Re: [Haskell-cafe] The container problem

On Sat, Sep 27, 2008 at 12:23 PM, Andrew Coppin wrote:

Can anybody actually demonstrate concretely how FDs and/or ATs would solve this problem? (I.e., enable you to write a class that any container can be a member of, despite constraints on the element types.)

Sure! Using type-families:

class Container c where type Elem c insert :: Elem c -> c -> c

instance Container [a] where type Elem [a] = a insert = (:)

instance Container ByteString where type Elem ByteString = Word8 insert = BS.cons

instance Ord a => Container (Set a) where type Elem (Set a) = a insert = Set.insert

In GHCi:

:t insert insert :: forall c. (Container c) => Elem c -> c -> c

Now the hard part is coming up with a proper API and class hierarchy. -Antoine