How does Swift let us compare many types of data?

Paul Hudson    @twostraws    October 25th 2021

Updated for Xcode 13.3

Swift lets us compare many kinds of values out of the box, which means we can check a variety of values for equality and comparison. For example, if we had values such as these:

let firstName = "Paul"
let secondName = "Sophie"

let firstAge = 40
let secondAge = 10

Then we could compare them in various ways:

print(firstName == secondName)
print(firstName != secondName)
print(firstName < secondName)
print(firstName >= secondName)

print(firstAge == secondAge)
print(firstAge != secondAge)
print(firstAge < secondAge)
print(firstAge >= secondAge)

Behind the scenes, Swift implements this in a remarkably clever way that actually allows it to compare a wide variety of things. For example, Swift has a special type for storing dates called Date, and you can compare dates using the same operators: someDate < someOtherDate, for example.

We can even ask Swift to make our enums comparable, like this:

enum Sizes: Comparable {
    case small
    case medium
    case large
}

let first = Sizes.small
let second = Sizes.large
print(first < second)

That will print “true”, because small comes before large in the enum case list.

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