On Sat, Mar 26, 2005 at 03:13:07AM -0700, Luke Palmer wrote: : Chip Salzenberg writes: : > I'm working on enhancing Perl6::Subs[*] to support more parameter : > traits than just C<is required>. I have some questions about : > parameters and traits. (These questions all apply to pure Perl 6, : > which I know I won't be able to translate completely, but I want to : > know which target I'm missing.) : > : > * Given a parameter C<Array @a>, it's obvious I'm not allowed to : > C<push @a,1>, because I didn't say C<is rw>. But am I allowed to : > C<@a[0]++>? How about C<@a[0][0]++>? How deep is read-only-ness? : : I believe that the default constant parameters is so we don't have to : construct lvalues out of our arguments when we call. So that probably : means that it's very shallow. : : On the language level, I've been thinking that it would be good to go : C's way and not allow any parameter modification whatsoever. The : problem is that in the presence of methods, we can't tell whether we : have to lvaluize anymore, so we're back to the Perl 5 trap of lvaluizing : everything.
I think only predeclared simple subs are allowed to have "rw" parameters. Methods can have "is ref" parameters, but those don't require enforced lvaluehood on the caller end like rw does. : So if you want things modified, you'd have to pass in a reference. : Arrays and hashes would not generally have this restriction, since we : pass references of those guys anyway. Yes. : > * Similarly, how deep is the copy implied by C<is copy>? : : I think it's exactly as deep as read-only-ness. And both may be exactly as deep as COW. : > * Do traits attach syntactically to the variable name, or to the : > declaration as a whole? : > : > variable: @a is rw of Array : > Array @a is rw : > : > declaration: @a of Array is rw : > Array @a is rw : : Well, from this example it seems like `of` should be tighter than `is`. Traits are not allowed on ordinary rvalues, only on declarations. "is" traits always attach to the main declaration. "of" types always attach to the container on their immediate left. : > * As far as I can tell, the choice of spelling an array parameter : > C<Array @a> or C<Array $a> is entirely cosmetic: both @a and : > $a are capable of holding an Array reference. Is there actually : > a difference, e.g. in how they handle an undefined value? : : Hmmm... well I think all scalars are allowed to be undef. Arrays : aren't. So yeah, if you give @a undef, it probably gives you [] (or : croaks, but I don't think that's a good idea). If you give $a undef, it : gives you undef. As I mentioned in my other message, I think we should not assume that Perl 6 works the same in this regard as Perl 5 does. There needs to be something we can return that not only means (), but means also means "You're hosed! (And here's why.)" And I think we can make undef mean that if we make it lazily sensitive to scalar/list context (much like @a itself can be lazily sensitive to context). Hmm, maybe it would simpler to just tell everyone undef is a special empty lazy array that refuses to produce a value no matter how you ask. Larry
