Playing Perl 6

Goto the Last Fifo


I always admired the elegance of the sysfs. You take a text and write it to a file to talk to a function running in kernel space. As soon as you know how to work with files, you can change the systems behaviour and reuse access control mechanism without any special tools. It’s very easy to script and dump (parts) of the systems state.

Linux and friends come with fifos that serve the same purpose. Create a fifo, set access rights and start reading from that pseudo-file. Very easy to do in Perl 5.

my $fifo; open($fifo, "+);
while (<$fifo>) {
    do-things-with $_;

Rakudo doesn’t really know about fifos yet, as a result it doesn’t block on a read of a fifo that don’t got data anymore. After a bit of fiddeling I found a way around that problem.

      1 use v6.c;
      3 # `mkfifo radio-fifo-in`
      4 # `echo "foo" > radio-fifo-in`
      5 # `echo "foo^D" > radio-fifo-in`
      7 my $fifo-in = open(„radio-fifo-in“, :r);
      9 LABEL: loop {
     10     react {
     11         whenever supply { .emit for $fifo-in.lines } {
     12             say .Str;
     13             last LABEL if /‚^D‘/;
     14         }
     15     }
     16 }

I learned that whenever reacts to last and will teach the docs about it later today. Luckily Perl 6 got labels so we can tell last where to goto.

UPDATE: scovit found a short expression that gets very close to the behaviour of Perl 5.

my $fifo = open("radio-fifo-in", :r);
while defined $_ = $fifo.get { .say }