Overwrite built-in wantarray()

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

Hi there, I want to "decorate" perls built-in wantarray-function, but I can't find a way to do that. I want to do something like this:

my $class = 'MyClass';
*CORE::GLOBAL::wantarray = sub {
    return CORE::wantarray(@_) if(!${"$class\::PRETEND_CONTEXT"});
    return 1 if(${"$class\::PRETEND_CONTEXT"} == 3);
    return 0 if(${"$class\::PRETEND_CONTEXT"} == 2);
    return undef if(${"$class\::PRETEND_CONTEXT"} == 1);
};
[download]

But, not as for the build-in function caller(), this doesn't work and it throws the following error:

Bareword found where operator expected at /usr/lib/perl/5.8/Data/Dumpe
+r.pm line 267, near "$realtype = $realpack ? Scalar::Util::reftype"
  (Might be a runaway multi-line ?? string starting on line 237)
        (Missing operator before Scalar::Util::reftype?)
[download]

It seems to be, that wantarray isn't in CORE. But where it is? If I only redefine the function (in CORE::GLOBAL::) without calling the original function, all is ok. But I don't want that. How can I decorate it? Any suggestions? Tobiwan PS: if you want to try the code-snippet, you have to put it into the "import()" function of a class and you have to "use TheClass;". There is a write protection for the CORE:: Classes.

Comment on Overwrite built-in wantarray() Select or Download Code

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Re: Overwrite built-in wantarray() by merlyn (Sage) on Aug 21, 2007 at 00:03 UTC
Why are you trying to fake the context? What will that gain you? It's not like you can cheat a routine to return a different thing in an "incorrect" context. It simply won't make sense. If you need to pass out-of-band information between cooperating routines, you can always use a localized package variable. -- Randal L. Schwartz, Perl hacker	[reply]
What I want to do with that by Tobiwan (Beadle) on Aug 21, 2007 at 12:19 UTC
I want to fake the context for tests. I want to do something like this `FakeContext('void'); my @response = tested_sub(); is(scalar(@response), 0, 'void return'); FakeContext('scalar'); my @response = tested_sub(); is(scalar(@response), 1, 'scalar return');` [download] I only want to fake the context for this one sub-call. All contexts in calls in this tested_sub() should be real and unmodified. When I overwrite the caller() function and change the wantarray-flag, this doesn't have any effect for wantarray(). And I've to decorate ist, because when I overwrite it complete, I can't fake only the first level of my sub-call. The only (not really god) idea I've is, to overwrite the wantarray() and use caller() to read out the context. But I think this will cause stragne side-effects. Any ideas? With PadWalker or Devel::Caller I can read out many things, but can I change the context? I don't know (yet). PS: with CORE::GLOBAL::wantarray I can overwrite the function, but I can't get any referene to the original sub.	[reply] [d/l]
Re: What I want to do with that by merlyn (Sage) on Aug 21, 2007 at 16:53 UTC
Ewww. Why not just: `tested_sub(); ok(some side effect, "expected side effect"); my $scalar_response = tested_sub(); is($scalar_response, $expected, "gives proper scalar value"); my @list_response = tested_sub(); is_deeply(\@list_response,\@expected_response, "gives proper list valu +e");` [download] Don't be trying to freak Perl out. I'd never expect the right response from giving it the wrong context. That's just asking for hurt. -- Randal L. Schwartz, Perl hacker	[reply] [d/l]
Re: Overwrite built-in wantarray() by chromatic (Archbishop) on Aug 20, 2007 at 22:48 UTC
It looks unoverrideable to me: `my $p = prototype 'wantarray'; print defined $p ? $p : "(not overrideable)";` [download]	[reply] [d/l]
Re^2: Overwrite built-in wantarray() by almut (Canon) on Aug 20, 2007 at 23:06 UTC
I think the conclusion is correct, though the test is not done properly ;) — the function name argument to `prototype` should've been prefixed with `CORE::`. Compare the results using a function that is overrideable, like `open` `for my $func qw(CORE::open open) { my $p = prototype $func; print "$func: ", defined $p ? $p : "(not overrideable)", "\n"; }` [download] Output: `CORE::open: *;$@ open: (not overrideable)` [download]	[reply] [d/l] [select]