How can I lift an attribute constraint depending on another attribute from a trigger into a type refinement? - perl

Question

This works:
use Moops;
class Foo :ro {
use Types::Common::Numeric qw(PositiveOrZeroInt);
has from => required => true, isa => PositiveOrZeroInt;
has to => required => true, isa => PositiveOrZeroInt, trigger => method($to) {
die 'must be from ≤ to' unless $self->from <= $to
};
}
Foo->new(from => 0, to => 1); # ok
Foo->new(from => 1, to => 0); # "must be from ≤ to at …"
I would like to be able to make the constraint part of the type, somehow.
use Moops;
class Bar :ro {
use Types::Common::Numeric qw(PositiveOrZeroInt);
has from => required => true, isa => PositiveOrZeroInt;
has to => required => true, isa => PositiveOrZeroInt->where(sub {
$self->from <= $_
});
}
Bar->new(from => 0, to => 1);
# Global symbol "$self" requires explicit package name
# (did you forget to declare "my $self"?)
I checked that the where sub only receives one parameter.

Answer 1

If you wanted to do it in a type check, you could combine the two attributes into one attribute which would be an arrayref holding both numbers.
use Moops;
class Foo :ro {
use Types::Standard qw(Tuple);
use Types::Common::Numeric qw(PositiveOrZeroInt);
has from_and_to => (
required => true,
isa => Tuple->of(PositiveOrZeroInt, PositiveOrZeroInt)->where(sub {
$_->[0] <= $_->[1];
}),
# Provide `from` and `to` methods to fetch values
handles_via => 'Array',
handles => {
'from' => [ get => 0 ],
'to' => [ get => 1 ],
},
);
# Allow `from` and `to` to be separate in the constructor
method BUILDARGS {
my %args = ( #_==1 ? %{$_[0]} : #_ );
$args{from_and_to} ||= [ $args{from}, $args{to} ];
\%args;
}
}
Foo->new(from => 0, to => 1); # ok
Foo->new(from => 1, to => 0); # "must be from ≤ to at …"
I wouldn't do it in a type check though. I'd do it in BUILD (no, not BUILDARGS) if the attribute were read-only, or trigger if it were read-write.