Validation shouldn't have Empty

The type Validation belongs to the Alt abstraction (operation <|>) but not to the Alternative abstraction (operation <|> and empty element).

Why?

Because it doesn't obey the Alternative rule empty <*> f = empty

Here's a repro:

> let (e: Validation<string list,int>) = empty
val e : Validation<string list,int> = Failure []

// but

> let (x: Validation<string list,int>) = empty <*> Failure [""] ;;
val x : Validation<string list,int> = Failure [""]

> e = x ;;
val it : bool = false

I can't see the Alternative instance in the Haskell implementation, if it ever was there now they removed it.

What to do?

We can't remove it now, because of backwards compatibility issues. We'll have to wait until v2.0

Start by reviewing the Validation type and its Alternative operations, especially empty and <*>; compare the reported behavior with the Haskell implementation mentioned in the issue. The work is complete when the v2.0 compatibility plan resolves the invalid Alternative behavior without leaving the documented law violation.