Question

Is there any way to have a subroutine send data back while still processing? For instance (this example used simply to illustrate) – a subroutine reads a file. While it is reading through the file, if some condition is met, then “return” that line and keep processing. I know there are those that will answer – why would you want to do that? and why don’t you just …?, but I really would like to know if this is possible.

Answer 1

A common way to implement this type of functionality is with a callback function:

{
    open my $log, '>', 'logfile' or die $!;
    sub log_line {print $log @_}
}

sub process_file {
    my ($filename, $callback) = @_;
    open my $file, '<', $filename or die $!;
    local $_;
    while (<$file>) {
        if (/some condition/) {
             $callback->($_)
        }
        # whatever other processing you need ....
    }
}

process_file 'myfile.txt', \&log_line;

or without even naming the callback:

process_file 'myfile.txt', sub {print STDERR @_};