Add Raise#fromEither and friends

[armanbilge]

Inspired by discussion with @benhutchison in typelevel/cats-effect#3765 (reply in thread). The idea is to add a bunch of methods for lifting from the "primitive" monads Either, Reader, Writer, State into F[_]. This complements #489 which pushes towards using IO for F[_] instead of the monad transformers EitherT, Kleisli, WriterT, and StateT. So we need a mechanism for lifting.

• Ask#fromReader
• Raise#fromEither (analogous to ApplicativeError#fromEither)
• Stateful#fromState
• Tell#fromWriter

[armanbilge]

Andrew raised a great question about use-cases, which made me realize we can do even better 🙂

For example suppose you have a Ask[IO, Request[IO]] and you want to interop with an http4s thing Kleisli[IO, Request[IO], Response[IO]]. Well, now you can!

[benhutchison]

As one user of this library, I'd like to "bias towards action" and see you merge this @armanbilge, and some version of the error submarine if you see fit.

I think currently the risks of stagnation exceed the risks from lightly reviewed code getting merged, so my preference is for low barriers to contribution, for non-breaking changes at least.

[armanbilge]

As one user of this library, I'd like to "bias towards action" and see you merge this @armanbilge

@benhutchison I literally cannot merge my own PRs :) I can however review and merge work submitted by others.

[igor-ramazanov]

Suggested change

	def fromValidated[E2 <: E, A](va: Validated[E2, A])(implicit F: Applicative[F]): F[A] =
	va.fold(raise, F.pure)

[igor-ramazanov]

I wonder if we are not accidentally adding a performance penalty comparing with, for example:

  def fromValidated[E2 <: E, A](va: Validated[E2, A])(implicit F: Applicative[F]): F[A] =
    va match {
      case Validated.Invalid(e) => raise(e)
      case Validated.Valid(a) => F.pure(a)
    }

[armanbilge]

Actually we are. Using fold requires allocating lambdas, pattern matching does not. I don't think it's important in practice.

[djspiewak]

Bike-shedding a bit, the from prefix threw me for quite a while as I was trying to parse this. I can't think of a better name though. This is absolutely a nice addition.