Battle of the patterns: part 1

Ali Churcher · 22 Jul 2019

pug crossed with an owl


The members of Liefery’s Flutter development team come from a wide range of backgrounds - from React and Elm to Java and Scala. While rewriting our mobile application in Flutter we were able to pick and choose different design patterns that we knew from experience were good for a particular problem. Two of the patterns we chose were well established and solved clear problems that we had. After some time however, we found ourselves battling with these patterns, or rather, these patterns battling each other. When we needed to write code that bypassed the best practices of one pattern to satisfy the other we realized we needed to rethink our design. In this blog post I want to describe two patterns that we are using in our codebase, how they are currently conflicting in our state management code, and some ideas that the wider team has discussed for improvements. I hope that part 2 of this blog post will describe the approach we end up taking to solve this problem and create a slick new state management design.

The two patterns in question: Redux Selectors and Option Types

Background information: we use flutter_redux for our state management. It allows us to centralize our application state in a single place and control how it is updated. It behaves mostly like the popular Redux library from JavaScript, and a very nice explanation of the main concepts can be found here.

Redux Selectors

A pattern of writing helper functions that take the entire state object and return a specific part of the state.

When using flutter_redux, all application state is stored in a single place - the state object. Rather than access this information by being aware of its structure, we encapsulate away this information and access the data with selectors. This provides the benefit of allowing the underlying state structure to change without having to change all our code.

Without selectors:

const int count = state.session.user.orders.unpaid.length  // directly access the state field

With selectors:

const int count = getNumberOfUnpaidBills(state) // call a selector 
int getNumberOfUnpaidBills(state) {
  return state.session.user.orders.unpaid.length  // selector implementation

Option Types

A way to remove nulls by providing a type to represent an optional/unknown value

Option types are a language feature implemented in many function languages such as Haskell (called maybe), Rust, Swift and Scala. Our project uses a Dart implementation of Scala Option types. Option types provide a safe way to handle the absence of a value. The concept of null/nil/undefined is completely avoided and replaced with a type that is optional - i.e. a value that maybe present or not.

For example, if we have a variable comment that might be present or not, we can model it like this:

const Option<String> comment;

If the value is present, then the Option<String> holds an instance of Some<String>.

comment = Some('thanks for the speedy delivery');

If the value is absent then comment holds an instance of None.

comment = None();

Implemented correctly, Option types prevent the errors and surprises caused by unhandled nulls. The type forces you to handle both the presence and absence of the value at compile time.

Where the pattern battle begins

We encountered some situations where Option types and selectors do not play nicely together. Let’s look at an example involving one important part of our application state – the session. For this example, the session will have just three fields: an apiKey, the name of the logged in user, and some orders:

class SessionState {
  final String ApiKey;
  final String username;
  final List<Order> userOrders;

The session is present in the application state, the centralized state object that we access and update with Redux. Since a session only exists when a user is logged in, this variable is a perfect candidate to use an Option type.

class State  {
    final Option<SessionState> session; // Option type
    String languageCode;
    final Route currentRoute;
    . . .

When the user is logged out the session variable will hold None(). When the user is logged in, session becomes Some(SessionState) where SessionState is our session object defined above containing the ApiKey and other information.

When we go to use this session, we handle it with a Fold function that takes two callbacks - one for if the Option variable holds None() and one for if there is a value.

Route currentRoute = session.fold(
        () => loginScreen(), // when session is None
        (session) => userHomepage(session) // when session is Some

You can see that these two callbacks allow us to direct the user to the LoginScreen if there is no session, and proceed straight to the userHomepage if a session is present.

Adding selectors into the mix

Selectors take the entire state and return the requested value. A selector to get the username might look like this:

String getLoggedInUsername(state) {
   return state.session.username;

however, this code wouldn’t compile yet. As session is an Option we must handle the case where the value is None:

String getLoggedInUsername (FlutteryState state) =>
      () => '', // if session is None return an empty string
      (session) => session.username // if it exists return the username

You can see that if the session doesn’t exist we return an empty string.

This is where our two patterns first clash

By returning an empty string we have thrown away the Option pattern – we now have something that appears to be a value, but is effectively not present (empty string). This is especially dangerous in an application that uses Option types because the need for vigilant null checking is usually avoided.

