test.hs

{-# LANGUAGE ScopedTypeVariables #-}

type Speed = Double
type Name = String

class Ship s c where
  sail :: s -> c -> Speed

data Sea s c = Sea {
  ships :: M.Map (Name) s
  }

data Cartesian = Cartesian

class Coordinate c where
  toCartesian :: c -> Cartesian
  fromCartesian :: Cartesian -> c

sailShip :: forall s c. (Coordinate c, Ship s c) => Sea s c -> Name -> Cartesian -> Speed
sailShip sea name c = let
  cord1 = fromCartesian c::c
  cord2 = toCartesian cord1
  cord3 = fromCartesian cord2::c
  ship = ships sea M.! name
  in sail ship cord3