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if for postfix member expressions

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0308 allows Swift to use #if conditions with postfix member expressions. This sounds a bit obscure, but it solves a problem commonly seen with SwiftUI: you can now optionally add modifiers to a view.

For example, this change allows us to create a text view with two different font sizes depending on whether we’re using iOS or another platform:

import SwiftUI

Text("Welcome")
#if os(iOS)
    .font(.largeTitle)
#else
    .font(.headline)
#endif

You can nest these if you want, although it’s a bit hard on your eyes:

Text("Welcome")
#if os(iOS)
    .font(.largeTitle)
    #if DEBUG
        .foregroundColor(.red)
    #endif
#else
    .font(.headline)
#endif

You could use wildly different postfix expressions if you wanted:

let result = [1, 2, 3]
#if os(iOS)
    .count
#else
    .reduce(0, +)
#endif

print(result)

Technically you could make result end up as two completely different types if you wanted, but that seems like a bad idea. What you definitely can’t do is use other kinds of expressions such as using + [4] instead of .count – if it doesn’t start with . then it’s not a postfix member expression.

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