NAME

    Object::Pad::FieldAttr::Checked - apply value constraint checks to
    Object::Pad fields

SYNOPSIS

    With Types::Standard:

       use Object::Pad;
       use Object::Pad::FieldAttr::Checked;
       use Types::Standard qw( Num );
    
       class Point {
          field $x :param :reader :Checked(Num);
          field $y :param :reader :Checked(Num);
       }
    
       Point->new( x => 123, y => "456" );         # this is fine
    
       Point->new( x => "hello", y => "world" );   # throws an exception

    Or, standalone:

       use Object::Pad;
       use Object::Pad::FieldAttr::Checked;
    
       package Numerical {
          use Scalar::Util qw( looks_like_number );
          sub check { looks_like_number $_[1]; }
       }
    
       class Point {
          field $x :param :reader :Checked('Numerical');
          field $y :param :reader :Checked('Numerical');
       }
    
       ...

DESCRIPTION

    This module provides a third-party field attribute for
    Object::Pad-based classes, which declares that values assigned to the
    field must conform to a given constraint check.

    WARNING The ability for Object::Pad to take third-party field
    attributes is still new and highly experimental, and subject to much
    API change in future. As a result, this module should be considered
    equally experimental.

    Additionally, the behaviour provided by this module should be
    considered more of a work-in-progress stepping stone. Ideally,
    constraint syntax ought to be provided in a much more fundamental way
    by Perl itself, allowing it to be used on my lexicals, subroutine
    parameters, and other places as well as object fields. This module is
    more of a placeholder to allow some part of that behaviour to be
    specified for object fields, while not getting in the way of a more
    general, more powerful system being added in future.

FIELD ATTRIBUTES

 :Checked

       field $name :Checked(EXPRESSION) ...;

    Declares that any value assigned to the field during the constructor or
    using an accessor method must conform to the constraint checker
    specified by the expression. Attempts to assign a non-conforming value
    will throw an exception and the field will not be modified. Currently
    only scalar fields are supported.

    At compiletime, the string given by EXPRESSION is eval()'ed in scalar
    context, and its result is stored as part of the field's definition.
    The expression must yield either an object reference, or a string
    containing the name of a package. In either case, a method called check
    must exist on it.

    During the string eval() of the expression, the strict 'subs' pragma is
    specifically disabled, allowing a bareword to be used as the name of a
    package. This feature may be removed in future, so be sure to quote
    package names if required.

       field $x :Checked('CheckerPackage');

    At runtime, this constraint checker is used every time an attempt is
    made to assign a value to the field from outside the object class,
    whether that is from :param initialisation, or invoking a :writer or
    :accessor. The checker is used as the invocant for invoking a check
    method, and the new value for the field is passed as an argument. If
    the method returns true, the assignment is allowed. If false, it is
    rejected with an exception and the field itself remains unmodified.

       $ok = $checker->check( $value );

    (For performance reasons, the check method is actually resolved into a
    function at compiletime when the :Checked attribute is applied, and
    this stored function is the one that is called at assignment time. If
    the method itself is replaced later by globref assignment or other
    trickery, this updated function will not be used.)

    As this is the interface supported by Types::Standard, any constraint
    object provided by that module is already supported here.

       use Types::Standard qw( Str Num );
    
       field $name :Checked(Str);
       field $age  :Checked(Num);

    Note carefully that direct assignment into the field variable by code
    within the class is not checked. This is partly because of design
    considerations, and partly because any way to implement that would be
    horribly slow, or flat-out impossible. Prior to version 0.04 this
    module used to claim that even direct assignments would be checked. but
    this gave a false sense of safety if deeply-nested containers were
    involved and modified from within.

TODO

      * Support :mutator accessors as well. This is a bit harder because
      lvalue magic.

SEE ALSO

      * Object::Pad::FieldAttr::Isa - apply class type constraints to
      Object::Pad fields

AUTHOR

    Paul Evans <leonerd@leonerd.org.uk>