Validation as a ZipFunctor?

[NickDarvey]

I'm not even sure if I'm phrasing this right, but, is Validation a ZipFunctor? And would this be a valid implementation of it?

module Validation =
  let zip = liftA2 tuple2

I can open a PR if it actually is a zippable functor.

[gusty]

That's a very interesting question.

Unfortunately I think the answer is No and I'll tell you why:

Let's suppose Validation is a ZipFunctor which means that it has a zip operation which is the inverse of Functor's unzip (which is defined as unzip s = map fst s, map snd s).

Then we can do:

> zip (Validation<string,int>.Success 42) (Validation<string,float>.Failure "Error") ;;
val it : Validation<string,(int * float)> = Failure "Error"

and now

> unzip it ;;
val it : Validation<string,int> * Validation<string,float> =
  (Failure "Error", Failure "Error")

but now

> it ||> zip;;
val it : Validation<string,(int * float)> = Failure "ErrorError"

so we came back to a different value !

Note however that Result seems to be well behaved in this regard:

> zip (Result<int,string>.Ok 42) (Result<float,string>.Error "No") ;;
[<Struct>]
val it : Result<(int * float),string> = Error "No"

> unzip it ;;
val it : Result<int,string> * Result<float,string> = (Error "No", Error "No")

> it ||> zip;;
[<Struct>]
val it : Result<(int * float),string> = Error "No"

Does it makes sense?

[NickDarvey]

That does make sense. Thank you for the accessible explanation @gusty 😊