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When to use default parameters for functions

Paul Hudson    @twostraws   

Updated for Xcode 12.0

Default parameters let us make functions easier to call by letting us provide common defaults for parameters. So, when we want to call that function using those default values we can just ignore the parameters entirely – as if they didn’t exist – and our function will just do the right thing. Of course, when we want something custom it’s there for us to change.

Swift developers use default parameters very commonly, because they let us focus on the important parts that do need to change regularly. This can really help simplify complex function, and make your code easier to write.

For example, imagine some route finding code such as this:

func findDirections(from: String, to: String, route: String = "fastest", avoidHighways: Bool = false) {
    // code here
}

That assumes that most of the time folks want to drive between two locations by the fastest route, without avoiding highways – sensible defaults that are likely to work most of the time, while giving us the scope to provide custom values when needed.

As a result, we can call that same function in any of three ways:

findDirections(from: "London", to: "Glasgow")
findDirections(from: "London", to: "Glasgow", route: "scenic")
findDirections(from: "London", to: "Glasgow", route: "scenic", avoidHighways: true)

Shorter, simpler code most of the time, but flexibility when we need it – perfect.

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