How can I require a Moose constructor arg that is not a attribute?

Question

I have a Moose object module that should accept a relatively large data structure (ds) as one of its constructor arguments. It is used to calculate some of the object's attributes. However, I do not wish to store ds itself as an attributes -- it is only needed during the construction of the object.

I thought of using BUILDARGS but then I'm not sure how to define that ds is a required argument.

How can I work around this?

Answer 1

I'd be inclined to have a constructor that takes only the calculated values derived from your data structure. Then use a different method to take limited params plus the data structure as args.

sub generate_foo_from_ds {
    my $class = shift; 
    my %arg = @_;

    my $ds = $arg{foo_data};

    # Get attributes from args
    my %attrib;
    for (qw(foo bar baz ) {
        croak "Attrib '$_' is required" unless exists $arg{$_};
        $attrib{$_} = $arg{$_};
    }

    # calculate some more attributes here.
    $attrib{griz} = $ds->{whee} * $ds->{whoosh} / $ds->{whiz}[12];

    my $foo = $class->new( %attrib );

    return $foo;
}

Then make your objects like so:

my $foo = Foo->generate_foo_from_ds( foo_data => $bar, foo => 1, bar => 2, baz => 2 );

Now you don't have to worry about weird serialization issues or BUILDARGS or even BUILD. You have a simple method and that is all.