The original motive here was efficiency. That's taken more-than-usually
seriously in this case, because array descriptors are one of the few
upper-layer features which are mostly about efficiency -- everything
they do could be done as well or better in Perl code, but at a
substantial penalty in efficiency.
In XS, a reference is a pointer copy (one machine instruction) and
incrementing the SV's reference count. A scalar means a copy of another
scalar, which means creating a new SV, a much more costly operation.
Depending on context, creating an SV is not a huge overhead -- a Perl
script is creating SV's all the time. But here the context is that
avoiding this kind of overhead is *exactly* why array descriptors were
created.
In addition, with the release not long ago of the CPAN indexed version,
there is now the issue of backward compatibility.
This makes these scalars, for most purposes, read-only. (Though you
could do your own copy-on-write.) A workaround might be to keep the
scalars in their original state, and add a new field to the array. In
this case it would mean you have "cooked" and "raw" versions of the
name. This also might have other advantages: It's a more functional
(fewer side effects) style of coding, and the "raw" version may even
come in handy later.
Hope this helps, jeffrey
On 03/01/2014 04:41 PM, Ruslan Shvedov wrote:
As in script <https://gist.github.com/rns/9299823#file-refs-in-ast-t>,
output
<https://gist.github.com/rns/9299823#file-refs-rather-than-scalars-in-ast>.
A symbol, e.g. 'bracketed' in one ast node is followed by
${\$VAR1->[0]} in its place in all ast nodes downwards.
My use case where I'd appreciated scalars rather than refs in
/ast_symbol_name_to_id/ sub below in my transpiler
<https://github.com/rns/MarpaX-Regex/blob/master/t/translation.t>, this assignment
$ast->[0] = "$name/$symbol"
changes all downward nodes and the below subs stops working.
Is it possible that symbol and name descriptors always produce scalars
in AST's?
#
# convert nodes' symbol, name tuples to id's
#
sub ast_symbol_name_to_id {
my ($ast) = @_;
if (ref $ast){
my ($name, $symbol, @nodes) = @$ast;
warn "ast_symbol_name_to_id:", "'$name', '$symbol'";
if ($name eq $symbol){
$ast->[0] = $name;
}
else{
$ast->[0] = "$name/$symbol";
}
splice @$ast, 1, 1;
map { ast_symbol_name_to_id ( $_ ) } @nodes;
}
}
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