ModularM
Modular•2y ago•
5 replies
drujensen

dynamic traits are not supported yet

Hello. Nooby question. I am working on some middleware for the lightbug project and want to build a chain-of-responsibility design pattern to handle the different aspects of processing a request / response:
## Middleware is an interface for processing HTTP requests.
## Each middleware in the chain can modify the request and response.
trait Middleware:
    fn set_next(self, next: Middleware):
        ...
    fn call(self, context: Context) -> HTTPResponse:
        ...

## MiddlewareChain is a chain of middleware that processes the request.
## The chain is a linked list of middleware objects.
struct MiddlewareChain(HTTPService):
    var root: Middleware

    fn add(self, middleware: Middleware):
        if self.root == nil:
            self.root = middleware
        else:
            var current = self.root
            while current.next != nil:
                current = current.next
            current.set_next(middleware)

    fn call(self, request: HTTPRequest) raises -> HTTPResponse:
        var context = Context(request)
        return self.root.call(context)

This is how we would build the MiddlewareChain:
  var middleware = MiddlewareChain()
  middleware.add(CompressionMiddleware())
  middleware.add(ErrorMiddleware())
  middleware.add(LoggerMiddleware())
  middleware.add(CorsMiddleware(allows_origin = "*"))
  middleware.add(BasicAuthMiddleware("admin", "password"))
  middleware.add(StaticMiddleware("static"))
  middleware.add(router)
  middleware.add(NotFoundMiddleware())


I'm getting the TODO: Dynamic traits are not supported yet. please use a compile time generic instead. I have tried using a Pointer[Middleware]` but you need the type to cast and make the call. I also tried passing in a concrete middleware type as a parameter at compile time but it requires some really ugly code to chain them together.

Are there any other options? Let me know if anyone has a solution that doesn't look completely crazy.
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