I like this idea! It's related to a more general idea I have been thinking about for providing syntactic sugar for anonymous structs. The idea is to use double braces {{ }} to denote an anonymous struct, a capture list to initialize properties, and support trailing anonymous types similar to how we support trailing closures.
When the protocol only has a single method (or possibly subscript) requirement, the body could be used to fulfill the requirement:
protocol Monoid {
associatedtype Value
var empty: Value
func combine(_ lhs: Value, _ rhs: Value) -> Value
}
extension Sequence {
func fold<M: Monoid>(_ monoid: M) -> Element
where M.Value == Element
}
[1, 2, 3].fold {{ [empty = 0] in $0 + $1 }}
Ideally this could be made to work in cases such as Monoid & Hashable where memberwise-synthesizable conformances are assumed and the compiler understands that the single requirement being fulfilled is combine.
In cases where multiple requirements must be implemented manually, the body could also support explicit declarations:
[1, 2, 3].fold {{
typealias Value = Int
var empty: Int { 0 }
func combine(_ lhs: Int, _ rhs: Int) -> Int {
lhs + hrs
}
}}
To provide a concrete real world example applied to SwiftUI:
NavigationLink("Details") {{
VStack {
// details view content
}
}}
I haven't thought much about whether that exact syntax is viable or not, but I think sugar along these lines is very interesting. Declaring a nominal type can be sometimes be inconvenient, especially considering the restriction on declaring them inside a function in generic contexts.