On Mon, 9 Dec 2002, Dan Sugalski wrote:
> At 12:27 AM +0000 12/7/02, Alex Gough wrote:
> >That is to say, in "if ( !exp1 ) { ... }", !exp1 merely has to be true
> >or false, while $foo = !exp1 leaves !exp1 needing to be all manner of
> >things.
>
> D'oh! Now it's obvious.
> I've been conflating the two, which is wrong.
> The get_bool vtable method is specifically to retrieve the boolean
> state of a PMC. It returns 1 or 0, true or false, and we use it to
> check truthfulness.
> We need a logical_not vtable entry that returns a full-fledged PMC
> that's the logical complement of the not-ted variable.
As in one that actually allows creation of one, or the same as already
exists (which does not let one [easily] return a full PMC, just call vtable
methods on the one passed in).
> So "if (!$foo)" probably translates to "unless $foo, BAR" and under
> the hood does a get_bool on $foo. On the other hand, "$foo = !$bar"
> calls "not $foo, $bar" and $bar is in a position to decide what it
> isn't.
What about "if($a = !$foo)"? Or: "if(!!$foo)" or "if($a = !!$foo)"? Or if
it's part of a bigger expression: "if(($a && !$b) || ($c && !$d))"? Or
"if((!$some_set).union($other_set))"? I think using "unless" is more of an
optimization than a "default" encoding for not, even if it isn't assigned
anywhere permanent.
> If a language really needs "if (!$bar)" to get a full-fledged not-ted
> $bar and then test it for truthfulness, it can emit "not $$temp,
> $bar; if $temp, FOO" or something very much like it.
What type is $temp?
Or are logical_not et al. to be changed to be able to actually _return_
a PMC rather than just modify a given destination PMC?
I'm sorry to be asking a lot of pesky questions; it's not personal, just
business ;).
Dave
Isa. 40:31
