map for Result
Panicking in the previous example's multiply does not make for robust code.
Generally, we want to return the error to the caller so it can decide what is
the right way to respond to errors.
We first need to know what kind of error type we are dealing with. To determine
the Err type, we look to parse(), which is implemented with the
FromStr trait for i32. As a result, the Err type is
specified as ParseIntError.
In the example below, the straightforward match statement leads to code
that is overall more cumbersome.
Luckily, Option's map, and_then, and many other combinators are also
implemented for Result. Result contains a complete listing.