Are nested/recursive Macros supported in Swift 5.9?

Evolution Discussion
1

I just had an idea for an architectural-level macro where my macro would help structure features of an app. I have not had the time to play around with the idea yet (to figure out if a Macro is even a good choice), but some of the "expanded" code of my Macro could potentially involve other macros, such as @Observable. I wonder if macro expansion works even with nested macros?

I haven't found anything on the topic in this forum or in the Macros Vison document (I searched for "nested" and "recursive" macros). Would be really useful if nested/recursive macros were supported. :crossed_fingers:

2

It's working.

As an example the Model macro from SwiftData expansion contains others macros like PersistedProperty macro.

Xcode also supports expanding nested macro in the IDE.

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3

Awesome! :muscle::tada:

4

I think it's worth to mention that recursive macros are not supported. I asked about this at WWDC and received the following response:

The compiler will emit an error if there is a cycle in the expansion of macros.

5

Nested macro expansion is supported. You will definitely be interested in member-attribute macros which primarily exist to facilitate nested macro expansion like you're describing. This is what @Observable uses to apply accessor macros to all properties within a type.

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6

Are there any examples of this in action for reference?

7

For example, the @Model macro from SwiftData, among other things, adds the @PersistedProperty macro to all stored properties.

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8

Is it possible for one macro to nest more macros on the declaration itself, for example, would it be possible to have something like this:

@MacroA
enum Fruit {
    case apple, banana
}

On expanding MacroA:

@MacroB
@MacroC
enum Fruit {
    case apple, banana
}
9

No.

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