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Swift Typealias – Make Your Code Understandable

In all of its simplicity, typealias in Swift is just an alias for an existing type. Using typealias smartly can make your Swift code more readable. To declare a new one, just use this syntax:

typealias NewType = ExistingType

Next up, I will show you some real-life use cases for typealiases in your Swift program.

By the way, if you want to watch a short video about this topic, look no further:

Table of Contents

Typealias Examples

Let’s say you’re dealing with weights in your code. These weights are essentially just doubles. What is really cool about typealiases is you can create an alias Weight that is actually Double under the hood:

typealias Weight = Double

Now you can use Weight instead of Double:

let mass1: Weight = 152.3
let mass2: Weight = 43.2

print(mass1 + mass2)

Write an Extension for a Typealias

Writing an extension for a typealias also extends the underlying type. For example, assuming that the Weight is in kilos, let’s create an extension toPounds() for the Weight alias:

typealias Weight = Double

extension Weight {
    func toPounds() -> Weight {
        return self * 2.205
    }
}

let mass: Weight = 100
print(mass.toPounds())

// The extension affects Doubles too:
let massDoubles: Double = 120
print(massDoubles.toPounds())

Output:

220.5
264.6

As you can see, both Double and Weight instances now have the toPouds() conversion.

Combine Protocols using Typealias

Assume you have two access protocols for reading and writing students’ grades:

protocol ReadAccess {}
protocol WriteAccess {}

Your teacher should be able to both read and write grades. Using typealias, you can neatly combine these two protocols to create a new protocol dedicated to teacher admins only:

typealias AdminAccess = ReadAccess & WriteAccess
struct Teacher: AdminAccess {}

Closures and Typealias

Let’s say we have a simple method that takes a closure as its input:

func method(operation: (Double) -> (Double)) {}

You can now create a typealias for the (Double) -> (Double) function type to highlight that it is a math operation:

typealias MathOperation = (Double) -> (Double)

Now the method can be updated to use this typealias in its argument’s type:

func method(operation: MathOperation) {}

Conclusion

Typealias is just an alias to an existing type. The main goal of typealiases is to make code more readable and understandable. As an example, here is a typealias Weight that essentially is just a Double: