Using The Option Type Effectively

In a previous blog post, craftsman Dave Torre showed how optional types can alleviate common problems with null values. Bulding on that post, we are going to dive deeper into the API of optional types.

For examples I will be using the Option type provided in Rust, but everything shown here can be accomplished in Java, Scala, Haskell, Swift, OCaml, or any language that has a similar API for optional values.

In Dave Torre's blog post, he built a function get_shortest to retrieve the shortest string from a list. Its signature looks like this:

  fn get_shortest(names: Vec<String>) -> Option<String>

We see that the return type is an Option<String>. The reason for that is that the shortest element of an empty list should be None, as shown here:

let names = vec!["Uku", "Felipe"];
get_shortest(names) //=> Some("Uku")

let empty = Vec::new();
get_shortest(empty) //=> None

We can build on this function and use it to show the shortest name to the user. If the list is empty we should just show "Not found". How might we implement this?

It is very common to use pattern matching to get the job done:

fn show_shortest(names: Vec<&str>) -> String {
  match get_shortest(names) {
    Some(shortest) => shortest,
    _              => "Not Found",
  }
}

show_shortest(vec!["Uku", "Felipe"]) //=> "Uku"

show_shortest(Vec::new()) //=> "Not Found"

The function now works as advertised, but the implementation is more complex than it needs to be. Rust provides a much easier way to grab the boxed value of an Option with a default fallback for None

fn show_shortest(names: Vec<&str>) -> String {
  get_shortest(names).unwrap_or("Not Found")
}

This behaves exactly like our first pattern-matching solution, returning the shortest string or "Not Found" if the list was empty. I find this version easier to read and understand since we don't have to interpret as many elements of syntax (match, => and _). The result of the operation is also evident in the name of the function: either unwrap the inner value of Some or use "Not Found" instead.

Map

At first it is natural to unwrap the value of an Option as quickly as possible. Getting burned by the infamous NullPointerException has created a culture of avoiding nulls, but the Option type is different. As opposed to null, optional values are part of the type system. Hence they can safely be passed around and operated on like any other first-class value.

For example, say our users wanted a function to find out the length of the shortest name in a list. Given that we already have get_shortest, it should be a matter of just calling length on its result. However, we know that get_shortest returns an optional value, so what is the length of None? Without support for optional values, we might come up with a special constant or throw an exception to handle this case, but luckily we don't have to. If optional values are supported, we can just say that the length of None is None.

Here is how we might implement it:

fn get_shortest_length(names: Vec<&str>) -> Option<usize> {
  match get_shortest(names) {
    Some(shortest) => Some(shortest.len()),
    None           => None,
  }
}

get_shortest_length(vec!["Uku", "Felipe"]) //=> Some(3)

get_shortest_length(Vec::new()); //=> None

Again, this works just fine, but we force readers to untangle the pattern-matching expression in their heads. Have a closer look at the pairing of possible inputs on the left of => and outputs on the right of it. Notice that the container does not change: None maps to None and Some to Some. That seems to be enough of a pattern to extract it into a separate function. Rust provides exactly such a map function that we can make use of instead:

fn get_shortest_length(names: Vec<&str>) -> Option<usize> {
  get_shortest(names).map(|shortest| shortest.len())
}

get_shortest_length(vec!["Uku", "Felipe"]) //=> Some(3)

get_shortest_length(Vec::new()); //=> None

As you can see, map takes care of all of the boilerplate and allows us to express intent more concisely. In many ways, mapping over an optional is similar to mapping over a collection. The difference is in how the lamdba expression is handled: for collections it is called once for each element, for optionals it is only called if the value exists.

You may also notice that we don't have to manually wrap the return value of shortest.len(). The map function maintains the contract that we discussed earlier: None maps to None and Some to Some.

And Then

Our imaginary shortest name feature is starting to gain more popularity and the users' demand is growing. In addition to the shortest name and its length, they would like to see more information about the user with that name. This information must be pulled from another service and parsed from JSON. Both of these functions aready exist:

fn find_user_by_name(name: String) -> Option<JSON>
fn json_to_user(json: JSON) -> Option<User>

Since mapping worked so well the last time, why not use it for this problem:

fn get_user_with_shortest_name(names: Vec<&str>) -> Option<User> {
  get_shortest(names)
    .map(|shortest| find_user_by_name(shortest))
    .map(|user| json_to_user(user))
}

Running this through the compiler gives us an error:

<anon>:18:37: 18:41 error: mismatched types:
 expected `collections::string::String`,
    found `core::option::Option<collections::string::String>`
(expected struct `collections::string::String`,
    found enum `core::option::Option`) [E0308]
<anon>:18     .map(|user|     json_to_user(user))
                                           ^~~~

The compiler is actually right — mapping is not the appropriate operation in this case. Since map wraps its return value in an Option, it does not compose well with functions that already return optional values like find_user_by_name and json_to_user. Trying to combine these functions results in double-wrapping the value in every call to map, which turns our user record into something like Some(Some(Some(User{name: "Uku"}))).

So it looks like our types are not lining up properly. Let's just write what we want to do manually and refactor later.

fn get_user_with_shortest_name(names: Vec<&str>) -> Option<User> {
  let shortest = get_shortest(names);
  if shortest.is_some() {
    let json = find_user_by_name(shortest.unwrap());
    if json.is_some() {
      json_to_user(json.unwrap())
    } else {
      None
    }
  } else {
    None
  }
}

let names = vec!["Uku", "Felipe"];
get_user_with_shortest_name(names) //=> Some(User { name: "Uku" })

This seems to work, but there's one problem with it: nobody wants to write code like this. There's a clear pattern in the way we check for presence every step of the way, so abstracting this chain should be trivial. Luckily, Rust has a built-in method called and_then that can help us out.

fn get_user_with_shortest_name(names: Vec<&str>) -> Option<User> {
   get_shortest(names)
    .and_then(|shortest| find_user_by_name(shortest))
    .and_then(|user| json_to_user(user))
}

let names = vec!["Uku", "Felipe"];
get_user_with_shortest_name(names) //=> Some(User { name: "Uku" })

and_then works similarly to map, but it makes less assumptions about the function passed into it. Instead of wrapping the return value in an Option automatically, it lets the function choose the return type. This grants us the power to create a chain of operations where any link can possibly return None.

It may be easier to see the subtle difference between map and and_then by composing simple arithmetic functions. For example, we know that addition always produces a result, while dividing by zero is meaningless. If we encode this with optionals, we get these two signatures:

fn add(num1: i32, num2: i32) -> i32

fn div(num1: i32, num2: i32) -> Option<i32>

In a chain of functions applied to an optional integer, these two functions must be treated differently:

Some(3)
  .map(|n| add(n, 5))
  .and_then(|n| div(16,n)) // => Some(2)

The general rule of thumb is to use map for plain functions from value to value, but use and_then for functions from value to an optional value.

A smart box

I like to think of the Option type as a box and pattern matching as opening the box. It turns out that Option is intelligent enough that we usually don't have to peek inside of it to achieve the desired effect. They actually know how to unwrap themselves, run operations on the contained value, combine themselves with other optionals, and much more. I encourage everyone to explore this standard API and challenge yourselves to not look inside the box.

Combined with the fact that optional values are part of the type system, these functions make them very different from regular null handling. In Java or Ruby it is important to check for null as early as possible to avoid invalid states. However, in languages like Rust you can leverage optional values in high-level business logic without fear of running into an unexpected null.

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