Support deriving typeclasses with an effectful return type

[Fristi]

Like a Decoder[A] returning a Either[String, A], for each member of a type class we need to decode it. Currently construct is deficient to do that.

Current

def construct[Return](makeParam: Param[Typeclass, Type] => Return): Type

How should this look like for effects?

def construct[Effect[_] : Applicative, Return](makeParam: Param[Typeclass, Type] => Effect[Return]): Type

Yet this would imply that for type classes with multiple methods the effect type will be the same (like codecs, in scodec this holds though)

Where do we get Applicative from? I wouldn't favor depending on cats or scalaz

[propensive]

I don't think the API needs to be this specific, does it? Could you work with a new construct method which takes a Seq[Any] of the parameters? i.e.

def construct(paramValues: Seq[Any]): Type

You could then fold the ctx.parameters however you like, to get the effect and then pass the Seq[Any] of the parameters to the new construct.

[propensive]

We would, of course, lose some typesafety with this version. The sequence is no longer length-safe, and it would be possible to treat the parameters non-uniformly. I would like to keep the old construct method around, maybe under a different name.

[sirthias]

+1 for adding something like

def construct(paramValues: Seq[Any]): Type

I'm currently trying to write a "conserving tree rewriter" typeclass that is supposed to return the original case class instance if all its members have been rewritten to the identical instance.
With this new construct overload this would be easy. The current design makes this quite hard.

[fommil]

oh, I just fixed this in #120 for Either... but doing it in the general case for flatMap would be good.

[Fristi]

I think this is already done right?