Re: [Haskell-cafe] Type Directed Name Resolution

11 Nov 2010

      On Thu, Nov 11, 2010 at 3:10 PM, Lauri Alanko  wrote:
...
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.)
Well, it's not exactly the same. For example:

myCat = Cat { owner = michael }

versus

myCat = Cat { catOwner = michael }

Not to mention that with TDNR, there is much less typing involved: no
need to declare a type class, declare instances, export the type
class.

Michael