NAME

       Readonly - Facility for creating read-only scalars, arrays, hashes

Synopsis

           use Readonly;

           # Deep Read-only scalar
           Readonly::Scalar    $sca => $initial_value;
           Readonly::Scalar my $sca => $initial_value;

           # Deep Read-only array
           Readonly::Array    @arr => @values;
           Readonly::Array my @arr => @values;

           # Deep Read-only hash
           Readonly::Hash    %has => (key => value, key => value, ...);
           Readonly::Hash my %has => (key => value, key => value, ...);
           # or:
           Readonly::Hash    %has => {key => value, key => value, ...};

           # You can use the read-only variables like any regular variables:
           print $sca;
           $something = $sca + $arr[2];
           next if $has{$some_key};

           # But if you try to modify a value, your program will die:
           $sca = 7;
           push @arr, 'seven';
           delete $has{key};
           # The error message is "Modification of a read-only value attempted"

           # Alternate form (Perl 5.8 and later)
           Readonly    $sca => $initial_value;
           Readonly my $sca => $initial_value;
           Readonly    @arr => @values;
           Readonly my @arr => @values;
           Readonly    %has => (key => value, key => value, ...);
           Readonly my %has => (key => value, key => value, ...);
           Readonly my $sca; # Implicit undef, readonly value

           # Alternate form (for Perls earlier than v5.8)
           Readonly    \$sca => $initial_value;
           Readonly \my $sca => $initial_value;
           Readonly    \@arr => @values;
           Readonly \my @arr => @values;
           Readonly    \%has => (key => value, key => value, ...);
           Readonly \my %has => (key => value, key => value, ...);

Description

       This is a facility for creating non-modifiable variables. This is useful for configuration
       files, headers, etc. It can also be useful as a development and debugging tool for
       catching updates to variables that should not be changed.

Variable Depth

       Readonly has the ability to create both deep and shallow readonly variables.

       If you pass a $ref, an @array or a %hash to corresponding functions "::Scalar()",
       "::Array()" and "::Hash()", then those functions recurse over the data structure, marking
       everything as readonly. The entire structure is then non-modifiable. This is normally what
       you want.

       If you want only the top level to be readonly, use the alternate (and poorly named)
       "::Scalar1()", "::Array1()", and "::Hash1()" functions.

       Plain "Readonly()" creates what the original author calls a "shallow" readonly variable,
       which is great if you don't plan to use it on anything but only one dimensional scalar
       values.

       "Readonly::Scalar()" makes the variable 'deeply' readonly, so the following snippet kills
       over as you expect:

        use Readonly;

        Readonly::Scalar my $ref => { 1 => 'a' };
        $ref->{1} = 'b';
        $ref->{2} = 'b';

       While the following snippet does not make your structure 'deeply' readonly:

        use Readonly;

        Readonly my $ref => { 1 => 'a' };
        $ref->{1} = 'b';
        $ref->{2} = 'b';

The Past

       The following sections are updated versions of the previous authors documentation.

   Comparison with "use constant"
       Perl provides a facility for creating constant values, via the constant pragma. There are
       several problems with this pragma.

       • The constants created have no leading sigils.

       • These constants cannot be interpolated into strings.

       • Syntax can get dicey sometimes.  For example:

             use constant CARRAY => (2, 3, 5, 7, 11, 13);
             $a_prime = CARRAY[2];        # wrong!
             $a_prime = (CARRAY)[2];      # right -- MUST use parentheses

       • You have to be very careful in places where barewords are allowed.

         For example:

             use constant SOME_KEY => 'key';
             %hash = (key => 'value', other_key => 'other_value');
             $some_value = $hash{SOME_KEY};        # wrong!
             $some_value = $hash{+SOME_KEY};       # right

         (who thinks to use a unary plus when using a hash to scalarize the key?)

       • "use constant" works for scalars and arrays, not hashes.

       • These constants are global to the package in which they're declared; cannot be lexically
         scoped.

       • Works only at compile time.

       • Can be overridden:

             use constant PI => 3.14159;
             ...
             use constant PI => 2.71828;

         (this does generate a warning, however, if you have warnings enabled).

       • It is very difficult to make and use deep structures (complex data structures) with "use
         constant".

Comparison with typeglob constants

       Another popular way to create read-only scalars is to modify the symbol table entry for
       the variable by using a typeglob:

           *a = \'value';

       This works fine, but it only works for global variables ("my" variables have no symbol
       table entry). Also, the following similar constructs do not work:

           *a = [1, 2, 3];      # Does NOT create a read-only array
           *a = { a => 'A'};    # Does NOT create a read-only hash

   Pros
       Readonly.pm, on the other hand, will work with global variables and with lexical ("my")
       variables. It will create scalars, arrays, or hashes, all of which look and work like
       normal, read-write Perl variables. You can use them in scalar context, in list context;
       you can take references to them, pass them to functions, anything.

