Numeric string representations in Swift

Jun 3, 2020 · Follow on Twitter and Mastodon swiftextensions

In this post, we’ll create string representations of numeric Swift types and extend them with convenience functionality to make them easier to use.

The basics

You can use String(format:_:) to create strings from serializable types. Different formats apply to different value types.

For instance, you can create a two decimal string from a Double value like this:

let value = 1.2345
let result = String(format: "%0.2f", value)    // => "1.23"

While this is easy, I find it hard to remember formats. I also think it’s nasty to scatter magic formatting strings all over the code base.

Extending numeric types

For instance, say that we want to add an extension to make it easier to serialize a decimal value with any number of decimals easier.

We could create extensions for the numeric types that we want to support:

public extension CGFloat {
    
    func string(withDecimals decimals: Int) -> String {
        String(format: .decimals(decimals), self)
    }
}

public extension Double {
    
    func string(withDecimals decimals: Int) -> String {
        String(format: .decimals(decimals), self)
    }
}

public extension Float {
    
    func string(withDecimals decimals: Int) -> String {
        String(format: .decimals(decimals), self)
    }
}

private extension String {
    
    static func decimals(_ decimals: Int) -> String { "%0.\(decimals)f" }
}

However, this duplicates the same code over and over. We can do a lot better than this.

Creating a shared extension

If we look at String(format:,), we can see that it takes a list of CVarArg arguments. This is a protocol that is implemented by all the numeric types above.

We could thus make the extension above more general by applying it to CVarArg instead:

public extension CVarArg {
    
    func string(withDecimals decimals: Int) -> String {
        String(format: "%0.\(decimals)f", self)
    }
}

This is however not a good idea. CVarArg is implemented by some types where “decimals” don’t make sense. For instance, with the extension above, we could do this:

let string = "Hello, world!"
let result = string.string(withDecimals: 2)

While this is exciting (right?), it just doesn’t make sense. We need to restrict this somehow and can do this by introducing a new protocol:

public protocol NumericStringRepresentable: CVarArg {}

We can then let the numeric types we want to support implement this protocol:

extension CGFloat: NumericStringRepresentable {}
extension Double: NumericStringRepresentable {}
extension Float: NumericStringRepresentable {}

We can then apply the extension to this protocol instead of CVarArg:

public extension NumericStringRepresentable {
    
    func string(withDecimals decimals: Int) -> String {
        String(format: "%0.\(decimals)f", self)
    }
}

With this, we have constrained the extension to pnly NumericStringRepresentable types.

Source Code

I have added these extensions to my SwiftUIKit library. You can find the source code here. Feel free to try it out and let me know what you think!

Discussions & More

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