use v6;

sub _NL_Iter(@loops, $when) returns Ref {
    my @list;
    my $i = -1;
    my @idx;

    return  sub { return }
        if  ! @loops;

    my @vals = @loops;

    return  sub {
        while 1 {
            # Prepare to append one more value:
            if  $i < @loops.end {
                @idx[++$i]= -1;
                if @loops[$i].does(Code) {
                    @vals[$i]= @loops[$i](@list[-1], @list);
                 }
            }
            ## return   if  $i < 0;
            # Increment furthest value, chopping if done there.
            while  @vals[$i].elems <= ++@idx[$i] {
                pop @list;
                return   if  --$i < 0;
            }
            @list[$i]= @vals[$i][@idx[$i]];

            if $when.does(Code) {
               return @list if $when(@list[-1], @list);
            } else {
               return @list;
            }
        }
     };
}

sub NestedLoop(++@loops, +$code, +$when) {
   my $onlywhen = $when;
   my $count_elems = @loops.elems + 1;
   $onlywhen //= sub { @_ == $count_elems }; # Default case - when we have a full count.

    my $iter= _NL_Iter(@loops, $onlywhen );

    if ! $code {
        # There seeems to be no way to check for void context, since
        # want.count is not working.  I wanted to test want.count == 0.
        return $iter;
    }

    my @ret;
    my @list;
    while @list = $iter() {
        @list = $code( @list );
        if want.List {
            push @ret, @list;
        } else {
            @ret[0] += @list;
        }
    }
    return want.List ?? @ret :: ( $ret[0] // 0 );
}

my $next = NestedLoop(loops => ([0..2], [0..2], [0..2]));
my @group;
while @group = $next() { say ~@group; };

say "\nsecond case:";
my $N = 4;
my $depth = 3;
my $i2 = NestedLoop(loops => ( [ 0..$N ],
                                 ( sub { [@_[0]+1..$N] } ) xx ($depth-1),
                              ) );
while @group = $i2() { say ~@group; };