       Readonly.pm also works well with complex data structures, allowing you to tag the whole
       structure as nonmodifiable, or just the top level.

       Also, Readonly variables may not be reassigned. The following code will die:

           Readonly::Scalar $pi => 3.14159;
           ...
           Readonly::Scalar $pi => 2.71828;

   Cons
       Readonly.pm used to impose a performance penalty. It was pretty slow. How slow? Run the
       "eg/benchmark.pl" script that comes with Readonly. On my test system, "use constant"
       (const), typeglob constants (tglob), regular read/write Perl variables (normal/literal),
       and the new Readonly (ro/ro_simple) are all about the same speed, the old, tie based
       Readonly.pm constants were about 1/22 the speed.

       However, there is relief. There is a companion module available, Readonly::XS.  You won't
       need this if you're using Perl 5.8.x or higher.

       I repeat, you do not need Readonly::XS if your environment has perl 5.8.x or higher.
       Please see section entitled Internals for more.

Functions

       Readonly::Scalar $var => $value;
           Creates a nonmodifiable scalar, $var, and assigns a value of $value to it. Thereafter,
           its value may not be changed. Any attempt to modify the value will cause your program
           to die.

           A value must be supplied. If you want the variable to have "undef" as its value, you
           must specify "undef".

           If $value is a reference to a scalar, array, or hash, then this function will mark the
           scalar, array, or hash it points to as being Readonly as well, and it will recursively
           traverse the structure, marking the whole thing as Readonly. Usually, this is what you
           want. However, if you want only the $value marked as Readonly, use "Scalar1".

           If $var is already a Readonly variable, the program will die with an error about
           reassigning Readonly variables.

       Readonly::Array @arr => (value, value, ...);
           Creates a nonmodifiable array, @arr, and assigns the specified list of values to it.
           Thereafter, none of its values may be changed; the array may not be lengthened or
           shortened or spliced. Any attempt to do so will cause your program to die.

           If any of the values passed is a reference to a scalar, array, or hash, then this
           function will mark the scalar, array, or hash it points to as being Readonly as well,
           and it will recursively traverse the structure, marking the whole thing as Readonly.
           Usually, this is what you want. However, if you want only the hash %@arr itself marked
           as Readonly, use "Array1".

           If @arr is already a Readonly variable, the program will die with an error about
           reassigning Readonly variables.

       Readonly::Hash %h => (key => value, key => value, ...);
       Readonly::Hash %h => {key => value, key => value, ...};
           Creates a nonmodifiable hash, %h, and assigns the specified keys and values to it.
           Thereafter, its keys or values may not be changed. Any attempt to do so will cause
           your program to die.

           A list of keys and values may be specified (with parentheses in the synopsis above),
           or a hash reference may be specified (curly braces in the synopsis above). If a list
           is specified, it must have an even number of elements, or the function will die.

           If any of the values is a reference to a scalar, array, or hash, then this function
           will mark the scalar, array, or hash it points to as being Readonly as well, and it
           will recursively traverse the structure, marking the whole thing as Readonly. Usually,
           this is what you want. However, if you want only the hash %h itself marked as
           Readonly, use "Hash1".

           If %h is already a Readonly variable, the program will die with an error about
           reassigning Readonly variables.

       Readonly $var => $value;
       Readonly @arr => (value, value, ...);
       Readonly %h => (key => value, ...);
       Readonly %h => {key => value, ...};
       Readonly $var;
           The "Readonly" function is an alternate to the "Scalar", "Array", and "Hash"
           functions. It has the advantage (if you consider it an advantage) of being one
           function. That may make your program look neater, if you're initializing a whole bunch
           of constants at once. You may or may not prefer this uniform style.

           It has the disadvantage of having a slightly different syntax for versions of Perl
           prior to 5.8.  For earlier versions, you must supply a backslash, because it requires
           a reference as the first parameter.

               Readonly \$var => $value;
               Readonly \@arr => (value, value, ...);
               Readonly \%h   => (key => value, ...);
               Readonly \%h   => {key => value, ...};

           You may or may not consider this ugly.

           Note that you can create implicit undefined variables with this function like so
           "Readonly my $var;" while a verbose undefined value must be passed to the standard
           "Scalar", "Array", and "Hash" functions.

