G'day all. On Wed, Aug 21, 2002 at 02:31:05PM +0100, Guest, Simon wrote:
Please could someone explain the meaning of | in this class declaration (from Andrew's example):
class (Ord k) => Map m k v | m -> k v where lookupM :: m -> k -> Maybe v
Others have answered the question about what it means. However, this doesn't explain why I used a fundep when Haskell has perfectly good constructor classes. I could have written: class (Ord k) => Map m k v where lookupM :: m k v -> k -> Maybe v instance (Ord k) => Map FiniteMap k v where lookupM = lookupFM However, this would not work for the other two cases (the assoc list and the function). For that, I'd have to introduce a new type, such as: newtype MapFunc k v = MapFunc (k -> Maybe v) instance (Ord k) => Map MapFunc k v where lookupM (MapFunc f) = f A good Haskell compiler would optimise the representation of the type, so it wouldn't cost much (or possibly _anything_) at run time, but it's still a pain to program with. You need to pack and unpack the MapFunc type at awkward places, when all you really want to do is rearrange type variables for one declaration. Fundeps let you avoid many of these "artificial" constructors. Unfortunately, I don't think that fundeps will help you to get rid of all of them. For example, the standard state transformer monad: newtype State s a = State { runState :: s -> (a, s) } I don't think you can get rid of the constructor here. Cheers, Andrew Bromage