Erlang and OTP in Action.pdf - Synrc

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2.8.2 – Using try…catch
In modern Erlang, you use a try-expression to handle exceptions that may occur in a piece
of code. In most ways it works like a case-expression, and in the simplest form it looks like
this:
try
some_unsafe_function(…)
catch
oops
-> got_throw_oops;
throw:Other -> {got_throw, Other}
exit:Reason -> {got_exit, Reason}
error:Reason -> {got_error, Reason}
end
Between try and catch, we have the body or protected section. Any exception that
occurs within the body and tries to propagate out from it will be caught and matched against
the clauses listed between catch and end. If it doesn’t match any of the clauses, the
exception will continue as if there was no try-expression at all around the body. Similarly, if
the body is evaluated without raising an exception, its result will become the result of the
whole expression, as if the try and catch…end had not been there. The only difference is
when there is an exception, and it matches one of the clauses. In that case, the result
becomes whatever the matching clause returns.
The patterns of these clauses are a bit special—they may contain a colon (:) to separate
the exception class (error, exit, or throw) from the thrown term. If you leave out the
class, it defaults to throw. You shouldn’t normally catch error and exit unless you know
what you are doing—it goes against the idea of failing early, and you could be masking a real
problem. Sometimes, though, you just want to run some code that you don’t trust too much,
and catch anything that it throws. You can use the following pattern for catching all
exceptions:
_:_ -> got_some_exception
(or “Class:Term -> …” if you want to inspect the data in the exception).
Also, note that once you get to the catch part, the code is no longer protected. If a new
exception happens in a catch-clause, it will simply propagate out of the entire tryexpression.
2.8.3 – try…of…catch
There is a longer form of try that is useful when you need to do quite different things in the
successful cases and the exception cases. For instance, if you want to continue doing some
work with the value you got in the successful case, but want to print an error message and
give up in the exception case, you can add an “of…” section, like this:
try
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