First fix idea

Selectors should not return empty values such as '' and []

Where previously our selector returned '' when the value was not present, what if we returned an Option type?

Option<String> getLoggedInUsername (FlutteryState state) =>
    state.session.fold(() => None(), (session) => Some(session.username));

The problem here is that this selector is not reflecting the true value in the state. If we look at the original state again:

class state  {
    final Option<SessionState> session;
    String langugeCode;
    final Route currentRoute;
    . . .

we can see that if session is not there, then the concept of session.username does not even exist. It’s not None() it’s just not there at all! This design also means added hassle of dealing with the None() scenario of an Option type in situations when we know for sure that the value is present - when the user is logged in.

cat gifintermission cat

Second fix idea

Favour selector pattern over Option types when the patterns clash

To better fit with the fact that our selectors returned empty strings etc, we can remodel our state to match. Rather than the session being an Option type, we can make it as an object that was always present. When the user is logged out the session object has empty values. When the user is logged in the values are present.

class state  {
    final SessionState session; // previously was Option<SessionState> session
    String langugeCode;
    final Route currentRoute;
    . . .
class SessionState {
  final String ApiKey;
  final String username;
  final List<Order> userOrders;
  . . .

  apiKey = '',
  username = '',
  userOrders = [],
 // this constructor provides default values for when no value is present 
 // i.e. before any user has logged in.

With this change, our selector can be rewritten:

String getLoggedInUsername(state) {
   return state.session.username;

and the selector return value will always match exactly what is in the state.

The problem with this? We now have to handle absence of values ourselves. We need to check if things are empty or present when we use them – exactly what Option types were added to avoid. We are dealing with the baggage of Option types without the benefits of safety. Let’s see if we can improve on this.

A compromise?

Follow the Option pattern strictly and be more lenient with selectors

A lot of the original problems with our getLoggedInUsername selector came from the fact that our session object was an Option type. The selector had to handle either the absence or the presence of the session. We could avoid this problem by passing a subset of the state into the selector:


String getLoggedInUsername (FlutteryState state) =>
      () => '', 
      (session) => session.username 
      // pass full state, session could be None()


String getLoggedInUsername(SessionState session) => 
// function only be called when we have a session

This approach restricts us to only ever calling this selector when the session object exists, but it means we are guaranteed an accurate return value. By passing something other than the full state however, we are no longer hiding away the state structure. For example, if the state was refactored and session was renamed to userDetails and restructured, we would have to refactor every selector. We will have lost the main benefit of redux selectors - to encapsulate away the inner structure of the state object.

Bonus idea:

There is one more idea to try to get Option types and selectors to play nicely.

If session is None() then we could argue that our application should never be asking for the username in the first place. Our application logic should be smart enough to only ask for the username when the user is logged in and we have a session. With this philosophy, if session is None() then asking for the session’s username could be modelled as an error rather than a None().

Either is another Scala device, very similar to Option types, but rather than a type, or null, it’s any two types. In this case, a String or an error.

Either<String, error> getLoggedInUsername (FlutteryState state) =>
      () => Right('error no session'),
      (session) => Left(session.username)

This pattern fits well with our Option types pattern, but means we must have selectors that return values that are not actually in the state object - namely errors.

What next?

So now we’ve seen our two very different patterns. The first was the selector pattern, which values access to all application state at any time in the lifecycle of the application. The second was the Option types pattern, which encourages using application knowledge to only ask for information when it makes sense, for example only asking for the session’s username if we have a session.

Although these two patterns nicely solve design problems, we can see that sometimes the way they interact with each other can cause trouble. For example, with the selector pattern we sometimes need to forego Option types and instead return (and handle) nulls and other ‘empty’ values such as ''. And when using Option types we sometimes weren’t able to successfully access all redux state values at all times, for example not being able to access the username when there is no logged in user.

We know that patterns come with trade-offs. Sometimes it’s an added level of indirection, a high learning curve, or boilerplate code. We can now see that sometimes the trade-off of adding a new pattern could even mean giving up existing pattern.

So are we seconds away from finding a Redux + Option type utopia or will we need to accept defeat and cut one from our application? Join us after our tech design meeting to find out what we choose in part 2 - a shiny new solution!