NAME
    constant::more - Constants and Enumerations. Assign constant values from
    the command line

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,
            };

    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 C

    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
    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.

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
    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.

  Simple Form
    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.

  Enumeration Form
    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};

  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) 2023 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.

