Perl: When DWIM Doesn't

> Both above spot the undef gotcha.

The builtin open() even tries to spot when it's not intentional. The documentation puts "undef" in quotations in the section where it introduces the feature (suggesting it has to be the literal), and the implementation goes to some effort to agree with it. For example, the following code dies:

  my $config = { file_paht => "/opt/config" };
  my $path = $config->{file_path};

  open (my $fh, "<", $path)
    or  die "can't open $path: $!";

In the case in the article it's a bug in the implementation, combined with a typo, which introduces the little known feature.

Using Path::Class obfuscates the open, to my eyes. Using IO::File is a nice suggestion. Your lock_keys suggestion below will probably be our update to the code for future protection, though.

!!! Supplementary for future readers of this comment thread !!!

The pragma autodie also spots this problem!

So....

    use strict;
    use warnings;
    use autodie;

    our $config;
    $config->{file_paht} = "somefile";

    open my $fh, '<', $config->{file_path};

Will throw an exception at open(). NB. Bizarrely the error is Use of uninitialized value $file in sprintf... but hey the problem is still caught :)

Another solution could be to lock the keys of $config after its been configured:

  use Hash::Util 'lock_keys';

  our $config;
  $config->{file_paht} = "/opt/app/data_file";
  lock_keys( %$config );

Now any attempt to use an undefined key will throw an error. So on the open() it would now throw error: Attempt to access disallowed key 'file_path' in a restricted hash at...

While that can be helpful, that comes with its own set of problems. It's harder to say $session->ab_variation_overrides ||= {inital => "values"}. I think it's possible, but I've been warned off by the huge EXPERIMENTAL warning around the lvalue subroutine return documentation [1] and the fact that it's been labelled "experimental" for a long time now, so I don't dare put it in production code, or use it where I can't count on someone knowing everything in those docs. I've been in tight loops where the cost of a function call was noticeable vs. the hash lookup. You can't avoid the fact that lots of other libraries and existing code expects hashes in one form or another (the calling convention that essentially puts a hash in a list is just another variant of that).

You're not "wrong", but it doesn't solve very much of the problem in practice. Still, it can be useful where appropriate, and there are libraries that use such approaches for safety; here's one example off the top of my head [2].

This is where I really prefer Python over Perl; Perl nominally has bullet-point feature compatibility with Python in most respects, but where in Perl operator overloading, tying, and the various other ways of overloading things always come with caveat lists a mile long and often go together poorly, in Python they Just Work, and actually can be used together. In this case, properties. They work. I've even made them dance and sing before with nonstandard behaviors and they still work.

[1]: http://perldoc.perl.org/perlsub.html#Lvalue-subroutines

[2]: http://search.cpan.org/~mlehmann/JSON-XS-2.32/XS.pm

Well, that's embarrassing. I fat-fingered those while I was transcribing code into the article. "Surprise Two" was supposed to be about the autovivification surprise, not me mistyping things (article updated).

Even still, your point about using objects and encapsulation stands. If the session were being assigned to via setters, and read from with getters, the grep aliasing wouldn't do its damage.

Cheers.

I think it's a pretty weak reason to give up on a language because you were using a global variable and expecting it to automatically be treated as a local variable within your function.

As others have mentioned, you are using a global variable ($_) inside a function. This is generally considered a bad practice.

The standard perl idiom of getting your function arguments via shift would make this a non-issue.

This is a feature, not a bug. And this is not _requirement_, it is up to you to use it. You are always free to pass parameters the way it would not happen..

This is not much different than mutating a global variable inside a function.

I would write your code as:

    foo($_) foreach @bar;

    sub foo {
        my $var = shift;
        $var =~ s/foo/bar/;
        print $var;
    }

You can also fix this is to localize $_ inside the function, if you don't want to pass $_ as a parameter.

    foo foreach @bar;

    sub foo {
        my $var = $_;
        $var =~ s/foo/bar/;
        print $var;
    }

You can even use $_ instead of $var (although it looks a little weird assigning $_ to itself):

    foo foreach @bar;

    sub foo {
        my $_ = $_;
        s/foo/bar/;
        print;
    }

You're funny. If this really pushed you over the edge, you were born -on- the edge, and didn't give Perl a fair shake.

Frankly, the first one is a clear explanation of why you should have a config object not a hash.

Don't use a freeform data structure for data that isn't actually freeform if you want the perl5 VM to be able to catch errors for you.

Shutterstock are legendary round the perl community for being a hive of badly written ancient code that never gets significant refactoring effort though; combine this with a significant tendency not to use libraries due to NIH syndrome and their experience of gotchas becomes more an inevitability than bad luck.

Or: You might love your programming language, but if your relationship with it tends towards the abusive, the problems end up cutting both ways.

(though it's only in the past half decade that the perl community as a whole has seriously moved to trying to make the sensible techniques standard - and any project started during the dot-com era is almost certainly a hive of badly written now-ancient code; there's similarly timed spikes of awful PHP and then rails code due to their popularity spikes since)

In the cases in the article, strict and warnings won't save you.

Assuming you're not a troll, you have bad timing who left just before Moose and dependable grammar extensions. I've been doing some Python lately and regularly scream when I lack usable anonymous funs (>1 instruction) or can't find a lib I know exist on CPAN, etc.

ymmv.