Modular•4mo ago

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)

## 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())

  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.

3 Replies

tkeitt•3mo ago

Dunno. But I sure could use generic traits already.

Ryulord•3mo ago

Currently you need to explicitly do runtime type checking yourself using the variant type like so:

if middleware.isa[CompressionMiddleware]():
    middleware[CompressionMiddleware].set_next(next)

if middleware.isa[CompressionMiddleware]():
    middleware[CompressionMiddleware].set_next(next)

If you're going to be doing lots of these often you could make a helper struct like the following:

@value
struct DynamicMiddleware:
    var _middleware: Variant[CompressionMiddleware, ErrorMiddleware]

    fn set_next(inout self, owned next: DynamicMiddleware):
        if self._middleware.isa[CompressionMiddleware]():
            self._middleware[CompressionMiddleware].set_next(next)
        elif self._middleware.isa[ErrorMiddleware]():
            self._middleware[ErrorMiddleware].set_next(next)

@value
struct DynamicMiddleware:
    var _middleware: Variant[CompressionMiddleware, ErrorMiddleware]

    fn set_next(inout self, owned next: DynamicMiddleware):
        if self._middleware.isa[CompressionMiddleware]():
            self._middleware[CompressionMiddleware].set_next(next)
        elif self._middleware.isa[ErrorMiddleware]():
            self._middleware[ErrorMiddleware].set_next(next)

Then you can just use DynamicMiddleware any place you have some kind of Middleware but don't know which one until runtime. It can also be useful to create an __init__ on DynamicMiddleware method for each type in the variant. This would let you write code like middleware.add(ErrorMiddleware()) instead of stuff like middleware.add(DynamicMiddleware(ErrorMiddleware()))

a2svior•3mo ago

@Ryulord thanks! This is helpful!

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dynamic traits are not supported yet