Error handling

Error handling is the process of handling the possibility of failure. For
example, failing to read a file and then continuing to use that bad input
would clearly be problematic. Noticing and explicitly managing those errors
saves the rest of the program from various pitfalls.

There are various ways to deal with errors in Rust, which are described in the
following subchapters. They all have more or less subtle differences and different
use cases. As a rule of thumb:

An explicit panic is mainly useful for tests and dealing with unrecoverable errors.
For prototyping it can be useful, for example when dealing with functions that
haven’t been implemented yet, but in those cases the more descriptive unimplemented
is better. In tests panic is a reasonable way to explicitly fail.

The Option type is for when a value is optional or when the lack of a value is
not an error condition. For example the parent of a directory - / and C: don’t
have one. When dealing with Options, unwrap is fine for prototyping and cases
where it’s absolutely certain that there is guaranteed to be a value. However expect
is more useful since it lets you specify an error message in case something goes
wrong anyway.

When there is a chance that things do go wrong and the caller has to deal with the
problem, use Result. You can unwrap and expect them as well (please don’t
do that unless it’s a test or quick prototype).

For a more rigorous discussion of error handling, refer to the error
handling section in the official book.