Two types of Swift macros

Explain # and @ in Swift - your next interview may have this question! Don’t miss it out!

Shortly - # and @ are prefixes for macros in Swift.

What is macro? 💡 Macro is a feature that generates code during compilation. Unlike macros in C, which work like “find and replace”, Swift macros are type-safe and context aware, making them powerful tools reducing boilerplate code.

Two types of macros 1️⃣ attached - use @ prefix, tied to a declaration adding extra logic to it, like: @Test, @Model, @Observable 2️⃣ freestanding - use # prefix, standalone code like #expect, #Predicate, #warning

Example of attached macro ⤵️

@Test func addition() { // tied to the declaration
    ...
}

Example of freestanding macro ⤵️

@Test func addition() {
    #require(1 + 2 == 3) // not attached to a declaration
}

Bonus: It’s possible to expand macros, especially those defined by you, and check their implementation by right-clicking on a macro and selecting the “Expand Macro” option. ⤵️

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