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We’ve modeled side effects in our architecture, but it’s not quite right yet: a reducer can write to the outside world, but it can’t read data back in! This week our architecture’s dedication to unidirectional data flow will lead us there.
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We have extracted our first effect into our architecture. Let’s reflect on what we just accomplished.
We had a side effect in our view, saving favorite primes to disk, that we knew we needed some way to control. For one thing it’s untestable code, but for another we have found that a useful way to simplify our views was to move all of their logic into our reducers, and simply make the view responsible for sending user actions to the store.
Elm is a pure functional language wherein applications are described exclusively with unidirectional data flow. It also has a story for side effects that closely matches the approach we take in these episodes. This document describes how commands (like our effect functions) allow for communication with the outside world, and how the results can be mapped into an action (what Elm calls a “message”) in order to be fed back to the reducer.
The Redux documentation describes and motivates its “strict unidirectional data flow.”
Redux, at its core, is very simple and has no single, strong opinion on how to handle side effects. It does, however, provide a means of layering what it calls “middleware” over reducers, and this third-party extension point allows folks to adopt a variety of solutions to the side effect problem.
Redux Thunk is the recommended middleware for basic Redux side effects logic. Side effects are captured in “thunks” (closures) to be executed by the store. Thunks may optionally utilize a callback argument that can feed actions back to the store at a later time.
ReSwift is one of the earliest, most popular Redux-inspired libraries for Swift. Its design matches Redux, including its adoption of “middleware” as the primary means of introducing side effects into a reducer.
An early example of Redux in SwiftUI. Like ReSwift, it uses “middleware” to handle side effects.
Elm is both a pure functional language and framework for creating web applications in a declarative fashion. It was instrumental in pushing functional programming ideas into the mainstream, and demonstrating how an application could be represented by a simple pure function from state and actions to state.
The idea of modeling an application’s architecture on simple reducer functions was popularized by Redux, a state management library for React, which in turn took a lot of inspiration from Elm.
A talk that Brandon gave at the 2017 Functional Swift conference in Berlin. The talk contains a brief account of many of the ideas covered in our series of episodes on “Composable State Management”.
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