Perl Modules: AnyEvent::ReadLine::Gnu

2013-09-06

REPLs (Read-Eval–Print Loop) can be handy little things. In UAV::Pilot, the uav shell takes arbitrary Perl expressions and eval()‘s them.

Before integrating with AnyEvent, handling the prompt was done by Term::ReadLine::Gnu. When AnyEvent was integrated, I wanted the shell to use AnyEvent’s non-blocking I/O, so it was migrated to AnyEvent::ReadLine::Gnu.

This also handles command history. Hit the ‘Up’ arrow to get your previous command. No code is necessary; AnyEvent::ReadLine::Gnu does it for you.

ReadLine also has options for tab-completion. I would like to add this to the uav shell eventually.

Using the AnyEvent version is quite simple. You pass a callback that takes input. In the uav callback, we only run the code when it ends with a semicolon (ignoring trailing whitespace). If it doesn’t, we save it in a buffer and wait for more input.

Here’s how this is implemented in UAV::Pilot:

    my $readline; $readline = AnyEvent::ReadLine::Gnu->new(
        prompt => 'uav> ',
        on_line => sub {
            my ($line) = @_;
            add_cmd( $line );
            if( $line =~ /; \s* \z/x ) {
                my $cmd = full_cmd;
                $readline->hide;
                my $do_continue = run_cmd( $cmd, $repl );
                $readline->show;
 
                $cv->send( $do_continue ) unless $do_continue;
            }
        },
    );

The add_cmd() method adds the input to the buffer. If that did end with a semicolon, then we call full_cmd() to get back the text of the code. It also clears the buffer. run_cmd() then eval()‘s the code. If we’re meant to exit the program, it returns false, which we handle with the $cv->send.

The $readline->hide and $readline->show calls stop ReadLine from outputting the prompt when we might have other output going.