[OT] Should exist / does exist?

[Matt Lawrence]

Andy Armstrong wrote:
> On 6 Dec 2006, at 21:36, Paul Johnson wrote:
>> #!/usr/bin/perl
>>
>> use strict;
>> use warnings;
>> no warnings "exiting";
>>
>> sub X::withQuery
>> {
>>     my $self = shift;
>>     my ($loop, $sub) = @_;
>>     eval <<EOE;
>>     $loop:
>>     for my \$l (1 .. 3)  # or whatever your loop looks like
>>     {
>>         \$sub->(\$l);
>>     }
>> EOE
>>     die "argh: $@" if $@;
>> }
>>
>> my $db = bless {}, "X";
>>
>> $db->withQuery(ROW => sub {
>>     my $row = shift;
>>     print "-- $row\n";
>>     if ($row > 1) {
>>         last ROW;
>>     }
>> });
>
> Aha. I'm a dumbass - I didn't realise that the last|redo|next would 
> percolate up to the loop within the iterator.
>
> That pretty much satisfies what I originally asked but doesn't help in 
> the case where the iterator isn't implemented as a single loop. I'd 
> really like to have the kind of loop exit communicated back to the 
> iterator so it can handle it in whatever way is appropriate. For 
> example the iterator might actually be recursively walking some data 
> structure without even containing an explicit loop
>
> sub iter {
>     my $node = shift;
>     my $sub  = shift;
>
>     iter($node->{left}, $sub) if exists $node->{left};
>     $sub->($node);
>     # Should redo $sub call if redo thrown
>     #        carry on       if next thrown
>     #        throw 'last' up the call stack
>     #                       if last thrown
>     iter($node->{right}, $sub) if exists $node->{right};
> }
>
> --Andy Armstrong, hexten.net
>
>
Couldn't you write methods in your iterator class called next, last, 
redo or whatever to signal the outer loop via die and eval {}?

You wouldn't get the nice natural Perl syntax, but you would have the 
freedom to implement any kind of loop control you wanted, even ones that 
Perl doesn't support.

Matt