Author: autrijus
Date: Fri Feb 24 14:16:49 2006
New Revision: 7858
Modified:
doc/trunk/design/syn/S09.pod
Log:
* S09: Autovivification no longer happens under rvalue context.
Suggested and contributed by Yuval Kogman.
Modified: doc/trunk/design/syn/S09.pod
==============================================================================
--- doc/trunk/design/syn/S09.pod (original)
+++ doc/trunk/design/syn/S09.pod Fri Feb 24 14:16:49 2006
@@ -672,3 +672,33 @@
a list of keys, the size of which is the number of declared dimensions
of the hash. [XXX but this seems to imply another lookup to find the
value. Perhaps the associated value can also be bundled in somehow.]
+
+=head1 Autovivification
+
+Autovivification will only happen if the vivifiable path is used as a
+container, by binding, assigning, or taking a reference. On the other hand,
+value extraction does not autovivify.
+
+This is as opposed to Perl 5, where autovivification could happen
+unintentionally, even when the code looks like a non-destructive test:
+
+ my %hash;
+ exists $hash{foo}{bar}; # creates $hash{foo} as an empty hash reference
+
+In Perl 6 these read-only operations are indeed non-destructive:
+
+ my %hash;
+ exists $hash{foo}{bar}; # %hash is still empty
+
+But these ones I<do> autovivify:
+
+ my %hash;
+ my $val := $hash{foo}{bar};
+
+ my @array;
+ my $ref = \$array[0][0];
+
+ my %hash;
+ $hash{foo}{bar} = "foo"; # duh
+
+This rule applies to dereferencing arrays, hashes, and scalar references.
