HaloO,
John M. Dlugosz wrote:
First, consider the stated examples for Generic type parameters, from
the passage which defines the terminology in S02:
I fear we need a rigorous definition of generic first.
sub max (Num ::X @array)
> {
> push @array, X.new();
> }
Here you have a nice example of the variance problem.
Assuming Num <: Any the question is if push is callable
with Array[Num]. If Array[::T] is invariant on ::T
push has to have exactly this type. This is easily achievable
if push is itself parametric and the above instanciates
push[Array of X, X [EMAIL PROTECTED] on the fly.
Question 1 - can the Any be left off? That is, ::X as an undeclared
type name used as the only type, syntactically correct?
Any can be dropped. But note that ::X captures the *actual*
type of @array. That is
my Num @a = (1,2,3); # actually Array of Int
my Num @b = (1.2, 2.2); # actually Array of Num
compare( @a, @b ); # type error
Question 2 - is $x constrained to ::T or does that just note what it
was originally? (in cases where $x is not read only. In this example
that is moot.)
It captures the actual type. To put a contraint on $x you need to
evaluate the type, that is use it without the :: sigil. For rw
parameters we can't allow variance if we want type safety anyway.
BTW, I wonder if the trait 'is ref' means write-only parameter?
So, we might want to capture that concrete type to, and write
Document Storable Positional ::DocType $doc;
Fine.
Now the fact that this is a generic type notwithstanding, it is just
another type in the list. It just happens to be the same type that
is the actual type $doc was initialized with. But, it implies that
yes, $doc is constrained to that type from then on. The generic type
just customizes the list at run-time, but it is still a list of
juxaposed types, so they are ANDed together.
No.
Now we have a list of juxaposed type names, that are ANDed together.
The generic type is the one that wasn't defined (as a type) before.
It needs the :: so it doesn't flag as an undefined symbol, but the
others could have the sigil if you wanted to be explicit:
::Document ::Storable ::Positional ::DocType $doc;
Hopefully not. This just captures the type $doc into four variables.
::T $x;
No, this is just like &foo is different from foo. It does not
constrain $x to T but captures Any in this case. And if T is already
declared in this scope it's a redeclaration error. S02 seems to say
that this just rebinds ::T but that means to loose it as a unique
type parameter for the scope! So we should change that.
BTW, C# raises an error for the following case
sub foo ($a)
{
my $x = 3;
if $a > 0
{
say $x; # error usage before declaration
my $x = 17; # in effect from start of scope
say $x;
}
}
and I see some merit in it. How is Perl 6 supposed to handle this?
That answers the two questions, based only on other things in the
Synopses.
Ups, my reading is different.
Now for a proposal, and the observation of an issue.
The only thing that makes a generic type parameter generic is that it
was previous undefined as a type. So, what if you have code that's
working just fine, and then some other change puts a symbol T into
the lexical scope, perhaps as a global import? Boom! The code
changes meaning drastically.
Which is no problem if ::T defines it in the lexical scope. Any
outer definition is hidden. I guess re-using ::T in the same scope
again for capturing a new type is an error. Just like redeclaring
a value variable. BTW is
sub foo ($x)
{
my $x; # redeclaration error?
}
just funnily hiding the parameter $x or an error?
Contrast that to the normal meaning of declaring a variable, in which
case it hides anything with the same name in the outer scopes.
Introducing a global one does not change the code that uses lexical
variables. Except for generics. That is inconsistant and wrong for
the same reason that it would be wrong for all other kinds of
symbols.
Which is why it isn't so, or is it? Please correct me if I'm wrong!
To address this, I propose using a positive way to declare generic
parameters rather than having them implicit based on not previously
existing. I propose the triple colon, :::, for this purpose:
:::T $x;
Which is ugly.
Regards, TSa.
--
"The unavoidable price of reliability is simplicity"
-- C.A.R. Hoare
