Parameter list wierdness

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

Afternoon monks,

2 questions:
1: why does this not work:

print "not ref:\n";
test(dogs=>'bark', cats=>'miaow');

print "ref:\n";
test({dogs=>'bark', cats=>'miaow'});

sub test{
  my $params;                                  
  ref($_[0]) ? $params = shift : $params = {@_};
  print Dumper($params);
}
[download]

The output is

not ref:
$VAR1 = {
          'cats' => 'miaow',
          'dogs' => 'bark'
        };
ref:
Odd number of elements in anonymous hash at params.t line 16.
$VAR1 = {
          'HASH(0x81194e0)' => undef
        };
[download]

Which would seem to imply that the test is always false, but if I change $params = {@_} to print "hello" I get:

not ref:
hello
$VAR1 = undef;
ref:
$VAR1 = {
          'dogs' => 'bark',
          'cats' => 'miaow'
        };
[download]

So in this case, when it is a reference, the test correctly evaluates to true and doesn't print 'hello'

Also, if I do it like:

  if(ref($_[0])){
    $params = shift;
  }
  else{
    $params = {@_};
  }
[download]

I get exactly what I was expecting:

not ref:
$VAR1 = {
          'cats' => 'miaow',
          'dogs' => 'bark'
        };
ref:
$VAR1 = {
          'cats' => 'miaow',
          'dogs' => 'bark'
        };
[download]

?????

2. Is there a better way to allow a sub to be called like either test(dogs=>bark, cats=>miaow) or test({dogs=>bark, cats=>miaow})?

Cheers, Cxx

Edit by BazB retitle from ":? wierd"

Comment on Parameter list wierdness Select or Download Code

Replies are listed 'Best First'.
Re: Parameter list wierdness by dave_the_m (Monsignor) on Jul 01, 2004 at 15:31 UTC
try replacing `ref($_[0]) ? $params = shift : $params = {@_};` [download] with `$params = ref($_[0]) ? shift : {@_};` [download] Dave. update: Note that your line was getting parsed as `(ref($_[0]) ? $params = shift : $params) = {@_};` [download]	[reply] [d/l] [select]
Re^2: Parameter list wierdness by CassJ (Sexton) on Jul 02, 2004 at 01:17 UTC
Thanks for the explanation. That's what I get for only reading the first line of the description in perlop. Sorry! Cass	[reply]
Re: Parameter list wierdness by rir (Vicar) on Jul 01, 2004 at 15:33 UTC
Try writing your ternary operation like this: `sub test{ my $params; $params = ref($_[0]) ? shift : {@_}; print Dumper($params); }` [download] Be well.	[reply] [d/l]
Re^2: Parameter list wierdness by CassJ (Sexton) on Jul 01, 2004 at 15:39 UTC
aah, that works and it's much nicer. Still think I'm missing something about how the ternary operator works though! Thanks, Cxx	[reply]
Re^3: Parameter list wierdness by Grygonos (Chaplain) on Jul 01, 2004 at 16:45 UTC
`my $value = (if condition? then this: else this);` [download] That's about the quickest explanation. Ternary should never be used in void context. See my node Ternary in void context where I cleared up my ternary confusion as well. The ternary operator always returns the value present in its truth branch (for lack of a better term), that's why you shouldn't substitute it for if then constructs. an if then construct that warrants a ternary is similar to the following `if($some_var eq 'X') { $this_var = 'Run using X format'; } else { $this_var = 'Run using standard format'; }` [download] This would equate to the ternary `$this_var = ($some_var eq 'X'? 'Run using X format':'Run using standar +d format');` [download] Grygonos	[reply] [d/l] [select]
Re: Parameter list wierdness by davido (Cardinal) on Jul 01, 2004 at 16:58 UTC
This one took some fiddling for it to occur to me what was the problem. Trinary operators can be used as an R-value, or as an L-value. You've constructed a L-value. Here's what's happening: `ref($_[0]) ? $params = shift : $params = {@_};` [download] When the trinary's conditional is false (no reference), {@_} is assigned to $params. When the trinary's conditional is true, {@_} is assigned to $params = shift. Though the following behavior is undefined, here's why "it" doesn't work. `$params = shift` shifts the reference out of @_. But then next a hash reference is created using the now empty @_ with the " = {@_} " part. So the first value of $params is discarded, and replaced by a hash-ref to a hash containing only the empty array @_. There's some speculation in all that, but it seems to bear out in lightweight testing. Dave	[reply] [d/l] [select]