Dear monks,

this is not a serious problem, but I come across its variants quite frequently.

I have a method (say open_for_reading), which accepts hashref of parameters. Since it is quite important method I need all of its parameters to be understood. The open_for_reading method is overriden in a couple of subclasses, each of them understands more parameters.

my $context = $file->open_for_reading( {} );

my $context2 = $file->open_for_reading( { 'path' => $file } );
# OK
my $context_derived
    = $file_derived->open_for_reading( { 'path' => $file, 'join_longca
+lls' => 1 } );

# this should fail
my $context_derived = $file->open_for_reading( { 'join_longcalls' => 1
+ } );

[download]

Without the validation the code is something like:

package MyBaseClass;

sub open_for_reading {
    my ($this, $params_ref) = @_;

    $params_ref ||= {};
    return $this->get_context('path' => $params_ref->{'path'} || $this
+->path);
}

package MyDerivedClass;

sub open_for_reading {
    my ($this, $params_ref) = @_;

    $params_ref ||= {};
    my $context = $this->SUPER::open_for_reading();
    if ($params_ref->{'join_longcalls'}){
        ... modify context
    }
    return $context;
}

[download]

For parameter validation I found Params::Validate from CPAN. The only way I was able to add the validation is like this:

package MyBaseClass;

sub open_for_reading_validation_spec {
    return { 
        'path' => 0,
    };
}

sub open_for_reading {
    my ($this, $params_ref) = @_;

    $params_ref ||= {};
    Params::Validate::validate($params_ref, $this->open_for_reading_va
+lidation_spec);
    return $this->do_open_for_reading($params_ref);
}

sub do_open_for_reading {
    my ($this, $params_ref) = @_;

    return $this->get_context('path' => $params_ref->{'path'} || $this
+->path);
}


package MyDerivedClass;
use base qw(MyBaseClass);

sub do_open_for_reading {
    my ($this, $params_ref) = @_;

    my $context = $this->SUPER::do_open_for_reading($params_ref);
    if ($params_ref->{'join_longcalls'}){
        ... modify context
    }
    return $context;
}

sub open_for_reading_validation_spec {
    my ($this) = @_;

    return {
        %{$this->SUPER::open_for_reading_validation_spec},
        'join_longcalls' => 0,
    };
}

[download]

Instead of one method I have three:

I can live with three methods, but besides open_for_reading there are also open_for_writing, open_for_processing, ... so whole interface suddenly "triplicates".

Do you think is there a different solution?

Even without the validation there is a "naming" problem of its own. I have an interface method which does initialization and cleanup, and between these two blocks is a code to be overriden in subclasses.

sub method {
    my $this = shift;
    
    $this->do_some_initialization;
    $this->do_something_which_can_be_overriden_in_subclasses();
    $this->do_some_cleanup;
}

[download]

I put the inner block into separate method, but I often have problem to name it. Do you use some naming convention for the "executive" methods? Prefixes (do_, exec_)? Postfixes (_body)?

In reply to Method parameters validation and inheritance by roman

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