Cats: EitherT.fromMonadError

[tg44]

I think #1644 is related.

Basicly something like:

import cats.implicits._
import cats.data.EitherT
import cats.MonadError

def fromMonadError[F[_], A, B](me: F[B])(implicit ev: MonadError[F, A]) = {
  EitherT(me.map(Either.right[A, B]).recover{case ex => Either.left[A, B](ex)})
}

Or if we have this functionality already maybe it would worth a mention in the EitherT/MonadError documentation.

If others think this is useful, I try to PR it :)

[djspiewak]

I think this is neat, though I believe it could be more simply implemented with attempt. Something like this:

def fromMonadError[F[_], E, A](fa: F[A])(implicit F: MonadError[F, E]): EitherT[F, E, A] =
  EitherT(F.attempt(fa))

It's kind of like a mechanism for lifting into a redundant signal channel. I do stuff like this semi-frequently; I'm curious if others do as well.

[joroKr21]

It comes up surprisingly often for me as well.

[fthomas]

It seems this already exists and is called attemptT.

[tg44]

Nice! So can we add this to the EitherT docs?