[erlang-questions] When to catch (was: Module config data and functional programming)

Daniel Dormont <>
Sun Sep 4 03:31:33 CEST 2011


This was an earlier thread of mine that inspired a new question.

As the months go by I'm getting more comfortable with "let it crash"
as an idea and seeing its benefits. But it occurs to me: surely there
are exceptions. Or rather: in order for "let it crash" to work at one
level of abstraction or functionality, there must be actual
crash-handling code at a level beneath it. In Erlang, this seems to
often involve letting individual processes crash and their supervisors
react accordingly. But perhaps that's not always the right answer. So
my question is:

When is "catch" the right tool for the job? In the example below,
Ejabberd's "hooks" handlers use catch when calling functions in
foreign modules that are (probably) not critical path to whatever it's
doing. That seems like a pretty decent case to me. Would you agree?
What are some cases common in Erlang where exceptions need to be
caught and handled (or ignored) within a single process?

dan

On Fri, Apr 22, 2011 at 1:30 PM, Per Melin <> wrote:
> On Fri, Apr 22, 2011 at 4:32 PM, Daniel Dormont
> <> wrote:
>> It also occurs to me that switching to gen_server would offer another advantage in that it would be easy to recover from crashes in my own code. If I were not to do that though, is the recommended practice just to put a 'catch' call around code that might break?
>
> I don't have enough context, but it sounds absolutely unnecessary to
> create a gen_server for this.
>
> I would consider it ejabberd's responsibility to handle any crashes in
> your code. They have asked you for a callback function and they must
> consider that it may break. And they are in a better position than you
> to know how to proceed from a crash here. I did have a look in
> ejabberd_hooks and they do catch exits from your function. Let it
> crash.
>
> I'm hesitant to touch the question of what is recommended practice
> since this is the kind of thread where Dr Richard O'Keefe usually
> comments after a while and turns everyone that posted before him into
> fools.
>



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