Re^2: RFC: Is there more to alias?

[xmath]

The hard part was getting any sort of "trigger" on the occurrence of alias (which is just a declared but undefined sub). The breakthrough was the discovery of PL_check and the fact it's editable.

This array contains a list of "check functions" which are invoked whenever a node is created in the op-tree. I used it to trigger on OP_RV2CV (the occurrence of "alias" or "copy") and OP_ENTERSUB (the completion of the op-subtree that invokes it). I do some initial modifications of the op-tree, and in particular mark is so I can find it later.

When alias/copy has been seen I also temporarily override PL_peepp. There I scan the optree for markings and do the actual work, which consists mostly of setting the op_ppaddr of various ops inside alias to aliasing equivalents.

A final piece of the puzzle is supporting alias BLOCK which involves a lexer hack, but is mostly unrelated to the rest.

It may indeed be applicable in other situations. I was already thinking about how I could make this mechanism more reusable.

Any further details you want?

[fergal]

Interesting but scary! It looks like the same technique could be used to implement compile-time checking of method calls, so I could do something like

type Dog my $a;
$a->bark;
$a->$trick; # compile time warning maybe
$a->perform_brain_surgery; # compile time error

a bit like Java interfaces except far less painful. So it would pick up typos in method names but doesn't try to look at the Class of $a (although an optional assertion might be useful).

Rereading your original posting, I can't follow

$x = alias [$y, $z];
alias push @x, $y;

Doesn't something have to be aliased to something else? What is being aliased to what in these examples?

[xmath]

Doesn't something have to be aliased to something else? What is being aliased to what in these examples?

$x = alias [$y, $z];

$x->[0] is an alias to $y and $x->[1] is an alias to $z.

alias push @x, $y;

$x[-1] is an alias to $y.

[fergal]

Does the first 1 autovivify the arrayref in $x (as $x->[0] = 1 would) or must $x already have an array ref in it? What happens if I do shift @$x? Is $x->[0] now an alias to $z and the alias to $y disappears? Or (unlikely) does it effectively do @{$x} = @{$x}[1..($#$x-1)] which amounts to

$y = $z;
$z = $x->[2];
$x->[2] = $x->[3];
....
$x->[$#$x - 1] = $x->[$#$x];
pop @$x;

Fun and games!

The second example one seems very odd to me. To me, it reads as alias(push(@x, $y)) but I guess perl doesn't parse it as that. Why do this rather than just alias $x[-1] = $y or should that be alias $y = $x[-1]?

So you can use , or = to separate the aliased things? And you can put alias on either side of the =?

Anyway, time for bed. I dreamt about object pascal last night, I hope I don't dream about aliases to night!