A peer macro expanding generic struct or class to a specialized type. This helps Swift to better specialize your class/struct when your code introduces generics, protocols and type erasure. To some degree, this macro makes generics in Swift work more like C++ templates.
enum Namespace {
@WithSpecializedGenerics("public typealias Hola<S> = Hello<Int, S>;public typealias Hej<S> = Hello<String, S>")
final class Hello<T, S>: Identifiable where T: Hashable, S.ID == T, S: Identifiable {
let id: T
let children: Hello<T, S>
let name = #ReplaceWhenSpecializing("Hello!", "\"¡Hola!\"")
func greeting(with word: Hello<T, S>) -> Hello<T, S> {
let _: Hello<T, S> = Hello(id: word.id, children: word.children)
return Hello<T, S>(id: word.id, children: word.children)
}
init(id: T, children: Hello<T, S>) {
self.id = id
self.children = children.children
}
}
}will be expanded to
enum Namespace {
final class Hello<T, S>: Identifiable where T: Hashable, S.ID == T, S: Identifiable {
let id: T
let children: Hello<T, S>
let name = "Hello!"
func greeting(with word: Hello<T, S>) -> Hello<T, S> {
let _: Hello<T, S> = Hello(id: word.id, children: word.children)
return Hello<T, S>(id: word.id, children: word.children)
}
init(id: T, children: Hello<T, S>) {
self.id = id
self.children = children.children
}
}
final class Hola<S>: Identifiable where S.ID == Int, S: Identifiable {
let id: T
let children: Hola<S>
let name = "¡Hola!"
func greeting(with word: Hola<S>) -> Hola<S> {
let _: Hola<S> = Hola(id: word.id, children: word.children)
return Hola<S>(id: word.id, children: word.children)
}
init(id: T, children: Hola<S>) {
self.id = id
self.children = children.children
}
public typealias T = Int
}
final class Hej<S>: Identifiable where S.ID == String, S: Identifiable {
let id: T
let children: Hej<S>
let name = "¡Hola!"
func greeting(with word: Hej<S>) -> Hej<S> {
let _: Hej<S> = Hej(id: word.id, children: word.children)
return Hej<S>(id: word.id, children: word.children)
}
init(id: T, children: Hej<S>) {
self.id = id
self.children = children.children
}
public typealias T = String
}
}The enum Namespace is required since peer macros cannot introduce new name in global scope.
The macro @WithSpecializedGenerics("typealias ...") accepts a list of typealias syntaxes, with a tolerance of the final modifier. The string inside is also parsed with the SwiftSyntax package. This means it works as long as you are providing a list of parsable typealiases. The macro does 6 things for you:
- Put a specialized type alongside your generic definition.
- Add a typealias for each specialized parameter.
- Remove the specialized generic parameters.
- Remove redundant generic requirements in original
whereclause. - Copy the generic requirements from your
"typealias ... where ..."to the originalwhereclause. - Replace all recursive references to the original type with the newly specialized type.
This package also provides a #ReplaceWhenSpecialing(#OldExpr#, "NewExpr") macro, which replaces all occurences of #OldExpr# with NewExpr when specializing.#OldExpr# must be a valid expression, and NewExpr can be any string. This comes handy when you have some specific implementations.
For the usecase in a quadtree data structure (line 48), this macro can speed up the construction time of data structure by ~16%, compared to directly using a typealias syntax. (Usecases like traversing tree nodes can benefit way more from this)
Note
For generic functions try @_specialize attribute.
Note
Currently this macro does not take special care for AnotherNamespace.StructOrClassWithSameName, and hence it might introduce undesired code when encountering this.
Note
If you’re adding an extension to the original generic type in the same file, and encountered error Circular reference, moving the extension to another file can solve this. This is probably similar to the issue discussed here: swiftlang/swift#66450 .