[OT] Should exist / does exist?
Matt Lawrence
matt.lawrence at virgin.net
Thu Dec 7 10:12:05 GMT 2006
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
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