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Error Handling

How do you do error handling in Erlang?
As I understand it, it’s somewhat different than in other languages.

The inter-process part certainly is.

As for intra-proces error handling, it’s basically the same as in other modern languages: you can signal errors with special return values, or you can throw exceptions.

Error-signalling return values

In typed languages, common special return values are null or negative integers, depending on the type. Or none if the language has an option type.

In Erlang, which is dynamically-typed and symbolic, you typically use an “error tuple”. I.e. {error, Tag}, where Tag is an atom describing the kind of error, or — if you want to add details — 

 — then you’d probably add it like this: {error, Tag, Details}, right?

Well, no. Typically you wouldn’t do that — because the custom is to expect a 2-tuple, to catch it with the pattern {error, Reason}. And a 3-tuple wouldn’t match that.

Ah, right. So you’d say {error, {Tag, Details}} instead. Even though the extra tupling might look superfluous.

Anything else to be aware of?

You certainly shouldn’t pair just anything with error and call it an error tuple. In particular, don’t wrap one error tuple in another — like it is done here:

case file:open(FileName, Modes) of
  %% open() returns {ok,_} | {error,_}
  {ok, Fd} -> something_good(Fd);
  Error    -> {error, Error}

That would result in a value like {error, {error, …​}} which is more confusing than informative.

You do get to know a measure of how many levels deep the original error were, though…​

Yes, but it is still debatable whether it’s better or worse than nothing.

So instead, I should just return Error — the original error tuple?

Yes; either that or tag it anew with something providing a bit of context:

  {error,_}=Error -> Error
%% or:
  {error,Reason} -> {error, {opening_the_foo_file_failed, Reason}}
%% or, if you want to add details:
  {error,Reason} -> {error, {opening_the_foo_file_failed, Reason, Filename}}

That way, the error message will tell not only what went wrong (file not found), but also what the program was trying to do at the time — a description at a higher level.


Understanding error messages