Re^4: RFC: Tutorial: use strict; now what!?

[Xiong]

I am trying to follow this as I use hashes all over the place. What exactly is a soft reference.

tangent, you'll get plenty of explanations of symbolic references. But even after you understand them well it may not be clear how hashes come into this, in almost the same breath.

To recap, moritz offered hashes as an alternative to hard references. There was some discussion of this and JavaFan noted hashes can invite some of the same issues strict 'refs' is meant to avoid. You may not see the connection; it took me a moment to pick it up.

Seen one way, use strict; is a typo-catcher. In the simple case:

our $x13 = 'foo';
print $x31;

... may not work as you expect because you really meant to type $x13 both times. But use strict; will complain that you did not declare $x31 and so the typo is caught.

If you attempt to take a hard reference and make a typo, you'll get an error, too. But if you take a symbolic reference and don't use strict; then you will not get early notice of your typo.

Where hashes begin to resemble (in a certain mental state) symbolic references is when you start stuffing a lot of unrelated data into a hash with literal keys; and then later you want to take something out:

my %hash;
$hash{boy}             = 'Arnold';
$hash{pension_balance} = 172.50;
$hash{filehandle}      = $fh;
print $hash{pensionbalance};

The typo will not be caught, with or without use strict; instead, perl will happily autovivify $hash{pensionbalance} for you and immediately print it... although it's undefined. This can be an annoying thing to track down.

For more on this, see Re: Accessing hash from within module, where I demonstrate exactly this kind of risky approach... and note that it's almost as evil as global variables. I would not even have mentioned it if the OP didn't seem solidly committed to globals in the first place. As questionable as it is, I still think it's better than no strict 'refs';

[tangent]

Thanks for taking the time to explain Xiong, and I get the connection now, even though I had to read through everything a few times. Being an obedient student, I always use warnings as well as strict, and note that I get a warning if I try to print $hash{pensionbalance} and also if I say something like:

my $debts = 100;
my $money_left = $debts - $hash{pensionbalance};
# Use of uninitialized value in subtraction (-)

Am I safe to assume then that use warnings will always keep me safe?

[runrig]

In this case it will. Or if you set all warnings to be fatal then you're extra "safe". Depending on how you use the value though, you may or may not get a warning. If you just use it as a boolean you do not get a warning:

if ($hash{pensionbalance}) {
  print "Got a pension balance!\n";
}

[tangent]

I kinda knew there would be something. And "if ($hash{pensionbalance})" is exactly the sort of thing I would put in to avoid all those "uninitialized value" errors. The plot thickens... I could change those conditions to:

if (exists $hash{pensionbalance}) {
    print "It's there, there's just nothing in it\n"
}

[tangent]

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