On Thu, Jan 25, 2007 at 11:34:55AM +1100, John Ky wrote:
A while back I asked about OO programming in Haskell and discovered existential types. I understood that existential types allowed me to write heterogeneous lists which seemed sufficient at the time.
Now trying to combine those ideas with records:
data AnyNode = forall a. Node a => AnyNode a
class Node -- yadda yadda
data Branch = Branch { name :: String, description :: String, children :: [AnyNode] } data Leaf = Leaf { name :: String, value :: String }
The problem here is I can't use the same 'name' field for both Branch and Leaf. Ideally I'd like the name field in the Node class, but it doesn't seem that Haskell classes are for that sort of thing.
They are - it's the record system that's biting you. Haskell-98 style records are widely acknowledged as sucking, and there are something like half a dozen proposals all of which are widely acknowledged as vastly superior. Expect to be stuck with H98 records for the remainder of time; see "bikeshed". (Plea to SPJ: stop deliberating and flip a coin!) Anyway, in the one of the proposals whose syntax I can mostly remember: data AnyNode = forall a. a \ name => AnyNode { a | name :: String } data Branch = Branch { name :: String, description :: String, children :: [AnyNode] } data Leaf = Leaf { name :: String, value :: String }