Add empty to State and StateT objects

[sderosiaux]

Hi,

I had some States where I needed to deal with empty values, such as:

def orEmpty[F[_]: MonadError[?[_], Unit]: SemigroupK, A: Monoid](s: Parser[F, A]) =
    s <+> StateT.pure(Monoid[A].empty)

In order to be explicit, why not add a empty constructor using a Monoid[A] that calls pure behind the scene? Note that IndexedStateT has runEmpty variants with the same Monoid[A] logic.

If you think it's an API overhead, feel free to discard and close the PR, no worry.

[kailuowang]

Could this be written as s <+> A.emtpy.pure? Finger crossed that ScalaC can infer the type.

[sderosiaux]

@kailuowang No, I get diverging and ambiguous implicits (cats.implicits._). It works with s <+> Monoid[A].empty.pure[Parser[F, ?]] (which is the inverse notation of my initial example).

[kailuowang]

yeah, unfortunately, scalac fails on inferring higher kinded types.

[larsrh]

Rebased in #2533.