# Class::Tiny::ConstrainedAccessor - Generate Class::Tiny accessors that apply type constraints

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[Class::Tiny](https://metacpan.org/pod/Class::Tiny) uses custom accessors if they are defined before the
`use Class::Tiny` statement in a package.  This module creates custom
accessors that behave as standard `Class::Tiny` accessors except that
they apply type constraints (`isa` relationships).  Type constraints
can come from [Type::Tiny](https://metacpan.org/pod/Type::Tiny), [MooseX::Types](https://metacpan.org/pod/MooseX::Types), [MooX::Types::MooseLike](https://metacpan.org/pod/MooX::Types::MooseLike),
[MouseX::Types](https://metacpan.org/pod/MouseX::Types), or [Specio](https://metacpan.org/pod/Specio).  Alternatively, constraints can be applied
using the technique described in
["Constraints without a type system"](#constraints-without-a-type-system).

Example of a class using this package:

    package SampleClass;
    use Scalar::Util qw(looks_like_number);

    use Type::Tiny;

    # Create the type constraint
    use vars::i '$MediumInteger' = Type::Tiny->new(
        name => 'MediumInteger',
        constraint => sub { looks_like_number($_) and $_ >= 10 and $_ < 20 }
    );

    use Class::Tiny::ConstrainedAccessor {
        medint => $MediumInteger,           # create accessor sub medint()
        med_with_default => $MediumInteger,
    };

    # After using ConstrainedAccessor, actually define the class attributes.
    use Class::Tiny qw(medint regular), {
        med_with_default => 12,
    };

# CONSTRAINTS WITHOUT A TYPE SYSTEM

If you don't want to use [Type::Tiny](https://metacpan.org/pod/Type::Tiny) or one of the other type systems listed
above, you can create your own constraints as two-element arrayrefs.  Example:

    use Class::Tiny::ConstrainedAccessor
        'field' => [ \&checker_sub, \&message_sub ];

`checker_sub` and `message_sub` are used as follows to check `$value`:

    checker_sub($value) or die get_message($value);

Therefore, `checker_sub` must return truthy if `$_[0]` passes the constraint,
or else falsy.  `get_message` must return something that can be passed to
`die()`, when given a `$_[0]` that has failed the constraint.

If your profile ever tells you that constraint-checks are on the critical
path, try custom constraints.  They may give you more control or opportunity
for optimization than general-purpose type systems.

# SUBROUTINES

## import

Creates the accessors you have requested.  Constraints can be passed as a list
or hashref of variable/constraint pairs.  Basic usage:

    # Constraints are passed as a list of pairs
    use Class::Tiny::ConstrainedAccessor
        name => constraint
        [, name2 => constraint ...]; # ... any number of name=>constraint pairs

    # Constraints are passed as a hashref
    use Class::Tiny::ConstrainedAccessor {
        name => constraint,
        [, name2 => constraint ...]; # ... any number of name=>constraint pairs
    };

This also creates a [BUILD()](https://metacpan.org/pod/Class::Tiny#BUILD) subroutine to check the
constructor parameters, if a `BUILD()` doesn't already exist.

If a `BUILD()` does exist (e.g., you said `use subs 'BUILD';`), this package
will create the same function, taking the same parameters as `BUILD()` would,
but call it `_check_all_constraints()`.   You can call this checker from your
own `BUILD()` if you want to.

# OPTIONS

To specify options, pass an **arrayref** as the first argument on the \`use\`
line.  This is because a hashref carries attributes and constraints.
For example:

    use Class::Tiny::ConstrainedAccessor [ OPTION=>value ],
        name => constraint ...;

Valid options are:

- NOBUILD

    If `NOBUILD => 1` is given, the constructor-parameter-checker
    is created as `_check_all_constraints` regardless of whether `BUILD()`
    exists or not.  Example:

        package MyClass;
        use Class::Tiny::ConstrainedAccessor
            [NOBUILD => 1],
            foo => SomeConstraint;
        # Now $object->_check_all_constraints($args) exists, but not BUILD().

# AUTHORS

Created by Christopher White, `<cxwembedded at gmail.com>`.  Thanks to
Toby Inkster (TOBYINK) and Ivan Butorin (FISHBONE) for code contributions.

# BUGS

Please report any bugs or feature requests through the GitHub Issues interface
at [https://github.com/cxw42/Class-Tiny-ConstrainedAccessor/issues](https://github.com/cxw42/Class-Tiny-ConstrainedAccessor/issues).  I will be
notified, and then you'll automatically be notified of progress on your bug as
I make changes.

# SUPPORT

You can find documentation for this module with the perldoc command.

    perldoc Class::Tiny::ConstrainedAccessor

You can also look for information at:

- GitHub (report bugs here)

    [https://github.com/cxw42/Class-Tiny-ConstrainedAccessor](https://github.com/cxw42/Class-Tiny-ConstrainedAccessor)

- MetaCPAN

    [https://metacpan.org/pod/Class::Tiny::ConstrainedAccessor](https://metacpan.org/pod/Class::Tiny::ConstrainedAccessor)

# LICENSE

Copyright 2019 Christopher White and contributors.

This program is free software; you can redistribute it and/or modify it
under the terms of the the Apache License (2.0). You may obtain a
copy of the full license at:

[https://www.apache.org/licenses/LICENSE-2.0](https://www.apache.org/licenses/LICENSE-2.0)

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
