Problem: Lazy Hash Evaluation

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

Hello, I'm trying to write some test code for the subroutine ScanDeps however, I'm seeing some odd behaviour that I can't explain. A cut-down version of my test code looks like this:

sub generic_rv_test {
    my $rv = shift;

    foreach my $key (keys %$rv) {
        if (exists($rv->{$key}{used_by})) {
            # test used_by ...
        } else {
            # fail test
        }
    }
}  

my $rv = scan_deps(
    files   => ['t/data/pluggable/Foo.pm'],
    recurse => 1,
);
generic_rv_test($rv);
[download]

However, my test was failing so I added the following lines after generic_rv_test($rv); to start debugging the issue:

use Data::Dump qw(dump);
print dump $rv;
[download]

However, adding these lines somehow caused my test to start working! What seems to be happening is that $rv is more complete with the dump lines than without. Can someone explain what is happening? Basically, I want to be able to ensure $rv is contains all it's information before calling the test function on it but without having to dump $rv ...

$rv without the dump lines:

{
  "Pluggable.pm" => {
                      file => "C:/Perl/site/lib/Module/Pluggable.pm",
                      key  => "Module/Pluggable.pm",
                      type => "module",
                    },
}
[download]

$rv with the dump lines:

{
  "Pluggable.pm" => {
                      file => "C:/Perl/site/lib/Module/Pluggable.pm",
                      key  => "Module/Pluggable.pm",
                      type => "module",
                      used_by => ["Foo.pm"],
                    },
}
[download]

Thanks, AdrianI

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Re: Problem: Lazy Hash Evaluation by diotalevi (Canon) on Feb 25, 2007 at 00:28 UTC
If the problem is that the value used_by is only present when you dump, there's clearly a missing piece here. There's nothing about perl to magically invent this used_by value. You ought to show your cards. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply]
Re^2: Problem: Lazy Hash Evaluation by Anonymous Monk on Feb 25, 2007 at 14:17 UTC
> If the problem is that the value used_by is only present when you dump That's exactly the problem. > There's nothing about perl to magically invent this used_by value I was expecting it to be produced as part of the output of scan_deps() and the subrountine generic_rv_test() is therefore testing that it was produced. The only thing that I can think of that might explain this is that the data stucture $rv hasn't been completely filled out, as a result of scan_deps(), before it's used by generic_rv_test() except in the case when I dump $rv. However, that strikes me as being unlikely. > You ought to show your cards. I assume you mean give you the full code so here it is: The test script (4-pluggable_fake.t)" Read more... (1077 Bytes) NB if I uncomment either of the two groups of comments in the above I get the `used_by => ["Foo.pm"],` line I'm testing for and all is OK. The complete generic_rv_test subrountine: Read more... (4 kB) And finally, the subroutine scan_deps comes from the CPAN module Module::ScanDeps. Thanks for the help, Adrian	[reply] [d/l] [select]
Re: Problem: Lazy Hash Evaluation by blazar (Canon) on Feb 24, 2007 at 22:40 UTC
Hello, I'm trying to write some test code for the subroutine ScanDeps however, I'm seeing some odd behaviour that I can't explain. A cut-down version of my test code looks like this: I have some trouble understanding what your actual problem may be: indeed you did good to prepare a cut-down version of you code, but you should go as far as making that into a minimal but complete example exhibiting the problem. You should specify what output you expect and what you get instead. `use Data::Dump qw(dump); print dump $rv;` [download] Data::Dumper exports `Dumper()` and AFAICT knows no `dump()`: `C:\temp>perl -MData::Dumper=dump -e "" "dump" is not exported by the Data::Dumper module Can't continue after import errors at -e line 0 BEGIN failed--compilation aborted.` [download] However, adding these lines somehow caused my test to start working! However, you failed to explain how it was failing to work before you added them. $rv without the dump lines: `[snip]` $rv with the dump lines: `[snip]` Very hard to believe... Update: misread Data::Dumper for Data::Dump, thanks to rhesa for pointing out.	[reply] [d/l] [select]