been defined by the top level script.
GetOpt::Long option specification is used for processing command line
options to give flexibility in how and what switches are used. To save on
memory, GetOpt::Long is only loaded if option processing is wanted (i.e. the
opt field not disabled).
In advanced form, a user subroutine can be supplied to give control of processing. This is used by Log::OK for example to generate multiple constants from a single input level.
From v0.3.0 constants can be also defined from a flat list and enumerations can be declared.
I use the constant and enum pragma frequently for unchanging values in my code. However, I would like to have the flexibility to configure constants at program start to enable debugging or platform specific code.
This module started as a way of disabling logging with no runtime overhead.
However goals changed and I made it more general purpose. I wrote the module
Log::OK to handle disabling of inactive logging statements. It uses this
module under the hood.
Constants are defined in a callers package unless the name includes a package. A name with '::' in it is classed as a full name for a variable. Use this to declare constants in a common namespace for example.
If you intend to set constants from the command line, it is important to use constant::more before other modules that also use constant::more. This
ensures that you can manipulate constant values from the top level of the
program. Otherwise you risk sub modules overriding your top level applications
logic.
In the case of the val field, command line and environment processing all
being enabled simultaneously, the precedence of a constant's value is: command
option, environment variable and lastly the val field.
Constant names and their values are set in a table (hash) before they are actually created. In the case of the Advanced Form usage (see below), a constant can have it's value updated multiple times, or multiple constants generated from the same command line option and added to the table. When processing is complete all entries in the table are created.
The usage of the pragma takes three forms, depending on how you want to set the value of your constants. These are detailed in the following sections.
In its simplest form, defining an constant (or multiple) is just like the use constant pragma. It also supports setting multiple constants from a flat list
of arguments:
use constant::more NAME=>"value"; #Set a single constant
use constant::more #Set multiple constants (from v0.3.0)
NAME=>"value",
ANOTHER=>"one",
;
use constant::more { #Set multiple constants
NAME=>"value",
ANOTHER=>"one",
};The key of the hash (or kv pair) becomes the name of the constant.
From v0.3.0, if the first item from the flat list looks like it has a '=' in the name, the list is treated as a enumeration declaration. Each time a '=' is seen the value after it is used as the value for the enumeration. The value before it is used as the constant name.
Each successive item in the list without a '=' causes the enumerated value to increment by 1:
use constant::more "my_enum=0"; #Single set of value
# Set enumeration value multiple times
use constant::more "my_enum=0", "another", "onemore=10";
# Use standard perl globbing to to add a prefix to each name
use constat::more <MY_PREFIX_{A=0,B,C,D,E=100,F};In its normal form, one or more anonymous hashes containing keys val,
opt, env, keep and sub are used to setup the processing of a
constant:
use constant::more {
MY_NAME=>{
val=>"john",
opt=>"name=s",
env=>"ENV_VAR_NAME",
},
ANOTHER=>{
value=>"one",
}
};The key for each anonymous hash is the name of the constant created (MY_NAME and ANOTHER from above).
The field values are all optional and include:
The (default) value set for the constant if no command line option or
environment variable is used/detected. If not provided the value of constant
generated will be undef.
The GetOpt::Long option specification to use in processing the command line. If not specified, no command line processing is performed and GetOpt::Long in not loaded.
The name of the environment variable to use in setting the constant's value. If not provided, environment variables are not processed.
A flag indicating if the @ARGV should be left as is (to keep) or consumed
when processing command line options. If not specified, @ARGV will have
options consumed and only remaining options passed through.
Advanced form has an additional field sub which changes behaviour of the
pragma dramatically.
If a child anonymous hash contains a CODE reference in the field sub, the top
level key in the hash is NOT used as the constant name, but only as a
label.
The actual constant names and values to be generated are returned as a key/value list from the CODE ref.
The CODE ref is called with a key/value pair. The first input argument is the name of the command line option, or undef if default or environment variable.
The second argument is the value from the command line, default or environment variable.
eg:
use constant::more {
just_a_label=>{ #this is just a label
val=>"john",
opt=>"name=s",
env=>"ENV_VAR_NAME",
sub=>sub{
my ($key,$value)=@_;
state $i=0;
#each time this sub is called it returns
#a new for a constant with value to set
("CONSTANT".$i++, $value);
}
},
};The code ref may be called multiple times if command line processing is enabled
(with the opt field). If multiple matching switches are present on the
command line, they are each passed in a call.
The names and values returned can be different each time to implement advanced use cases.
Please report feature requests and bugs via the github:
https://github.com/drclaw1394/perl-constant-more.git
Ruben Westerberg, drclaw@mac.com
Copyright (C) 2023 by Ruben Westerberg
Licensed under MIT
THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.