I call a flattened hash the list that is produced when a %-literal is used in a list context ; it can be thought as a "print representation" (LISP-speak) of a hash object. It is a list consisting of hash keys in odd position (0, 2, ...) with corresponding hash values in subsequent even position (1, 3, ...) (0 is first position in Perl lists, therefore odd :) ). It's used when copying hashes to/from other hashes or arrays or passed to arbitrary list contexts; also a hash can be initialized from an arbitrarily generated list, which is interpreted with flattened hash convention, as in the most common and idiomatic hash initialization construct: %h = (one => 1, two => 2);. Most monks are already familiar with these issues, so I won't press on.

While meditating on these flattened hashes, a question occurred to me: do they preserve lvalueness of value (even) positions when used in aliasing contexts (subroutine call and foreach)? A quick experiment revealed that they do: 

$ perl
my %h = 'a'..'f';

sub modify {
  for (my $i = 1 ; $i < @_ ; $i+=2) {
    $_[$i] .= '_modified';
  }
}

modify(%h);
print "$_ => $h{$_}\n" for sort keys %h;
^D
a => b_modified
c => d_modified
e => f_modified

$ perl
my %h = 'a'..'f';

# assignment to key (odd) positions has no side effects
$_ .= '_modified' for %h;
print "$_ => $h{$_}\n" for sort keys %h;
^D
[the same output]

[download]

Now I'm not advocating this as a technique for hash modification in called subroutines etc.! The usual method of call-by-reference is much cleaner and probably(?) more efficient. I only have a few questions for monks who are more intimate with Perl internals/development history:

  1. Is this intentional / documented behaviour or just incidental?
  2. If intentional, what's the reason? Historical? Incidental behaviour become standard with time? Implemented just to stay consistent? ...
  3. Do you think it conforms to the Principle of Least Surprise (I can't make up my mind on it)

LVALUEness of values

My experiments revealed that values is also lvaluable (and this is documented). With aliasing, it's useful for key-agnostic bulk value modifications.

In reply to LVALUEness in flattened hashes by calin

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