Perl And Type Checking

(aka known as which language am I using again?)

Dave Rolsky has a new post on perl 5 overloading. It’s fairly informative, but it contains this little gem (emphasis mine):

If you don’t care about defensive programming, then Perl 5′s overloading is perfect, and you can stop reading now. Also, please let me know so I can avoid working on code with you, thanks.

Defensive programming, for the purposes of this entry, can be defined as "checking sub/method arguments for sanity".

Blanket statements like this really get my gripe up. Let’s have a look at defensive argument checking in Perl taken to its illogical conclusion.

Defensive Primitive Type Checking

use strict;
use warnings;

use Carp;
use Scalar::Util 'looks_like_number';

sub can_legally_drink
{
    my $age = shift;
    croak "$age is not a number" unless looks_like_number($age);
    return $age >= 18;
}

print can_legally_drink('x'), "\n";

And fantastic news! can_legally_drink correctly detected that my argument isn’t a number.

x is not a number at age.pl line 10
        main::can_legally_drink('x') called at age.pl line 14

But hang on a minute. Not all integers are ages. Surely we want to check if a real age type was passed in.

Checking For A ‘Real’ Type

My stripped down defensive age type might look something like this.

package age;

use Carp;
use Scalar::Util 'looks_like_number';

sub isa_age
{
    my $arg = shift;
    return ref($arg) and blessed $arg and $arg->isa('age');
}

sub new {
    my ($class, $years) = @_;
    croak "$years is not a number" unless looks_like_number($years);
    bless { years => $years }, $class;
}

sub years {
    return $_[0]->{'years'}
}

sub less_than {
    my ($self, $other) = @_;
    croak "$other is not an age" unless isa_age($other);
    return $self->years() < $other->years();
}

And then my calling code can look like this:

package main;

sub can_legally_drink
{
    my $age = shift;
    croak "$age is not an age" unless $age->isa('age');
    return ! $age->less_than(age->new(18));
}

print can_legally_drink(age->new(18)), "\n";
print can_legally_drink(18), "\n";

And woohoo, the second line throws an error as I wanted.

Actually, I don’t write Perl like this. Dave, you probably want to avoid working on code with me, thanks.

Moose

To be fair, Rolsky is talking his own book. Moose has a bunch of stuff that handles all this type checking malarky nice and cleanly. If you’re building something big in Perl, you should take a look.

But if you really care about types, I mean defensive programming that much, you could use a statically typed language instead and then you even get fast code thrown in for free.

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