# NAME

Types::Const - Types that coerce references to read-only

# VERSION

version v0.5.0

# SYNOPSIS

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

...

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

# DESCRIPTION

This is a type library that provides types to coerce read-only hash
and array reference attributes to be deeply read-only.

See the [known issues](#known-issues) below for a discussion of
side-effects.

# TYPES

## `` Const[`a] ``

Any defined reference value that is read-only.

If parameterized, then the referred value must also pass the type
constraint, for example `Const[HashRef[Int]]` must a a hash reference
with integer values.

It supports coercions to read-only.

This was added in v0.3.0.

# KNOWN ISSUES

## Side-effects of read-only data structures

A side-effect of read-only data structures is that an exception will
be thrown if you attempt to fetch the value of a non-existent key:

```
Attempt to access disallowed key 'foo' in a restricted hash
```

The work around for this is to check that a key exists beforehand.

## Performance issues

Validating that a complex data-structure is read-only can affect
performance.  If this is an issue, one workaround is to use
[Devel::StrictMode](https://metacpan.org/pod/Devel%3A%3AStrictMode) and only validate data structures during tests:

```perl
has bar => (
  is      => 'ro',
  isa     => STRICT ? Const[ArrayRef[Str]] : ArrayRef,
  coerce  => 1,
);
```

Another means of improving performance is to only check the type
once. (Since it is read-only, there is no need to re-check it.)

## RegexpRefs

There may be an issue with regexp references. See
[RT#127635](https://rt.cpan.org/Ticket/Display.html?id=127635).

# SUPPORT FOR OLDER PERL VERSIONS

Since v0.5.0, 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 v0.4.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)

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

[Types::Standard](https://metacpan.org/pod/Types%3A%3AStandard)

[Types::ReadOnly](https://metacpan.org/pod/Types%3A%3AReadOnly)

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

# BUGS

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

# SOURCE

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

# AUTHOR

Robert Rothenberg <rrwo@cpan.org>

# CONTRIBUTOR

Mohammad S Anwar <mohammad.anwar@yahoo.com>

# 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)
```
