chained hash slices?

pc88mxer has asked for the wisdom of the Perl Monks concerning the following question:

Suppose %h is a HoH. Then @h{@a} is a hash slice. Is there a syntax for what is suggested by the (currently unrecognized) expression @h{@a}{@b}?

Code that computes what I'm trying to describe:

my @list;
for my $a (@a) {
  for my $b (@b) {
    push(@list, $h{$a}{$b});
  }
}
# return @list;
[download]

If not for a general list @b, how about if @b contains just a single element:

@h{@a}{$x} ≡ map { $_->{$x} } @h{@a}

Comment on chained hash slices? Select or Download Code

Replies are listed 'Best First'.
Re: chained hash slices? by ikegami (Patriarch) on Jun 03, 2008 at 05:06 UTC
Nope, but what's wrong with `map { @{$_}{@b} } @h{@a} map { $_->{$x} } @h{@a}` [download] They're both quite concise.	[reply] [d/l]
Re: chained hash slices? by ysth (Canon) on Jun 03, 2008 at 07:15 UTC
Expressions that read the double slice are easy to come by; aliases are harder. This seems to work though: `@{ my $sub; my @refs = @h{@a}; do {$sub = sub { @refs ? $sub->(@_,@{po +p @refs}{@b}) : \@_ }}->() }` [download] I would love to hear how it could be simplified. -- Online Fortune Cookie Search	[reply] [d/l]
Re: chained hash slices? by moritz (Cardinal) on Jun 03, 2008 at 06:11 UTC
`@h{@a}` returns a list, so adding curly braces for slicing doesn't make sense at all: `use strict; use warnings; use Data::Dumper; my %h = ( a => {b => 1}, c => {d => 2}, ); my @a = qw(a c); print Dumper @h{@a}; __END__ $VAR1 = { 'b' => 1 }; $VAR2 = { 'd' => 2 };` [download] If you want to slice that list, slice the indexing array instead: `my %h = ( a => {b => 1}, c => {d => 2}, ); my @a = qw(a b c); print Dumper @h{@a[0, 2]};` [download]	[reply] [d/l] [select]
Re^2: chained hash slices? by ikegami (Patriarch) on Jun 03, 2008 at 06:40 UTC
None of the following exist, so the result should be a list with 6 undef. $h{a}{0} $h{a}{2} $h{b}{0} $h{b}{2} $h{c}{0} $h{c}{2}	[reply]
Re^3: chained hash slices? by moritz (Cardinal) on Jun 03, 2008 at 06:52 UTC
So it seems the OP wants chained slice-dereferencing braces to calculate a cross product. That would be very unlike anything perl does at the moment. I don't see how `@hash{@list}{@list}` could work without being horribly magical, inconsistent and counter-intuitive. `@hash{@list}` returns a list of hash references, not a hash. If you want to change that, you'd have to invent a syntax like `%hash{@list}`, which merges all the hash refs into a single hash, which can be sliced in turn. And then you'd have to find a good way to deal with duplicates in the new hash.	[reply] [d/l] [select]