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Throw Now, Describe Later

String::Errf - 2013-12-22

A few years ago, I wrote about Throwable::X, a set of tools for making exceptions easier to work with. Part of the goal was to make it easy for catching code to identify the exception. That meant that rather than saying:


die "you can't set $attr to $value prior to $ready_date";

You say:


OurException->throw(ident => "tried to set attribute before ready");

The big deal is that later, your code can be clearer about identifying the exception. Instead of saying:


if ($error =~ /you can't set \S+ to \S+ prior to \S+/) { ... }

You can say:


if ($error->$_isa('OurException') && $error->ident eq 'tried to set attr before ready') { ... }

…or if you're thinking ahead…


if ($error->$_is_err('tried to set attr before ready')) { ... }

This works because we give each kind of exception a unique enough name. There's an obvious cost, though. The exception, when a string, contained three useful pieces of information that are now lost. We had to ditch them to make the string unique, no matter when it happened. The article on Throwable::X glossed over the solution. The idea is that in addition to an unchanging ident, the exception has a payload and maybe a message.



  ident => "tried to set attribute before ready",
  message => "you can't set %{attr}s to %{value}i before %{ready_date}t",
  payload => { attr => $attr, value => $value, ready_date => $ready_date },


The new parameters let us get the same data into the printed form of the exception that we had originally, without compromising its identifiability. What's up with that message string, though?


That string is an errf string. It's like the kind of thing you pass to sprintf, but different. It's meant to be simple to implement and to cover just the basic data you might need to stick in an error description.

Like Python's % operator, it can format based on a set of named parameters. It takes a totally different set of parameters, though. Above you see a demonstration of %s, for strings, %i, for integers, and %t for timestamps.

There are other codes, of course, each with its own options. There's %f for floats:


errf "%{x;prefix=+;precision=.2}f", { x => 10.1234 }; # returns "+10.12";

…and there are a number of useful options not seen for %t:



my $t = 1280530906;
errf "%{x}t", { x => $t }; # "2010-07-30 19:01:46"
errf "%{x;type=date}t", { x => $t }; # "2010-07-30"
errf "%{x;type=time}t", { x => $t }; # "19:01:46"
errf "%{x;type=datetime}t", { x => $t }; # "2010-07-30 19:01:46"

errf "%{x;tz=UTC}t", { x => $t }; # "2010-07-30 23:01:46 UTC"
errf "%{x;tz=UTC;type=date}t", { x => $t }; # "2010-07-30 UTC"
errf "%{x;tz=UTC;type=time}t", { x => $t }; # "23:01:46 UTC"
errf "%{x;tz=UTC;type=datetime}t", { x => $t }; # "2010-07-30 23:01:46 UTC"


The goal of errf strings isn't just to allow exceptions to be self-describing when caught and displayed, but to make their description easy to change in the presentation layer of code, which might not be handled by exactly the same team as the business logic. Presentation layer here might sound ominous, like these strings might be getting used to give reports to users — and they are! Many of our exceptions are flagged as user visible, and we show their description right to the user. That means it needs to be possible to make them look… well, not stupid.

  You can't add a new account.  You have 1 accounts already.

Ugh! Nobody likes an inflection bug. String::Errf has a fix for that:



errf "You can't add a new account. You have %{count;singular=account}n already.",
  { count => 1 };


The %n code will let you inflect words based on a count, including the count. %N is the same, but omits the number.

A secondary goal of String::Errf was to produce a format simple enough that it could be implemented in other languages, especially JavaScript, so that API errors could be provided with errf-format errors, then formated at the client side. So far this hasn't been done, but possibly soon…

See Also

Gravatar Image This article contributed by: Ricardo Signes <rjbs@cpan.org>