dmitri has asked for the wisdom of the Perl Monks concerning the following question:

Dear Perl Monks,

I have several modules that use large tables to generate code and other tables, etc. Usually, these tables look like this: 

use constant SOME_TABLE => {
  &SOMECONSTANT => [
     &ANOTHER1 => sub { ... },
     &CONST2   => sub { ... },
  ],
  .......
};

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These can go several levels deep. The problem I've run into is that I like the arrow notation, as it makes the tables easy and intuitive to read. However, since I use constants and left side of the arrow is placed in quotes, I have to call the constant explicitly, using ampersand.

The problem with this is that if such constant is undefined (due to a type-o, for instance), this error will not get caught until run-time. It would be neat to catch these bastards at compile-time, but then I have to lose the arrow and use a comma instead.

Question: is there a way to catch undefined subroutines at compile-time and keep using the arrow notation at the same time?

Replies are listed 'Best First'.
Re: Constants and arrow notation.
by davido (Cardinal) on Oct 28, 2004 at 18:45 UTC

    You can place the constant within parens, causing it to be checked at compiletime.

    use strict;
use warnings;

use Data::Dumper;

use constant THIS => 100;

print "Testing: \n";

my %hash = ( (THIS) => 200 );

print Dumper \%hash;

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    Try that code with the "use constant" line commented out, and you'll see it generates a compiletime error. The error you'll get is something along the lines of:

    Bareword "THIS" not allowed while "strict subs" in use at mytest.pl line 12. Execution of mytest.pl aborted due to compilation errors.


    Dave

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      Thank you so very much! That's exactly what I needed. Of course, this only works with use strict;.

      /dmitri dances the happy dance.

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Re: Constants and arrow notation.
by samtregar (Abbot) on Oct 28, 2004 at 18:35 UTC
    I think you can do this by putting the code in a BEGIN block. A BEGIN block asks Perl for a bit of run-time during compile-time. Thus, if you've got something happening at run-time that you'd rather have happen during compilation, BEGIN will usually do the trick.

    -sam

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Re: Constants and arrow notation.
by perrin (Chancellor) on Oct 28, 2004 at 18:41 UTC
    Maybe hashes with fixed keys would be a better approach here than constants.
[reply]

      I guess perrin is talking about Hash::Util, with this, it lets you lock hash keys. One example as how to use that module:

      use Data::Dumper;
use Hash::Util;

use strict;
use warnings;

my $constants;
$constants->{CONST_ABXCDEFG} = 1;
$constants->{CONST_ABCXDEFG} = 2;
$constants->{CONST_ABCDXEFG} = 3;
$constants->{CONST_ABCDEXFG} = 4;
$constants->{CONST_ABCDEFXG} = 5;
Hash::Util::lock_keys(%{$constants});

print Dumper($constants);

$constants->{CONST_ABCDEFGX} = 6;

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