NAME

Fixed - a readonly variable that you can assign to

SYNOPSIS

   use 5.012;
   use strict;
   use warnings;
   use Fixed;
   
   fix $x = 42;
   $x++;  # croaks

DESCRIPTION

Fixed is a little like Readonly; the main difference is that you can assign to fixed variables.

What?! Then how are they fixed?

Because you can only assign to them once!

   use 5.012;
   use strict;
   use warnings;
   use Fixed;
   
   fix $x;  # declared but not initialized
   
   given ($author) {
      when ("Adams")   { $x = 42    }  # ok
      when ("Heller")  { $x = 22    }  # ok
      default          { $x = undef }  # ok
   }
   
   $x = 99; # croaks, even when $x is undef

Note that Fixed differentiates between a variable which has no value, and a variable explicitly set to undef.

Fixed does not currently support arrays and hashes. (See "Internals" below for the reason.) You can of course assign an arrayref or hashref to a fixed variable, but this does not fix the contents of the array or hash. Use Readonly if you want readonly arrays and hashes.

Syntax

Fixed allows variables to be declared as fixed in several ways:

   fix $variable = $value;
   fix $variable;
   
   fix ($var1, $var2, ...) = ($val1, $val2, ...);
   fix ($var1, $var2, ...);

When a single variable is declared and initialized in the same statement (i.e. the first syntax), Fixed is able to use some optimizations, so this form should be preferred when possible.

Note that declaration of a variable with fix must be a statement on its own; fix cannot be slipped into the middle of an expression.

   if (fix $result = $search->get_result) {  # no!
      ...;
   }

This is a limitation inherited from Keyword::Simple.

Internals

The fix keyword is defined using Keyword::Simple and is parsed as if you'd witten:

   Fixed::Scalar(my $variable, $value);  # ... or ...
   Fixed::Scalar(my $variable);

If given a value, the Fixed::Scalar method will attempt to discover if Readonly's XS support is available, and if so will define the variable and use XS to set the scalar's SvREADONLY flag.

If XS is not available, or no initial value is provided, Fixed::Scalar will fall back to Perl's tie mechanism.

Arrays and Hashes do not have a SvREADONLY flag, plus the tie mechanism doesn't really have any way to differentiate between the initial list assignment to an uninitialized array or hash, and subsequent assignments. This is why Fixed does not support arrays or hashes.

Fixed without the Syntax Hacks

If you'd rather not enable the fix keyword and would prefer to just define fixed variables using Fixed::Scalar(my $variable => $value), then that's OK. Just include some empty parentheses when loading Fixed:

   use Fixed ();

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=Fixed.

SEE ALSO

Readonly, Readonly::XS, MooseX::SetOnce.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2013, 2014 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

cpanm Fixed

perl -MCPAN -e shell
install Fixed