Main.purs

module Main where

type Extensible a =
    { count :: Int | a }

data State
    = A (Extensible ( text :: String ))
    | B (Extensible ( whatever :: String ))

updatePosition :: (forall a. Extensible a -> Extensible a) -> State -> State
updatePosition f state =
    case state of
        A x ->
            A (f x)

        B x ->
            B (f x)