Re: PerlHash.get_pmc_keyed of non existing key

[Gordon Henriksen]

On Friday, August 22, 2003, at 02:52 , Leopold Toetsch wrote:

Gordon Henriksen wrote:

(PMCs have reference semantics[1])
Isn't that the job of Perl's \ operator?
Did you read on to [1] too?
I read

[1]
  new P1, .PerlHash
  new P3, .PerlString
  set P3, "yyy\n"
  set P1["a"], P3
  set P0, P1["a"]
  print P0
  set P0, "xxx\n"
  set P2, P1["a"]
  print P2
  end
yyy
xxx
as equivalent to...

$hash{a} = "yyy\n";
$y := $hash{a};  # Note :=lhs instead of =lhs or =\lhs.
print $y;       # yyy
$y = "xxx\n";
print $hash{a}; # xxx
Makes sense.

My point was only that element lookup in an rvalue (get) should be 
explicitly non-modifying—only element lookup in an lvalue context (put) 
should auto-vivify. But indirect puts like this are also necessary. (The 
rhs of := is an l-value. As is the operand to \.)

With that in mind, Perl could construct lvalue operations (those with 
auto-vivify) from rvalue operations (without auto-vivify). But the 
reverse isn't true. So the no-auto-vivify semantics are the one that 
must be preserved. Which contradicts your point that set Pn, Pm[...] 
should always auto-vivify to support reference semantics. But, really, 
both are significantly needed—$str = $hash{key} and $ref = \$hash{key} 
and $str := $hash{key} and $hash{key} := $str and $hash{key} = $str and 
$hashB{keyB}{keyC}{keyD} = $hash{key} should all come down to PIR simply 
and behave as in Perl 5 (where the construct exists in Perl 5). Element 
lookup in an rvalue context should return an anonymous PerlUndef, 
probably? In an lvalue context, it should do what you're asking for.

But my reading of [1] is not at all the same as this...

$hash{a} = "yyy\n";
$y = \$hash{a};   # = \ --> Brand new reference PMC!
print $$y;       # yyy
$$y = "xxx\n";
print $hash{a}; # xxx
Never mind that there's no PerlReference PMC for $$ to work on, or for \ 
to return. While Perl 6 could conceivably support reference copying 
without a reference-type PMC (using :=), it would be pretty darn 
fragile—one = instead of := and the reference is broken. I would not 
have translated your original Perl example (with its \) as you did—
actually, I wouldn't be able to translate it at all—\ wants to construct 
a PerlReference PMC IMO.

—

Gordon Henriksen
[EMAIL PROTECTED]