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<title>Swift Associated Types With Default Values</title>
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<h1>Swift Associated Types With Default Values</h1>
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Published on 16 Sep 2018
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<p>I was browsing the Swift forums when I stumbled across a <a href="https://forums.swift.org/t/a-question-about-associatedtype-in-protocols/16168">discussion</a> regarding an undocumented feature of Swift (as of this post's date): the ability to give associated types a default value.</p>
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<p>I've shown associated types in action before <a href="https://swiftrocks.com/writing-cleaner-view-code-by-overriding-loadview">in my article about loadView()</a> - they are a great tool to define generic behaviour inside protocols in the shape of a specific type. As the feature implies, setting a default value for an associated type will allow the children of the protocol to skip having to define a typealias for the type, unless they specifically want to use a different type.</p>
<p>This is the regular way to define a associated type:</p>
<pre><code>protocol Foo {
associatedtype FooType
}
struct Bar: Foo {
typealias Foo = BarType
}</code></pre>
<p>With just an equal sign, you can define a default type to it.</p>
<pre><code>protocol Foo {
associatedtype FooType = BarType
}
struct Bar: Foo {} //Automatically sets FooType to BarType
//unless another type is specified</code></pre>
<p>It is a very simple thing in nature, but I've found this undocumented feature very interesting. Having to set typealiases really bothered me - being forced to provide a typealias means that the type inherting the protocol is certainly going to do something unique, which might not <i>really</i> be true. Consider this structure used to define a HTTP request:</p>
<pre><code>/// The representation of a HTTPClient's request.
public protocol HTTPRequest {
/// The Value of a HTTPRequest is the response object retrieved after parsing the request's response.
associatedtype Value
/// The endpoint path of the request, to be appended after the HTTPClient's baseURL property.
var path: String { get }
/// Serializes the response of this request to its associated value type.
func serialize(data: Data) throws -> Value
}
extension HTTPRequest where Value: Unboxable {
public func serialize(data: Data) throws -> Value {
let value: Value = try unbox(data: data)
return value
}
}</code></pre>
<p>This structure works very well for my current project because defining <code>path</code> and the <code>Value</code> type is all a request needs to do in order to work, and if <code>Value</code> conforms to <code>Unboxable</code>, the object is even retrieved automatically. This works perfectly for requests such as this one that retrieves an user:</p>
<pre><code>struct UserRequest: HTTPRequest {
typealias Value = User // is Unboxable
let path: String = "v1/profiles"
}</code></pre>
<p>But that implementation is not perfect: Using associated types like that means that I can't test an endpoint without defining a fully fledgled response type to it.</p>
<p>Worse: What if I don't <i>need</i> my request to return something meaningful? Perhaps I don't care about the response, perhaps I just need to cache some <code>Data</code> on the device, or a plain dictionary is enough. For all these cases, I have to explicitly set <code>Value</code> to something and code a custom <code>serialize()</code> method.</p>
<p>If that situation was common, I would simply not use associated types for my requests. Luckily, the existence of default values for associated types solve this problem entirely. Now, I can solve it by making all <code>HTTPRequest</code> objects return a <code>Data</code> object by default:</p>
<pre><code>/// The representation of a HTTPClient's request.
public protocol HTTPRequest {
/// The Value of a HTTPRequest is the response object retrieved after parsing the request's response.
associatedtype Value = Data
/// The endpoint path of the request, to be appended after the HTTPClient's baseURL property.
var path: String { get }
/// Serializes the response of this request to it's associated value type.
func serialize(data: Data) throws -> Value
}
extension HTTPRequest where Value: Data {
public func serialize(data: Data) throws -> Value {
return data
}
}
extension HTTPRequest where Value: Unboxable {
public func serialize(data: Data?, error: Error?) throws -> Value {
let value: Value = try unbox(data: data)
return value
}
}</code></pre>
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<p>Plain requests can now work purely with a <code>path</code> property (and return a plain <code>Data</code> object) while still allowing the regular requests to provide their custom responses.</p>
<pre><code>struct ABTestDataRequest: HTTPRequest {
let path: String = "v1/abtest"
}</code></pre>
<p>Follow me on my Twitter - <a href="https://twitter.com/rockbruno_">@rockbruno_</a>, and let me know of any suggestions and corrections you want to share.</p>
<h2>Reference</h2>
<a href="https://bugs.swift.org/browse/SR-8761">SR-8761</a>
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