Autovivification

For the generation of life from non-living matter, see Abiogenesis.

Autovivification is a distinguishing feature of the Perl programming language involving the dynamic creation of data structures. Autovivification is the automatic creation of a variable reference when an undefined value is dereferenced. In other words, Perl autovivification allows a programmer to refer to a structured variable, and arbitrary sub-elements of that structured variable, without expressly declaring the existence of the variable and its complete structure beforehand.

In contrast, other programming languages either: 1) require a programmer to expressly declare an entire variable structure before using or referring to any part of it; or 2) require a programmer to declare a part of a variable structure before referring to any part of it; or 3) create an assignment to a part of a variable before referring, assigning to or composing an expression that refers to any part of it.

Perl autovivication can be contrasted against languages such as Python, PHP, Ruby, JavaScript and all the C style languages.

Hashes

The debugger session below illustrates autovivification of a hash:

  DB<1> $h{A}{B}{C}{D}=1
  DB<2> x \%h                                                                  
   0  HASH(0x83c71ac)
   'A' => HASH(0x837d50c)
      'B' => HASH(0x83c71e8)
         'C' => HASH(0x83c7218)
            'D' => 1
  DB<3>

Hashes several layers deep were created automatically without any declarations. Autovivification can prevent excessive typing. If Perl did not support autovivification, the structure above would have to be created as follows:

  DB<1> %h = (A => {B => {C => {D => 1}}})
  DB<2> x \%h                                                                  
  0  HASH(0x83caba4)
   'A' => HASH(0x83cfc28)
      'B' => HASH(0x83cab74)
         'C' => HASH(0x83b6110)
            'D' => 1
  DB<3>

File and Directory Handles

Perl 5.6.1 and newer support autovivification of file and directory handles. Calling open() on an undefined variable will set it to a filehandle. According to perl561delta, "[t]his largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

    for my $file ( qw(this.conf that.conf) ) {
        my $fin = open_or_throw('<', $file);
        process_conf( $fin );
        # no close() needed
    }
    use Carp;
    sub open_or_throw {
        my ($mode, $filename) = @_;
        open my $h, $mode, $filename
            or croak "Could not open '$filename': $!";
        return $h;
    }

Python

Python's collections module contains a defaultdict. These can be used to implement autovivificious dictionaries.

from collections import defaultdict
 
def tree():
    return defaultdict(tree)
 
lupin = tree()
lupin["express"][3] = "stand and deliver"

Ruby

Ruby hashes can take a block specifying an object to be returned for non-existing indexes. These can be used to implement autovivificious maps.

tree = proc { Hash.new { |hash, key| hash[key] = tree.call } }
 
lupin = tree.call
lupin["express"][3] = "stand and deliver"

PHP

PHP arrays are natively autovivificious.

$arr = array();
$arr["express"][3] = "stand and deliver";

However, this only applies to assignment, and not array access.

See also

• Evaluation strategy
• Variable

External links

• perl561delta: File and directory handles can be autovivified
• Autovivification in Perl: An In-Depth Tutorial
• Autovivification in Ruby - emulate Perl's autovivification
• A Use of the Y Combinator in Ruby - Implements autovivification in Ruby with the Y Combinator.