AsyncShift

AsynsShift is a marker base trait for typeclass, which provides 'shifted' variants of the hight-order methods of T, which called when we need to pass a cps-transformed function as an argument for this method.

The general convention is next:

• Let us have object O and method m(f: A=>B):R which accept hight-order argument f: A=>B. (for example - map in List).
• If we want to defined transformation of argument for any monad F, we should define the AsyncShift[O] with method m[F[_],...](o:O, m:CpsMonad[F])(f: A=>F[B]).
• Return type of this method can be F[R] or R or AsyncSubst[R].

Also we should define a given instance of AsyncShift[O], visible from our async block. I.e. implementation for our list will look as:

   class MyShiftedList[T] extentds AsyncShift[List[T]] {

     def map[F[_],S](m:CpsMonad[M], c:List[T])(f: T=>F[S]): F[List[T]] =
         ... // implementation here

   }

   transparent inline given myShiftedList[T]: AsyncShift[List[T]] = MyShiftedList[T]()

After this, you can freely use awaits inside "List.map":

  async {
    ....
    val fetched = uris.map(uri => await(fetch(uri)))
    ...
  }

see https://rssh.github.io/dotty-cps-async/HighOrderFunctions.html