+## Exceptions for error handling
+
+---
+class: center, middle
+
+### Problem: No exception handler up the call stack
+
+---
+class: ex-exceptions
+
+## Rule: Don't die/throw in event handlers.
+
+--
+### Error callback pattern
+
+```perl
+do_something_asynchronously(
+ callback => sub { ... },
+ on_error => sub { ... },
+);
+```
+
+---
+class: ex-exceptions
+
+## Rule: Don't die/throw in event handlers.
+
+### Use promises
+
+```perl
+my $promise = do_something_asynchronously();
+
+$promise->on_done(sub { ... });
+$promise->on_fail(sub { ... });
+```
+
+???
+More on this later.
+
+---
+class: center, middle
+
+## `SIGPIPE`
+
+---
+class: sigpipe
+## `SIGPIPE`
+
+- Sent to your program when it writes to a pipe that was closed.
+
+--
+- Default signal handler terminates the program.
+
+---
+class: ex-sigpipe
+
+## Solution: Ignore `SIGPIPE`
+
+```perl
+$SIG{PIPE} = 'IGNORE';
+```
+
+???
+Some event loops do this for you.
+
+--
+.big[
+Look for `EPIPE` from syscalls (like [`write`](http://man.he.net/man2/write)) instead.
+
+(You *are* checking return codes from your system calls... right?)
+]
+
+---
+class: center, middle
+
+## Debugging event-driven code
+
+---
+class: ex-debugging
+
+## Debugging event-driven code
+
+.big[
+- Print debug info to `STDERR`.
+]
+
+```bash
+export PERL_FUTURE_DEBUG=1
+# and
+export ANYEVENT_DEBUG=8
+# or
+export MOJO_IOLOOP_DEBUG=1
+# etc.
+```
+
+---
+## Debugging event-driven code
+
+.big[
+- Print debug info to `STDERR`.
+- Check out [`AnyEvent::Debug`](https://metacpan.org/pod/AnyEvent::Debug).
+]
+
+---
+## Debugging event-driven code
+
+.big[
+- Print debug info to `STDERR`.
+- Check out [`AnyEvent::Debug`](https://metacpan.org/pod/AnyEvent::Debug).
+- Use [`perl5db.pl`](https://metacpan.org/pod/perl5db.pl) and other `Devel::` debuggers.
+]
+
+???
+Traditional debuggers are still useful for event-driven code.
+- Be sure to carefully avoid memory leaks -- they are more devastating in long-lived programs which event-driven
+programs tend to be.
+
+---
+class: center, middle
+
+## Promises:
+### Proxy objects for future values
+
+???
+Proxy objects for values that have not yet been retrieved or computed.
+
+- In some cases, promises can provide you an alternative, often more useful interface to callbacks.
+
+---
+class: ex-future
+
+## Basic usage (using [`Future`](https://metacpan.org/pod/Future))
+
+```perl
+my $future = fetch_remote_file($url);
+
+$future->on_done(sub($filename, $contents) {
+ print "Fetched $filename\n";
+});
+```
+
+???
+The future is an object that stands in for the thing we actually want until the thing we want is available.
+
+--
+```perl
+$future->on_fail(sub($error) {
+ warn $error;
+});
+```
+
+???
+If the operation that provides the `Future` isn't able to fulfill its promise to provide us what we want, the `Future`
+may be set to a "fail" state.
+
+- For any operation that can fail (and that includes almost everything), it's a good idea to handle errors.
+
+---
+class: ex-future
+
+## Chaining futures
+
+```perl
+my $future = fetch_remote_file($url)
+ ->then(sub($filename, $contents) {
+ my $another_future = write_local_file("/tmp/$filename",
+ $contents);
+ return $another_future;
+ });
+
+$future->on_done(sub($filepath) {
+ print "Saved file to $filepath\n";
+});
+
+$future->on_fail(sub($error) {
+ warn $error;
+});
+```
+
+---
+class: ex-future2
+
+## Creating futures
+
+```perl
+use AnyEvent::HTTP;
+use Future;
+
+sub fetch_remote_file($url) {
+ my $future = Future->new;
+
+ http_get $url => sub($data, $headers) {
+ if ($headers->{Status} =~ /^[23]/) {
+ my ($filename) = get_filename($headers);
+ $future->done($filename, $data);
+ }
+ else {
+ $future->fail("Failed to fetch file: $headers->{Reason}");
+ }
+ };
+
+ return $future;
+}
+```
+
+---
+class: center, middle
+
+Check out [Paul Evan's blog](http://leonerds-code.blogspot.com/2013/12/futures-advent-day-1.html) for more things you can do with `Future`s.
+
+And, of course, [read the pod](https://metacpan.org/pod/Future).
+
+???
+Paul did an advent calendar with short posts detailing the things you can do with `Future`.
+
+- It's really well done.
+
+---
+class: center, middle
+
+## There's also [`Future::AsyncAwait`](https://metacpan.org/pod/Future::AsyncAwait).
projects / chaz / talk-event-driven-programming-in-perl / blobdiff