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Postings from August 2001
Dan suggested:
> The syntax for variable and value properties are going to be different, I
> think, I just can't remember what it's going to be. (I think the colon's
> involved, but isn't it always?)
I think you're now channelling my de specula, not Larry's de jure. :-)
In A2, Larry stated that both variable and value properties would be
specified using the C<is> keyword.
Subsequently, I drafted a private proposal that separated the property
notion into two distinct semantic concepts with distinct syntaxes. That
proposal was sent only to Larry, but I will now reproduce it here to
show my current thinking on the subject.
Damian
-----------cut-----------cut-----------cut-----------cut-----------cut----------
It seems that my property concept is becoming a little chunky.
So let me try again...
Suppose variables and subroutines had compile-time "traits"
whilst values had run-time "properties".
Subroutine and variable traits are specified in declarations via a colon
(i.e. they're colonic adverbs on the C<my>, C<our>, C<temp> declarator, whilst remaining
syntactically near-as-dammit to Perl 5 attributes):
my $foo : constant Wheel(4) Drive('transmission') = 'RV';
my %bar : persistent;
sub marine : Wet lvalue {...}
At run-time, traits may be read-accessed via a pseudo-method call:
if (%bar.persistent) { ... }
print "$foo.Drive() to all $foo.Wheel() wheels\n";
or (unambiguously) via the C<trait> built-in, which returns a hashref:
if (trait(%bar){persistent}) { ... }
my %t = trait $foo;
print "%t{Drive} to all %t{Wheel} wheels\n";
Value properties are set at run-time via an C<is>:
return 0 is true;
%bar{Jonah} is Called('son of fish food');
and may be accessed via a pseudo-method call:
if ($foo.true) { ... }
print "His nickname was ", %bar{Jonah}.Called;
or unambiguously via the C<prop> built-in, which returns a hashref:
if (prop($foo){true}) { ... }
print "His nickname was $(prop(%bar{Jonah}){Called})\n";
So we now have:
* backwards compatibility with Perl 5 attributes
* no compile-time C<is> vs run-time C<is> confusion
* no variable vs value ambiguity
* polymorphic access to variable or value annotations (via methods)
* monomorphic access to variable or value annotations (via builtins)
and, as a bonus:
* the ability to put a trait specifier *anywhere* in a declaration
(because it's an adverb), and
* the ability to use both types of annotation in one statement:
my $threshold : constant = 0 is true;
my $threshold = 0 is true : constant;
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