WIP: Type class that combines RefType and Validate

[fthomas]

This adds the RefinedType type class that combines RefType and Validate instances for a refined type F[T, P]. The motivating example for this type class was given by @benhutchison on Gitter. Abstracting over RefType.applyRef currently requires a lot of type ceremony:

def refineEff[S] = new ApplyRefPartiallyAppliedExt[S]

final class ApplyRefPartiallyAppliedExt[S] {
  def apply[E: Err, F[_, _], T, P](t: T)(
    implicit ev: F[T, P] =:= S, rt: RefType[F], v: Validate[T, P]
  ): Eff[E, S] =
    fromXor(RefType.applyRef[S].apply(t).toXor)
}

The goal is to allow calls like refineEff[PosInt](5) where PosInt is an alias for Int Refined Positive and the base type Int can be inferred from the parameter of apply. With RefinedType, this can be simplified to:

def refineEff[S] = new ApplyRefPartiallyAppliedExt[S]

final class ApplyRefPartiallyAppliedExt[S] {
  def apply[E: Err, T](t: T)(
    implicit rt: RefinedType.AuxT[S, T]
  ): Eff[E, S] =
    fromXor(rt.refine(t).toXor)
}

What has improved with RefinedType:

• apply has two fewer type parameters
• apply has two fewer implicit parameters
• RefType.applyRef[S].apply(t) became rt.refine(t)

In general I think this is a win. The only downside is that we still require the ApplyRefPartiallyAppliedExt class to partially apply the refined type S and to let the base type T be inferred.

Also the combination of RefType and Validate is very common (e.g., type class instances of 3rd-party libraries for refined types almost always require the combination of these two), so it is natural to combine them. It also makes the implementation of refined simpler in some cases.

It is unfortunate that the names RefType and RefinedType are very similar. This type class makes me regret the name RefType. Maybe something like CarrierType or WrapperType would have been a better choice.

[codecov-io]

Current coverage is 98.51% (diff: 95.00%)

Merging #193 into master will decrease coverage by 0.47%

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[benhutchison]

+1. Much simpler. Im keen to try it out in a project!

[fthomas]

Superseded by #369 which also contains the RefinedType type class.