# NAME

MooX::Const - Syntactic sugar for constant and write-once Moo attributes

# VERSION

version v0.6.2

# SYNOPSIS

```perl
use Moo;
use MooX::Const;

use Types::Standard -types;

has thing => (
  is  => 'const',
  isa => ArrayRef[HashRef],
);
```

# DESCRIPTION

This is syntactic sugar for using [Types::Const](https://metacpan.org/pod/Types%3A%3AConst) with [Moo](https://metacpan.org/pod/Moo). The
SYNOPSIS above is equivalent to:

```perl
use Types::Const -types;

has thing => (
  is     => 'ro',
  isa    => Const[ArrayRef[HashRef]],
  coerce => 1,
);
```

It modifies the `has` function to support "const" attributes.  These
are read-only ("ro") attributes for references, where the underlying
data structure has been set as read-only.

This will return an error if there is no "isa", the "isa" is not a
[Type::Tiny](https://metacpan.org/pod/Type%3A%3ATiny) type, if it is not a reference, or if it is blessed
object.

Simple value types such as `Int` or `Str` are silently converted to
read-only attributes.

As of v0.5.0, it also supports write-once ("once") attributes for
references:

```perl
has setting => (
  is  => 'once',
  isa => HashRef,
);
```

This allows you to set the attribute _once_. The value is coerced
into a constant, and cannot be changed again.

Note that "wo" is a removed synonym for "once". It no longer works in
v0.6.0, since "wo" is used for "write-only" in some Moose-like
extensions.

As of v0.4.0, this now supports the `strict` setting:

```perl
has thing => (
  is     => 'const',
  isa    => ArrayRef[HashRef],
  strict => 0,
);
```

When this is set to a false value, then the read-only constraint will
only be applied when running in strict mode, see [Devel::StrictMode](https://metacpan.org/pod/Devel%3A%3AStrictMode).

If omitted, `strict` is assumed to be true.

# KNOWN ISSUES

Accessing non-existent keys for hash references will throw an
error. This is a feature, not a bug, of read-only hash references, and
it can be used to catch mistakes in code that refer to non-existent
keys.

Unfortunately, this behaviour is not replicated with array references.

See [Types::Const](https://metacpan.org/pod/Types%3A%3AConst) for other known issues related to the `Const`
type.

## Using with Moose and Mouse

This module appears to work with [Moose](https://metacpan.org/pod/Moose), and there is now a small
test suite.

It does not work with [Mouse](https://metacpan.org/pod/Mouse). Pull requests are welcome.

# SUPPORT FOR OLDER PERL VERSIONS

Since v0.6.0, the this module requires Perl v5.14 or later.

Future releases may only support Perl versions released in the last ten years.

If you need this module on Perl v5.10, please use one of the v0.5.x
versions of this module.  Significant bug or security fixes may be
backported to those versions.

# SEE ALSO

[Const::Fast](https://metacpan.org/pod/Const%3A%3AFast)

[Devel::StrictMode](https://metacpan.org/pod/Devel%3A%3AStrictMode)

[Moo](https://metacpan.org/pod/Moo)

[MooseX::SetOnce](https://metacpan.org/pod/MooseX%3A%3ASetOnce)

[Sub::Trigger::Lock](https://metacpan.org/pod/Sub%3A%3ATrigger%3A%3ALock)

[Types::Const](https://metacpan.org/pod/Types%3A%3AConst)

[Type::Tiny](https://metacpan.org/pod/Type%3A%3ATiny)

# SOURCE

The development version is on github at [https://github.com/robrwo/MooX-Const](https://github.com/robrwo/MooX-Const)
and may be cloned from [git://github.com/robrwo/MooX-Const.git](git://github.com/robrwo/MooX-Const.git)

# BUGS

Please report any bugs or feature requests on the bugtracker website
[https://github.com/robrwo/MooX-Const/issues](https://github.com/robrwo/MooX-Const/issues)

When submitting a bug or request, please include a test-file or a
patch to an existing test-file that illustrates the bug or desired
feature.

# AUTHOR

Robert Rothenberg <rrwo@cpan.org>

This module was inspired by suggestions from Kang-min Liu 劉康民
<gugod@gugod.org> in a [blog post](http://blogs.perl.org/users/robert_rothenberg/2018/11/typeconst-released.html).

# CONTRIBUTOR

Kang-min Liu 劉康民 <gugod@gugod.org>

# COPYRIGHT AND LICENSE

This software is Copyright (c) 2018-2023 by Robert Rothenberg.

This is free software, licensed under:

```
The Artistic License 2.0 (GPL Compatible)
```