type Validation<'e,'v>
  | Failure of 'e
  | Success of 'v

let IsSuccess<'e, 'v> (v: Validation<'e, 'v>) =
  match v with
  | Success _ -> true
  | Failure _ -> false

let IsFailure<'e, 'v> (v: Validation<'e, 'v>) =
  match v with
  | Success _ -> false
  | Failure _ -> true

let MapFailure<'e, 'v, 'b> (v: Validation<'e, 'v>) (f: 'e -> 'b) =
  match v with
  | Success _ -> v
  | Failure x -> Failure(f x)

let MapSuccess<'e, 'v, 'b> (v: Validation<'e, 'v>) (f: 'v -> 'b) =
  match v with
  | Success x -> Success(f x)
  | Failure _ -> v

let Fold<'e, 'v, 'a> (v: Validation<'e, 'v>) (e: 'e -> 'a) (s: 'v -> 'a)
  match v with
  | Success x -> s x
  | Failure x -> e x