perl – 懒惰属性强制

使用 Moose,您可以在属性上使用惰性构建器,如果尚未填充属性,则首次访问属性时将调用构建器.您可以使用强制对属性进行类型强制,但只要设置了属性,就会应用此属性,因此即使在对象初始化时也是如此.

我正在寻找一种实现延迟强制的方法,其中一个属性可能最初被填充,但只有在首次访问时才被强制执行.当胁迫很昂贵时,这很重要.

在下面的示例中,我使用union类型和方法修饰符来执行此操作：

package My::Foo;
use Moose;
has x => (
    is => 'rw',isa => 'ArrayRef | Int',required => 1
);

around "x" => sub {
    my $orig = shift;
    my $self = shift;
    my $val = $self->$orig(@_);
    unless(ref($val)) {
        # Do the cocerion
        $val = [ map { 1 } 1..$val ];
        sleep(1); # in my case this is expensive
    }
    return $val;
}; 
1;

my $foo = My::Foo->new( x => 4 );
is_deeply $foo->x,[ 1,1,1 ],"x converted from int to array at call time";

但是这有一些问题：

>我不喜欢union类型方法修饰符方法.这与use coercion instead of unions的“最佳实践”建议背道而驰.这不是声明性的.
>我需要在许多类中使用许多属性来执行此操作.因此需要某种形式的DRY.这可能是元属性角色,类型强制,你有什么.

更新：
我遵循ikegami’s建议将昂贵的类型强制封装在一个对象内,并为这个对象提供外部强制：

package My::ArrayFromInt;
use Moose;
use Moose::Util::TypeConstraints;
subtype 'My::ArrayFromInt::Inner',as 'ArrayRef[Int]';
coerce 'My::ArrayFromInt::Inner',from 'Int',via { return [ (1) x $_ ] };
has uncoerced => (is => 'rw',isa => 'Any',required => 1); 
has value => (
    is      => 'rw',isa     => 'My::ArrayFromInt::Inner',builder => '_buildValue',lazy    => 1,coerce  => 1
);
sub _buildValue {
    my ($self) = @_; 
    return $self->uncoerced;
}
1;
package My::Foo;
use Moose;
use Moose::Util::TypeConstraints;
subtype 'My::ArrayFromInt::Lazy' => as class_type('My::ArrayFromInt');
coerce 'My::ArrayFromInt::Lazy',via { My::ArrayFromInt->new( uncoerced => $_ ) };
has x => (
    is => 'rw',isa => 'My::ArrayFromInt::Lazy',required => 1,coerce => 1
);
1;

如果调用$foo-> x->值,则此方法有效.但是这并没有解决第2点,因为我需要为我想要转换的每个属性创建My :: ArrayFromInt和:: Lazy子类型.如果可能的话,我想避免调用$foo-> x->值.

has _x => (
    is       => 'ro',isa      => 'Int|MyArrayOfInts',init_arg => 'x',);

has x => (
    is => 'ro',lazy => 1,isa => 'MyArrayOfInts',coerce => 1,default => sub { $_[0]->_x },);

has_lazily_coerced x => (
    is => 'ro',isa => 'TargetType',);