[Haskell-cafe] Type Directed Name Resolution

[Lauri Alanko]

On Thu, Nov 11, 2010 at 03:17:39PM +0200, Michael Snoyman wrote:
> data PetOwner
> data FurnitureOwner
> 
> data Cat = Cat { owner :: PetOwner }
> data Chair = Chair { owner :: FurnitureOwner }

These are clearly related uses, so as I said, you can use a type class
to overload the accessor name in a controlled fashion.


{-# LANGUAGE EmptyDataDecls, MultiParamTypeClasses, FunctionalDependencies #-}

data PetOwner
data FurnitureOwner

data Cat = Cat { catOwner :: PetOwner }
data Chair = Chair { chairOwner :: FurnitureOwner }

class Owned a b | a -> b where
  owner :: a -> b
  
instance Owned Cat PetOwner where  
  owner = catOwner
  
instance Owned Chair FurnitureOwner where
  owner = chairOwner


(You can also use associated type families for the same effect.)


Lauri