/ 
constant-more-v0.2.1
River stage two • 9 direct dependents • 12 total dependents
/ constant::more

NAME

constant::more - Assign values to constants from the command arguments

SYNOPSIS

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,
};

Parse 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

DESCRIPTION

Performs similar tasks as use constant, but adds features to assign values of constants from the command line or environment variables.

In addition, 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 non 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.

MOTIVATION

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.

USAGE

Implementation Details

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.

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.

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.

Simple Form

In its simplest form, defining an constant (or multiple) is just like the use constant pragma:

use constant::more NAME=>"value";	#Set a single constant
	
use constant::more {			#Set multiple constants
		NAME=>"value",
		ANOTHER=>"one",
};

The key of the hash becomes the name of the constant.

Normal Form

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:

val

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.

opt

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.

env

The name of the environment variable to use in setting the constant's value. If not provided, environment variables are not processed.

keep

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

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.

REPOSITOTY and BUGS

Please report feature requests and bugs via the github:

https://github.com/drclaw1394/perl-constant-more.git

AUTHOR

Ruben Westerberg, <drclaw@mac.com>

COPYRIGHT AND LICENSE

Copyright (C) 2022 by Ruben Westerberg

Licensed under MIT

DISCLAIMER OF WARRANTIES

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.