A simple struct for enabling fluent interface writing style in >= Swift 5.1.

FluentInterface.swift

@dynamicMemberLookup
public struct FluentInterface<Root> {
    public let root: Root
    public subscript<Value>(dynamicMember keyPath: WritableKeyPath<Root, Value>) -> (Value) -> Self {
        var root = self.root
        return { newValue in
            root[keyPath: keyPath] = newValue
            return FluentInterface(root: root)
        }
    }
}

postfix operator +
public postfix func + <T>(lhs: T) -> FluentInterface<T> {
    return FluentInterface(root: lhs)
}

postfix operator -
extension FluentInterface {
    @discardableResult
    public static postfix func - (lhs: FluentInterface) -> Root {
        return lhs.root
    }
}