Hi, On 28/08/13 21:05, Wvv wrote:
Let we have data in one module as this:
data Person = Person { personId :: Int, name :: String } data Address a = Address { personId :: Int, address :: String , way :: a}
It was discussed a lot in topics "OverloadedRecordFields"
This is an alternative: Let we have polymorphic typeclass:
class Record{f} a b | a -> b where f :: a -> b
so, compiler could create instances for our data as:
instance Record{personId} Person Int where personId (Person x _) = x
instance Record{personId} (Address a) Int where personId (Address x _ _) = x
instance Record{way} (Address Int) Int where way (Address _ _ x) = x
and we could use this:
p:: Record {personId} r Int => r -> Int p = personId
What do you think about this?
This more or less is what I am implementing as the OverloadedRecordFields extension. I define the following: class t ~ GetResult r f => Has r (f :: Symbol) t where getField :: proxy f -> r -> t type family GetResult (r :: *) (f :: Symbol) :: * instance Has Person "personId" Int where getField _ (Person x _) = x The `Has` class corresponds to your `Record` class, except it uses a type family instead of a functional dependency. Moreover, Haskell doesn't allow classes to be polymorphic in the names of their methods, so a single method `getField` is used. You can find out more about the design on the GHC wiki, which also has links to the development repositories and a prototype implementation: http://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Plan Regards, Adam