Re: [Haskell-cafe] classes for data structures

6 Feb 2008

      On Feb 6, 2008 9:40 AM, Henning Thielemann
 wrote:
...
On Wed, 6 Feb 2008, Denis Bueno wrote:
...
On Feb 6, 2008 9:00 AM, Henning Thielemann
 wrote:
...
On Wed, 6 Feb 2008, Matthew Pocock wrote:
...
class Buildable b where
  empty :: b a --makes an empty b with elements of type a
  insert :: a -> b a -> b a --inserts the new element into the buildable
How can this interface be used both for lists and maps? [...]
One solution is to change the class slightly:
class Buildable t x where
  empty :: t
  insert :: x -> t -> t
instance Buildable (Map k a) (k, a) where
    empty = Map.empty
    insert = uncurry Map.insert
ok, maybe with functional dependency t -> x
I'm not sure about that.  It's often convenient to have two instances,
one like the one I gave above, and others involving something that
embeds a key-value pair:

type SomethingWithKV k a = KV {getKV :: (k, a)}
instance Buildable (Map k a) (SomethingWithKV k a) where
  empty = Map.empty
  insert s m = uncurry Map.insert (getKV s) m

I have done this before -- it's very convenient, and I think makes the
code that uses empty and insert more robust, and easier to read.

-- 
                              Denis