type Either<'l, 'r> = abstract cata : ('l -> 'a) -> ('r -> 'a) -> 'a

let left  x = { new Either<_, _> with member this.cata l r = l x }
let right x = { new Either<_, _> with member this.cata l r = r x }

System.Console.WriteLine((right "lol").cata (fun _ -> "") id)