Applied Category Theory in Scala
source link: https://blog.oyanglul.us/grokking-monad/scala/en/choice
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Applied Category Theory in Scala
Choice
Table of Contents
Choice
Rationale
usually we deal with function more often, we're so familiar with A => B
says if we have two function A => C and B => C
how can we compose them into a single function that can take either A or B and produce a C?
in Scala the return type will be like Either[A, B] => C
This is exactly Choice typeclass for, replacing => with F, you
will get a Choice
trait Choice[F[_, _]] {
def choice(fac: F[A, C], fbc: F[B, C]): F[Either[A, B], C]
}
Applied
A very useful case of Choice typeclass would be like middleware in
HTTP server.
Take Http4s for example:
HttpRoutes[F]
http4s defined routes using Kleisli
type HttpRoutes[F[_]] = Kleisli[OptionT[F, ?], Request[F], Response[F]] def routes[F[_]]: HttpRoutes[F] = ???
But before going through routes, most request must pass middleware to
ensure the request has correct or not.
A authentication middleware could end up with 2 kinds of result
- return
Unauthorizedinstantly while token is invalid - Pass
Request[F]through if token if valid
So the return type of middleware will be like
Either[Response[F], Request[F]]
Middleware[F]
if we define middleware like
type Middleware[F[_]] = Kleisli[OptionT[F, ?], Request[F], Either[Response[F], Request[F]]] val passThrough: Kleisli[OptionT[F, ?], Response[F], Response[F]] = Kleisli.ask[OptionT[F, ?], Response[F]] def middleware[F[_]]: Middleware[F] = ???
Compose middleware and routes together is now easy thanks to Kleisli
has instance of Choice
middleware andThen passThrough.choice(routes)
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