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Postings from July 2004
On Wed, 2004-07-21 at 17:16, Dave Mitchell wrote:
> On Wed, Jul 21, 2004 at 04:58:54PM -0500, david nicol wrote:
> > How impossible is it to find out the size of an Lvalue array? I'm
> > imagining defining the true value of wantarray() function to be
> > a reference to the array that is to be filled by the result.
> >
> > Is that possible?.
>
> Eh? what would wantarray return a ref to in
>
> sub f { wantarray() ... }
> ($a, $b, @c) = f();
an array of aliases to $a, $b, and $c:
[${\$a},${\$b},${\$c}]
so that assigning to
@{wantarray()}
will be the same thing as returning values. Except then we need to
get out of the function somehow. So maybe optimizing the stack-copies
out of returning would only happen when assignment to @{wantarray()}
happens in a return:
return @{wantarray()} = qw/foo bar baz/;
Or better yet, assigning to @{wantarray()} or a slice of it would
imply returning.
This might get real confusing real fast, and I can't right away come up
with a use for the feature that wouldn't be more cleearly written using
C<map> and doing explicit dispatch outside of the function.
It would be a powerful implementation-hiding abstraction
sub f(){
my $target = wantarray() or die 'usage: ($a, $b, $c) = f()';
my @NeedsEffing = grep
{effthisp $target->[$_]}
(0..$#$target);
@$target[@NeedsEffing] =
map
{effthis $_}
@$target[@NeedsEffing];
}
but what would it give us that we can't already get by making the
assignee one of the function parameters instead of an l-value?
sub f(@){
wantarray() or die 'usage: f($a, $b, $c)';
my @NeedsEffing = grep {effthisp $_[$_]} (0..$#_);
effthis( $_ ) for @_[@NeedsEffing];
}
The real advantages are that access to details about the Lvalue allows
expected-type-based polymorphism. If we care about that.
For the purposes of implementing a truncated sort optimization, all we
care about is the length of the L-array, if any. If that's not going to
be available, truncated sorting is best left to the unwritten
Sort::Truncated module and however the invocation works it will take M,
or at least the target array, as a parameter.
--
david nicol
"People used to be able to read my thoughts, but
it doesn't appear to work any more. Should I eat less cheese?"
-- Elizabeth Woods
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