Question

In Perl, if I want to use named parameters in an object constructor, my code seems a bit clumsy if I wish to have some validation.

sub new {

   my $class = shift;
   my $self = {};

   my %args = @_;
   foreach my $argname (keys %args) {
     if    ($argname eq 'FOO') { $self->{$argname} = $args{$argname}; }
     elsif ($argname eq 'BAR') { $self->{$argname} = $args{$argname}; }
     elsif ($argname eq 'BAZ') { $self->{$argname} = $args{$argname}; }
     …
     else                      { die "illegal argument $argname\n"; }
   }

   bless $self;
   return $self;
}

Firstly it seems a bit clumsy to have a temporary hash (%args). Secondly the whole if chain seems verbose and tedious.

The latter can be simplified to

  if ('-FOO-BAR-BAZ-'=~m/-$argname-/) { $self->{$argname} = $args{$argname} }
  else { die "..."; }

but I imagine this can be improved.

If I need to check values, the if … elsif chain is still necessary?

I’ve searched a little but cannot find a better idiom. Is there one (other than using a Perl OO framework of some sort)

Answer 1

I found myself constantly writing unnecessary code which checked the given parameters. But then I discovered Params::Validate. It is easy to use and if the validation fails it provides very clear and user-friendly error messages. Covering all possible combinations of parameters and their error messages is a tedious task. I prefer this way instead:

use Params::Validate qw/:all/;
sub new {
    my $pkg = shift;
    validate(
        @_, {
            foo => { type => SCALAR | ARRAYREF },
            bar => { type => SCALAR, optional => 1},
            baz => { type => ARRAYREF, default => ['value'] },
            quux => { isa => 'CGI' }
        }
    );

    return bless { @_ }, $pkg;
}

And later this code

MyApp::Something->new(
    foo => 123,
    bbr => 'typo',
    quux => CGI->new()
);

becomes:

The following parameter was passed in the call to MyApp::Something::new but was not listed in the validation options: bbr
 at test.pl line 14.
    MyApp::Something::new(undef, 'foo', 123, 'bbr', 'typo', 'quux', 'CGI=HASH(0x7fd4fa1857e0)') called at test.pl line 27