       Readonly::Scalar1 $var => $value;
       Readonly::Array1 @arr => (value, value, ...);
       Readonly::Hash1 %h => (key => value, key => value, ...);
       Readonly::Hash1 %h => {key => value, key => value, ...};
           These alternate functions create shallow Readonly variables, instead of deep ones. For
           example:

               Readonly::Array1 @shal => (1, 2, {perl=>'Rules', java=>'Bites'}, 4, 5);
               Readonly::Array  @deep => (1, 2, {perl=>'Rules', java=>'Bites'}, 4, 5);

               $shal[1] = 7;           # error
               $shal[2]{APL}='Weird';  # Allowed! since the hash isn't Readonly
               $deep[1] = 7;           # error
               $deep[2]{APL}='Weird';  # error, since the hash is Readonly

Cloning

       When cloning using Storable or Clone you will notice that the value stays readonly, which
       is correct. If you want to clone the value without copying the readonly flag, use the
       "Clone" function:

           Readonly::Scalar my $scalar => {qw[this that]};
           # $scalar->{'eh'} = 'foo'; # Modification of a read-only value attempted
           my $scalar_clone = Readonly::Clone $scalar;
           $scalar_clone->{'eh'} = 'foo';
           # $scalar_clone is now {this => 'that', eh => 'foo'};

       The new variable ($scalar_clone) is a mutable clone of the original $scalar.

Examples

       These are a few very simple examples:

   Scalars
       A plain old read-only value

           Readonly::Scalar $a => "A string value";

       The value need not be a compile-time constant:

           Readonly::Scalar $a => $computed_value;

   Arrays/Lists
       A read-only array:

           Readonly::Array @a => (1, 2, 3, 4);

       The parentheses are optional:

           Readonly::Array @a => 1, 2, 3, 4;

       You can use Perl's built-in array quoting syntax:

           Readonly::Array @a => qw/1 2 3 4/;

       You can initialize a read-only array from a variable one:

           Readonly::Array @a => @computed_values;

       A read-only array can be empty, too:

           Readonly::Array @a => ();
           Readonly::Array @a;        # equivalent

   Hashes
       Typical usage:

           Readonly::Hash %a => (key1 => 'value1', key2 => 'value2');

       A read-only hash can be initialized from a variable one:

           Readonly::Hash %a => %computed_values;

       A read-only hash can be empty:

           Readonly::Hash %a => ();
           Readonly::Hash %a;        # equivalent

       If you pass an odd number of values, the program will die:

           Readonly::Hash %a => (key1 => 'value1', "value2");
           # This dies with "May not store an odd number of values in a hash"

Exports

       Historically, this module exports the "Readonly" symbol into the calling program's
       namespace by default. The following symbols are also available for import into your
       program, if you like: "Scalar", "Scalar1", "Array", "Array1", "Hash", and "Hash1".

Internals

       Some people simply do not understand the relationship between this module and Readonly::XS
       so I'm adding this section. Odds are, they still won't understand but I like to write
       so...

       In the past, Readonly's "magic" was performed by "tie()"-ing variables to the
       "Readonly::Scalar", "Readonly::Array", and "Readonly::Hash" packages (not to be confused
       with the functions of the same names) and acting on "WRITE", "READ", et. al. While this
       worked well, it was slow. Very slow. Like 20-30 times slower than accessing variables
       directly or using one of the other const-related modules that have cropped up since
       Readonly was released in 2003.

       To 'fix' this, Readonly::XS was written. If installed, Readonly::XS used the internal
       methods "SvREADONLY" and "SvREADONLY_on" to lock simple scalars. On the surface,
       everything was peachy but things weren't the same behind the scenes. In edge cases, code
       performed very differently if Readonly::XS was installed and because it wasn't a required
       dependency in most code, it made downstream bugs very hard to track.

       In the years since Readonly::XS was released, the then private internal methods have been
       exposed and can be used in pure perl. Similar modules were written to take advantage of
       this and a patch to Readonly was created. We no longer need to build and install another
       module to make Readonly useful on modern builds of perl.

       •   You do not need to install Readonly::XS.

       •   You should stop listing Readonly::XS as a dependency or expect it to be installed.

       •   Stop testing the $Readonly::XSokay variable!

Requirements

       Please note that most users of Readonly no longer need to install the companion module
       Readonly::XS which is recommended but not required for perl 5.6.x and under. Please do not
       force it as a requirement in new code and do not use the package variable
       $Readonly::XSokay in code/tests. For more, see "Internals" in the section on Readonly's
       new internals.

       There are no non-core requirements.

Bug Reports

       If email is better for you, my address is mentioned below but I would rather have bugs
       sent through the issue tracker found at http://github.com/sanko/readonly/issues.

Acknowledgements

       Thanks to Slaven Rezic for the idea of one common function (Readonly) for all three types
       of variables (13 April 2002).

       Thanks to Ernest Lergon for the idea (and initial code) for deeply-Readonly data
       structures (21 May 2002).

       Thanks to Damian Conway for the idea (and code) for making the Readonly function work a
       lot smoother under perl 5.8+.

Author

       Sanko Robinson <sanko@cpan.org> - http://sankorobinson.com/

       CPAN ID: SANKO

       Original author: Eric J. Roode, roode@cpan.org

License and Legal

       Copyright (C) 2013-2016 by Sanko Robinson <sanko@cpan.org>

       Copyright (c) 2001-2004 by Eric J. Roode. All Rights Reserved.

       This module is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.